From 8c0f6011057b5b2f7bd4d74ce63caf8d6630c39e Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Sat, 17 Aug 2019 12:57:44 +0200
Subject: [PATCH 01/18] fix Miri discriminant load/store when overflows are
 involved

---
 src/librustc_mir/interpret/operand.rs | 40 +++++++++++++++++++++----
 src/librustc_mir/interpret/place.rs   | 42 ++++++++++++++++++++-------
 2 files changed, 65 insertions(+), 17 deletions(-)

diff --git a/src/librustc_mir/interpret/operand.rs b/src/librustc_mir/interpret/operand.rs
index 9ad1542905b68..3b07202f898ba 100644
--- a/src/librustc_mir/interpret/operand.rs
+++ b/src/librustc_mir/interpret/operand.rs
@@ -609,15 +609,20 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
     ) -> InterpResult<'tcx, (u128, VariantIdx)> {
         trace!("read_discriminant_value {:#?}", rval.layout);
 
-        let (discr_kind, discr_index) = match rval.layout.variants {
+        let (discr_layout, discr_kind, discr_index) = match rval.layout.variants {
             layout::Variants::Single { index } => {
                 let discr_val = rval.layout.ty.discriminant_for_variant(*self.tcx, index).map_or(
                     index.as_u32() as u128,
                     |discr| discr.val);
                 return Ok((discr_val, index));
             }
-            layout::Variants::Multiple { ref discr_kind, discr_index, .. } =>
-                (discr_kind, discr_index),
+            layout::Variants::Multiple {
+                discr: ref discr_layout,
+                ref discr_kind,
+                discr_index,
+                ..
+            } =>
+                (discr_layout, discr_kind, discr_index),
         };
 
         // read raw discriminant value
@@ -634,7 +639,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                     .map_err(|_| err_unsup!(InvalidDiscriminant(raw_discr.erase_tag())))?;
                 let real_discr = if discr_val.layout.ty.is_signed() {
                     // going from layout tag type to typeck discriminant type
-                    // requires first sign extending with the layout discriminant
+                    // requires first sign extending with the discriminant layout
                     let sexted = sign_extend(bits_discr, discr_val.layout.size) as i128;
                     // and then zeroing with the typeck discriminant type
                     let discr_ty = rval.layout.ty
@@ -682,8 +687,31 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                         (dataful_variant.as_u32() as u128, dataful_variant)
                     },
                     Ok(raw_discr) => {
-                        let adjusted_discr = raw_discr.wrapping_sub(niche_start)
-                            .wrapping_add(variants_start);
+                        // FIXME: WTF, some discriminants don't have integer type.
+                        use layout::Primitive;
+                        let discr_layout = self.layout_of(match discr_layout.value {
+                            Primitive::Int(int, signed) => int.to_ty(*self.tcx, signed),
+                            Primitive::Pointer => self.tcx.types.usize,
+                            Primitive::Float(..) => bug!("there are no float discriminants"),
+                        })?;
+                        let discr_val = ImmTy::from_uint(raw_discr, discr_layout);
+                        // We need to use machine arithmetic.
+                        let niche_start_val = ImmTy::from_uint(niche_start, discr_layout);
+                        let variants_start_val = ImmTy::from_uint(variants_start, discr_layout);
+                        let adjusted_discr = self.binary_op(
+                            mir::BinOp::Sub,
+                            discr_val,
+                            niche_start_val,
+                        )?;
+                        let adjusted_discr = self.binary_op(
+                            mir::BinOp::Add,
+                            adjusted_discr,
+                            variants_start_val,
+                        )?;
+                        let adjusted_discr = adjusted_discr
+                            .to_scalar()?
+                            .assert_bits(discr_val.layout.size);
+                        // Check if this is in the range that indicates an actual discriminant.
                         if variants_start <= adjusted_discr && adjusted_discr <= variants_end {
                             let index = adjusted_discr as usize;
                             assert_eq!(index as u128, adjusted_discr);
diff --git a/src/librustc_mir/interpret/place.rs b/src/librustc_mir/interpret/place.rs
index 1a28548618206..0a23c1222b4bf 100644
--- a/src/librustc_mir/interpret/place.rs
+++ b/src/librustc_mir/interpret/place.rs
@@ -8,7 +8,9 @@ use std::hash::Hash;
 use rustc::mir;
 use rustc::mir::interpret::truncate;
 use rustc::ty::{self, Ty};
-use rustc::ty::layout::{self, Size, Align, LayoutOf, TyLayout, HasDataLayout, VariantIdx};
+use rustc::ty::layout::{
+    self, Size, Align, LayoutOf, TyLayout, HasDataLayout, VariantIdx, IntegerExt
+};
 use rustc::ty::TypeFoldable;
 
 use super::{
@@ -1027,7 +1029,7 @@ where
             }
             layout::Variants::Multiple {
                 discr_kind: layout::DiscriminantKind::Tag,
-                ref discr,
+                discr: ref discr_layout,
                 discr_index,
                 ..
             } => {
@@ -1038,7 +1040,7 @@ where
                 // raw discriminants for enums are isize or bigger during
                 // their computation, but the in-memory tag is the smallest possible
                 // representation
-                let size = discr.value.size(self);
+                let size = discr_layout.value.size(self);
                 let discr_val = truncate(discr_val, size);
 
                 let discr_dest = self.place_field(dest, discr_index as u64)?;
@@ -1050,6 +1052,7 @@ where
                     ref niche_variants,
                     niche_start,
                 },
+                discr: ref discr_layout,
                 discr_index,
                 ..
             } => {
@@ -1057,15 +1060,32 @@ where
                     variant_index.as_usize() < dest.layout.ty.ty_adt_def().unwrap().variants.len(),
                 );
                 if variant_index != dataful_variant {
-                    let niche_dest =
-                        self.place_field(dest, discr_index as u64)?;
-                    let niche_value = variant_index.as_u32() - niche_variants.start().as_u32();
-                    let niche_value = (niche_value as u128)
-                        .wrapping_add(niche_start);
-                    self.write_scalar(
-                        Scalar::from_uint(niche_value, niche_dest.layout.size),
-                        niche_dest
+                    // FIXME: WTF, some discriminants don't have integer type.
+                    use layout::Primitive;
+                    let discr_layout = self.layout_of(match discr_layout.value {
+                        Primitive::Int(int, signed) => int.to_ty(*self.tcx, signed),
+                        Primitive::Pointer => self.tcx.types.usize,
+                        Primitive::Float(..) => bug!("there are no float discriminants"),
+                    })?;
+
+                    // We need to use machine arithmetic.
+                    let variants_start = niche_variants.start().as_u32();
+                    let variants_start_val = ImmTy::from_uint(variants_start, discr_layout);
+                    let niche_start_val = ImmTy::from_uint(niche_start, discr_layout);
+                    let variant_index_val = ImmTy::from_uint(variant_index.as_u32(), discr_layout);
+                    let niche_val = self.binary_op(
+                        mir::BinOp::Sub,
+                        variant_index_val,
+                        variants_start_val,
+                    )?;
+                    let niche_val = self.binary_op(
+                        mir::BinOp::Add,
+                        niche_val,
+                        niche_start_val,
                     )?;
+                    // Write result.
+                    let niche_dest = self.place_field(dest, discr_index as u64)?;
+                    self.write_immediate(*niche_val, niche_dest)?;
                 }
             }
         }

From b73f1a51dcd1011129dc5c5c55a7577134410dbc Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Mon, 16 Sep 2019 13:08:21 +0200
Subject: [PATCH 02/18] better and more consistent variable names

---
 src/librustc_mir/interpret/operand.rs | 16 ++++++++--------
 src/librustc_mir/interpret/place.rs   |  8 ++++----
 2 files changed, 12 insertions(+), 12 deletions(-)

diff --git a/src/librustc_mir/interpret/operand.rs b/src/librustc_mir/interpret/operand.rs
index 3b07202f898ba..2a0bad2fa746c 100644
--- a/src/librustc_mir/interpret/operand.rs
+++ b/src/librustc_mir/interpret/operand.rs
@@ -698,28 +698,28 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                         // We need to use machine arithmetic.
                         let niche_start_val = ImmTy::from_uint(niche_start, discr_layout);
                         let variants_start_val = ImmTy::from_uint(variants_start, discr_layout);
-                        let adjusted_discr = self.binary_op(
+                        let variant_index_relative_val = self.binary_op(
                             mir::BinOp::Sub,
                             discr_val,
                             niche_start_val,
                         )?;
-                        let adjusted_discr = self.binary_op(
+                        let variant_index_val = self.binary_op(
                             mir::BinOp::Add,
-                            adjusted_discr,
+                            variant_index_relative_val,
                             variants_start_val,
                         )?;
-                        let adjusted_discr = adjusted_discr
+                        let variant_index = variant_index_val
                             .to_scalar()?
                             .assert_bits(discr_val.layout.size);
                         // Check if this is in the range that indicates an actual discriminant.
-                        if variants_start <= adjusted_discr && adjusted_discr <= variants_end {
-                            let index = adjusted_discr as usize;
-                            assert_eq!(index as u128, adjusted_discr);
+                        if variants_start <= variant_index && variant_index <= variants_end {
+                            let index = variant_index as usize;
+                            assert_eq!(index as u128, variant_index);
                             assert!(index < rval.layout.ty
                                 .ty_adt_def()
                                 .expect("tagged layout for non adt")
                                 .variants.len());
-                            (adjusted_discr, VariantIdx::from_usize(index))
+                            (variant_index, VariantIdx::from_usize(index))
                         } else {
                             (dataful_variant.as_u32() as u128, dataful_variant)
                         }
diff --git a/src/librustc_mir/interpret/place.rs b/src/librustc_mir/interpret/place.rs
index 0a23c1222b4bf..636b070a3d8f3 100644
--- a/src/librustc_mir/interpret/place.rs
+++ b/src/librustc_mir/interpret/place.rs
@@ -1073,19 +1073,19 @@ where
                     let variants_start_val = ImmTy::from_uint(variants_start, discr_layout);
                     let niche_start_val = ImmTy::from_uint(niche_start, discr_layout);
                     let variant_index_val = ImmTy::from_uint(variant_index.as_u32(), discr_layout);
-                    let niche_val = self.binary_op(
+                    let variant_index_relative_val = self.binary_op(
                         mir::BinOp::Sub,
                         variant_index_val,
                         variants_start_val,
                     )?;
-                    let niche_val = self.binary_op(
+                    let discr_val = self.binary_op(
                         mir::BinOp::Add,
-                        niche_val,
+                        variant_index_relative_val,
                         niche_start_val,
                     )?;
                     // Write result.
                     let niche_dest = self.place_field(dest, discr_index as u64)?;
-                    self.write_immediate(*niche_val, niche_dest)?;
+                    self.write_immediate(*discr_val, niche_dest)?;
                 }
             }
         }

From 75fe84f4fc764e0684e7fdafbe6139cc92082a44 Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Mon, 16 Sep 2019 13:15:23 +0200
Subject: [PATCH 03/18] factor getting the discriminant layout to a new method

---
 src/librustc/ty/layout.rs             | 11 +++++++++++
 src/librustc_mir/interpret/operand.rs | 10 ++--------
 src/librustc_mir/interpret/place.rs   | 11 ++---------
 3 files changed, 15 insertions(+), 17 deletions(-)

diff --git a/src/librustc/ty/layout.rs b/src/librustc/ty/layout.rs
index 5ec4754c4535b..e52feea1624c1 100644
--- a/src/librustc/ty/layout.rs
+++ b/src/librustc/ty/layout.rs
@@ -127,6 +127,7 @@ impl IntegerExt for Integer {
 
 pub trait PrimitiveExt {
     fn to_ty<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Ty<'tcx>;
+    fn to_int_ty<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Ty<'tcx>;
 }
 
 impl PrimitiveExt for Primitive {
@@ -138,6 +139,16 @@ impl PrimitiveExt for Primitive {
             Pointer => tcx.mk_mut_ptr(tcx.mk_unit()),
         }
     }
+
+    /// Return an *integer* type matching this primitive.
+    /// Useful in particular when dealing with enum discriminants.
+    fn to_int_ty(&self, tcx: TyCtxt<'tcx>) -> Ty<'tcx> {
+        match *self {
+            Int(i, signed) => i.to_ty(tcx, signed),
+            Pointer => tcx.types.usize,
+            Float(..) => bug!("floats do not have an int type"),
+        }
+    }
 }
 
 /// The first half of a fat pointer.
diff --git a/src/librustc_mir/interpret/operand.rs b/src/librustc_mir/interpret/operand.rs
index 2a0bad2fa746c..94fbd4ac7d7f3 100644
--- a/src/librustc_mir/interpret/operand.rs
+++ b/src/librustc_mir/interpret/operand.rs
@@ -5,7 +5,7 @@ use std::convert::TryInto;
 
 use rustc::{mir, ty};
 use rustc::ty::layout::{
-    self, Size, LayoutOf, TyLayout, HasDataLayout, IntegerExt, VariantIdx,
+    self, Size, LayoutOf, TyLayout, HasDataLayout, IntegerExt, PrimitiveExt, VariantIdx,
 };
 
 use rustc::mir::interpret::{
@@ -687,13 +687,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                         (dataful_variant.as_u32() as u128, dataful_variant)
                     },
                     Ok(raw_discr) => {
-                        // FIXME: WTF, some discriminants don't have integer type.
-                        use layout::Primitive;
-                        let discr_layout = self.layout_of(match discr_layout.value {
-                            Primitive::Int(int, signed) => int.to_ty(*self.tcx, signed),
-                            Primitive::Pointer => self.tcx.types.usize,
-                            Primitive::Float(..) => bug!("there are no float discriminants"),
-                        })?;
+                        let discr_layout = self.layout_of(discr_layout.value.to_int_ty(*self.tcx))?;
                         let discr_val = ImmTy::from_uint(raw_discr, discr_layout);
                         // We need to use machine arithmetic.
                         let niche_start_val = ImmTy::from_uint(niche_start, discr_layout);
diff --git a/src/librustc_mir/interpret/place.rs b/src/librustc_mir/interpret/place.rs
index 636b070a3d8f3..9a3d70144a586 100644
--- a/src/librustc_mir/interpret/place.rs
+++ b/src/librustc_mir/interpret/place.rs
@@ -9,7 +9,7 @@ use rustc::mir;
 use rustc::mir::interpret::truncate;
 use rustc::ty::{self, Ty};
 use rustc::ty::layout::{
-    self, Size, Align, LayoutOf, TyLayout, HasDataLayout, VariantIdx, IntegerExt
+    self, Size, Align, LayoutOf, TyLayout, HasDataLayout, VariantIdx, PrimitiveExt
 };
 use rustc::ty::TypeFoldable;
 
@@ -1060,14 +1060,7 @@ where
                     variant_index.as_usize() < dest.layout.ty.ty_adt_def().unwrap().variants.len(),
                 );
                 if variant_index != dataful_variant {
-                    // FIXME: WTF, some discriminants don't have integer type.
-                    use layout::Primitive;
-                    let discr_layout = self.layout_of(match discr_layout.value {
-                        Primitive::Int(int, signed) => int.to_ty(*self.tcx, signed),
-                        Primitive::Pointer => self.tcx.types.usize,
-                        Primitive::Float(..) => bug!("there are no float discriminants"),
-                    })?;
-
+                    let discr_layout = self.layout_of(discr_layout.value.to_int_ty(*self.tcx))?;
                     // We need to use machine arithmetic.
                     let variants_start = niche_variants.start().as_u32();
                     let variants_start_val = ImmTy::from_uint(variants_start, discr_layout);

From 61a28d5c881d81cca9156527851911bb5aa5b818 Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Mon, 16 Sep 2019 15:04:33 +0200
Subject: [PATCH 04/18] do the variant idx computations on the host
 (non-overflowing)

---
 src/librustc_mir/interpret/operand.rs | 14 ++++++--------
 src/librustc_mir/interpret/place.rs   | 16 +++++++---------
 2 files changed, 13 insertions(+), 17 deletions(-)

diff --git a/src/librustc_mir/interpret/operand.rs b/src/librustc_mir/interpret/operand.rs
index 94fbd4ac7d7f3..9e7ae32f1eacf 100644
--- a/src/librustc_mir/interpret/operand.rs
+++ b/src/librustc_mir/interpret/operand.rs
@@ -687,24 +687,22 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                         (dataful_variant.as_u32() as u128, dataful_variant)
                     },
                     Ok(raw_discr) => {
+                        // We need to use machine arithmetic to get the relative variant idx.
                         let discr_layout = self.layout_of(discr_layout.value.to_int_ty(*self.tcx))?;
                         let discr_val = ImmTy::from_uint(raw_discr, discr_layout);
-                        // We need to use machine arithmetic.
                         let niche_start_val = ImmTy::from_uint(niche_start, discr_layout);
-                        let variants_start_val = ImmTy::from_uint(variants_start, discr_layout);
                         let variant_index_relative_val = self.binary_op(
                             mir::BinOp::Sub,
                             discr_val,
                             niche_start_val,
                         )?;
-                        let variant_index_val = self.binary_op(
-                            mir::BinOp::Add,
-                            variant_index_relative_val,
-                            variants_start_val,
-                        )?;
-                        let variant_index = variant_index_val
+                        let variant_index_relative = variant_index_relative_val
                             .to_scalar()?
                             .assert_bits(discr_val.layout.size);
+                        // Then computing the absolute variant idx should not overflow any more.
+                        let variant_index = variants_start
+                            .checked_add(variant_index_relative)
+                            .expect("oveflow computing absolute variant idx");
                         // Check if this is in the range that indicates an actual discriminant.
                         if variants_start <= variant_index && variant_index <= variants_end {
                             let index = variant_index as usize;
diff --git a/src/librustc_mir/interpret/place.rs b/src/librustc_mir/interpret/place.rs
index 9a3d70144a586..5f4903d61e3b2 100644
--- a/src/librustc_mir/interpret/place.rs
+++ b/src/librustc_mir/interpret/place.rs
@@ -1060,17 +1060,15 @@ where
                     variant_index.as_usize() < dest.layout.ty.ty_adt_def().unwrap().variants.len(),
                 );
                 if variant_index != dataful_variant {
-                    let discr_layout = self.layout_of(discr_layout.value.to_int_ty(*self.tcx))?;
-                    // We need to use machine arithmetic.
                     let variants_start = niche_variants.start().as_u32();
-                    let variants_start_val = ImmTy::from_uint(variants_start, discr_layout);
+                    let variant_index_relative = variant_index.as_u32()
+                        .checked_sub(variants_start)
+                        .expect("overflow computing relative variant idx");
+                    // We need to use machine arithmetic when taking into account `niche_start`.
+                    let discr_layout = self.layout_of(discr_layout.value.to_int_ty(*self.tcx))?;
                     let niche_start_val = ImmTy::from_uint(niche_start, discr_layout);
-                    let variant_index_val = ImmTy::from_uint(variant_index.as_u32(), discr_layout);
-                    let variant_index_relative_val = self.binary_op(
-                        mir::BinOp::Sub,
-                        variant_index_val,
-                        variants_start_val,
-                    )?;
+                    let variant_index_relative_val =
+                        ImmTy::from_uint(variant_index_relative, discr_layout);
                     let discr_val = self.binary_op(
                         mir::BinOp::Add,
                         variant_index_relative_val,

From b21622cd6cc6c88cc6760e76ba1442aee1e2b9a8 Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Mon, 16 Sep 2019 16:25:39 +0200
Subject: [PATCH 05/18] add test

---
 .../miri_unleashed/enum_discriminants.rs      | 110 ++++++++++++++++++
 .../miri_unleashed/enum_discriminants.stderr  |  72 ++++++++++++
 2 files changed, 182 insertions(+)
 create mode 100644 src/test/ui/consts/miri_unleashed/enum_discriminants.rs
 create mode 100644 src/test/ui/consts/miri_unleashed/enum_discriminants.stderr

diff --git a/src/test/ui/consts/miri_unleashed/enum_discriminants.rs b/src/test/ui/consts/miri_unleashed/enum_discriminants.rs
new file mode 100644
index 0000000000000..9f34fc73953a4
--- /dev/null
+++ b/src/test/ui/consts/miri_unleashed/enum_discriminants.rs
@@ -0,0 +1,110 @@
+// compile-flags: -Zunleash-the-miri-inside-of-you
+// run-pass
+
+//! Make sure that we read and write enum discriminants correctly for corner cases caused
+//! by layout optimizations.
+
+const OVERFLOW: usize = {
+    // Tests for https://github.com/rust-lang/rust/issues/62138.
+    #[repr(u8)]
+    #[allow(dead_code)]
+    enum WithWraparoundInvalidValues {
+        X = 1,
+        Y = 254,
+    }
+
+    #[allow(dead_code)]
+    enum Foo {
+        A,
+        B,
+        C(WithWraparoundInvalidValues),
+    }
+
+    let x = Foo::B;
+    match x {
+        Foo::B => 0,
+        _ => panic!(),
+    }
+};
+
+const MORE_OVERFLOW: usize = {
+    pub enum Infallible {}
+
+    // The check that the `bool` field of `V1` is encoding a "niche variant"
+    // (i.e. not `V1`, so `V3` or `V4`) used to be mathematically incorrect,
+    // causing valid `V1` values to be interpreted as other variants.
+    #[allow(dead_code)]
+    pub enum E1 {
+        V1 { f: bool },
+        V2 { f: Infallible },
+        V3,
+        V4,
+    }
+
+    // Computing the discriminant used to be done using the niche type (here `u8`,
+    // from the `bool` field of `V1`), overflowing for variants with large enough
+    // indices (`V3` and `V4`), causing them to be interpreted as other variants.
+    #[allow(dead_code)]
+    pub enum E2<X> {
+        V1 { f: bool },
+
+        /*_00*/ _01(X), _02(X), _03(X), _04(X), _05(X), _06(X), _07(X),
+        _08(X), _09(X), _0A(X), _0B(X), _0C(X), _0D(X), _0E(X), _0F(X),
+        _10(X), _11(X), _12(X), _13(X), _14(X), _15(X), _16(X), _17(X),
+        _18(X), _19(X), _1A(X), _1B(X), _1C(X), _1D(X), _1E(X), _1F(X),
+        _20(X), _21(X), _22(X), _23(X), _24(X), _25(X), _26(X), _27(X),
+        _28(X), _29(X), _2A(X), _2B(X), _2C(X), _2D(X), _2E(X), _2F(X),
+        _30(X), _31(X), _32(X), _33(X), _34(X), _35(X), _36(X), _37(X),
+        _38(X), _39(X), _3A(X), _3B(X), _3C(X), _3D(X), _3E(X), _3F(X),
+        _40(X), _41(X), _42(X), _43(X), _44(X), _45(X), _46(X), _47(X),
+        _48(X), _49(X), _4A(X), _4B(X), _4C(X), _4D(X), _4E(X), _4F(X),
+        _50(X), _51(X), _52(X), _53(X), _54(X), _55(X), _56(X), _57(X),
+        _58(X), _59(X), _5A(X), _5B(X), _5C(X), _5D(X), _5E(X), _5F(X),
+        _60(X), _61(X), _62(X), _63(X), _64(X), _65(X), _66(X), _67(X),
+        _68(X), _69(X), _6A(X), _6B(X), _6C(X), _6D(X), _6E(X), _6F(X),
+        _70(X), _71(X), _72(X), _73(X), _74(X), _75(X), _76(X), _77(X),
+        _78(X), _79(X), _7A(X), _7B(X), _7C(X), _7D(X), _7E(X), _7F(X),
+        _80(X), _81(X), _82(X), _83(X), _84(X), _85(X), _86(X), _87(X),
+        _88(X), _89(X), _8A(X), _8B(X), _8C(X), _8D(X), _8E(X), _8F(X),
+        _90(X), _91(X), _92(X), _93(X), _94(X), _95(X), _96(X), _97(X),
+        _98(X), _99(X), _9A(X), _9B(X), _9C(X), _9D(X), _9E(X), _9F(X),
+        _A0(X), _A1(X), _A2(X), _A3(X), _A4(X), _A5(X), _A6(X), _A7(X),
+        _A8(X), _A9(X), _AA(X), _AB(X), _AC(X), _AD(X), _AE(X), _AF(X),
+        _B0(X), _B1(X), _B2(X), _B3(X), _B4(X), _B5(X), _B6(X), _B7(X),
+        _B8(X), _B9(X), _BA(X), _BB(X), _BC(X), _BD(X), _BE(X), _BF(X),
+        _C0(X), _C1(X), _C2(X), _C3(X), _C4(X), _C5(X), _C6(X), _C7(X),
+        _C8(X), _C9(X), _CA(X), _CB(X), _CC(X), _CD(X), _CE(X), _CF(X),
+        _D0(X), _D1(X), _D2(X), _D3(X), _D4(X), _D5(X), _D6(X), _D7(X),
+        _D8(X), _D9(X), _DA(X), _DB(X), _DC(X), _DD(X), _DE(X), _DF(X),
+        _E0(X), _E1(X), _E2(X), _E3(X), _E4(X), _E5(X), _E6(X), _E7(X),
+        _E8(X), _E9(X), _EA(X), _EB(X), _EC(X), _ED(X), _EE(X), _EF(X),
+        _F0(X), _F1(X), _F2(X), _F3(X), _F4(X), _F5(X), _F6(X), _F7(X),
+        _F8(X), _F9(X), _FA(X), _FB(X), _FC(X), _FD(X), _FE(X), _FF(X),
+
+        V3,
+        V4,
+    }
+
+    if let E1::V2 { .. } = (E1::V1 { f: true }) {
+        unreachable!()
+    }
+    if let E1::V1 { .. } = (E1::V1 { f: true }) {
+    } else {
+        unreachable!()
+    }
+
+    if let E2::V1 { .. } = E2::V3::<Infallible> {
+        unreachable!()
+    }
+    if let E2::V3 { .. } = E2::V3::<Infallible> {
+    } else {
+        unreachable!()
+    }
+
+    0
+};
+
+fn main() {
+    assert_eq!(OVERFLOW, 0);
+    assert_eq!(MORE_OVERFLOW, 0);
+}
diff --git a/src/test/ui/consts/miri_unleashed/enum_discriminants.stderr b/src/test/ui/consts/miri_unleashed/enum_discriminants.stderr
new file mode 100644
index 0000000000000..8ca81ad22b72b
--- /dev/null
+++ b/src/test/ui/consts/miri_unleashed/enum_discriminants.stderr
@@ -0,0 +1,72 @@
+warning: skipping const checks
+  --> $DIR/enum_discriminants.rs:23:13
+   |
+LL |     let x = Foo::B;
+   |             ^^^^^^
+
+warning: skipping const checks
+  --> $DIR/enum_discriminants.rs:25:9
+   |
+LL |         Foo::B => 0,
+   |         ^^^^^^
+
+warning: skipping const checks
+  --> $DIR/enum_discriminants.rs:88:28
+   |
+LL |     if let E1::V2 { .. } = (E1::V1 { f: true }) {
+   |                            ^^^^^^^^^^^^^^^^^^^^
+
+warning: skipping const checks
+  --> $DIR/enum_discriminants.rs:88:12
+   |
+LL |     if let E1::V2 { .. } = (E1::V1 { f: true }) {
+   |            ^^^^^^^^^^^^^
+
+warning: skipping const checks
+  --> $DIR/enum_discriminants.rs:108:5
+   |
+LL |     assert_eq!(OVERFLOW, 0);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: this warning originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
+
+warning: skipping const checks
+  --> $DIR/enum_discriminants.rs:108:5
+   |
+LL |     assert_eq!(OVERFLOW, 0);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: this warning originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
+
+warning: skipping const checks
+  --> $DIR/enum_discriminants.rs:108:5
+   |
+LL |     assert_eq!(OVERFLOW, 0);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: this warning originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
+
+warning: skipping const checks
+  --> $DIR/enum_discriminants.rs:109:5
+   |
+LL |     assert_eq!(MORE_OVERFLOW, 0);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: this warning originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
+
+warning: skipping const checks
+  --> $DIR/enum_discriminants.rs:109:5
+   |
+LL |     assert_eq!(MORE_OVERFLOW, 0);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: this warning originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
+
+warning: skipping const checks
+  --> $DIR/enum_discriminants.rs:109:5
+   |
+LL |     assert_eq!(MORE_OVERFLOW, 0);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: this warning originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
+

From 822393d690eb7c238c18c1bb0b1e7831c4776cd3 Mon Sep 17 00:00:00 2001
From: Aaron Hill <aa1ronham@gmail.com>
Date: Wed, 18 Sep 2019 18:22:13 -0400
Subject: [PATCH 06/18] Point at original span when emitting unreachable lint

Fixes #64590

When we emit an 'unreachable' lint, we now add a note pointing at the
expression that actually causes the code to be unreachable (e.g.
`return`, `break`, `panic`).

This is especially useful when macros are involved, since a diverging
expression might be hidden inside of a macro invocation.
---
 src/librustc_typeck/check/_match.rs           |  4 +--
 src/librustc_typeck/check/expr.rs             |  2 +-
 src/librustc_typeck/check/mod.rs              | 25 +++++++++++++------
 src/test/ui/dead-code-ret.stderr              |  5 ++++
 src/test/ui/if-ret.stderr                     |  5 ++++
 src/test/ui/issues/issue-2150.stderr          |  6 +++++
 src/test/ui/issues/issue-7246.stderr          |  5 ++++
 .../ui/lint/lint-attr-non-item-node.stderr    |  5 ++++
 src/test/ui/liveness/liveness-unused.stderr   |  5 ++++
 .../match-no-arms-unreachable-after.stderr    |  5 ++++
 src/test/ui/never-assign-dead-code.stderr     | 12 +++++++++
 src/test/ui/reachable/expr_add.stderr         |  5 ++++
 src/test/ui/reachable/expr_again.stderr       |  5 ++++
 src/test/ui/reachable/expr_array.stderr       | 11 ++++++++
 src/test/ui/reachable/expr_assign.stderr      | 17 +++++++++++++
 src/test/ui/reachable/expr_block.stderr       | 10 ++++++++
 src/test/ui/reachable/expr_box.stderr         |  5 ++++
 src/test/ui/reachable/expr_call.stderr        | 11 ++++++++
 src/test/ui/reachable/expr_cast.stderr        |  5 ++++
 src/test/ui/reachable/expr_if.stderr          | 10 ++++++++
 src/test/ui/reachable/expr_loop.stderr        | 15 +++++++++++
 src/test/ui/reachable/expr_match.stderr       | 10 ++++++++
 src/test/ui/reachable/expr_method.stderr      | 11 ++++++++
 src/test/ui/reachable/expr_repeat.stderr      |  5 ++++
 src/test/ui/reachable/expr_return.stderr      |  5 ++++
 src/test/ui/reachable/expr_return_in_macro.rs | 15 +++++++++++
 .../ui/reachable/expr_return_in_macro.stderr  | 22 ++++++++++++++++
 src/test/ui/reachable/expr_struct.stderr      | 23 +++++++++++++++++
 src/test/ui/reachable/expr_tup.stderr         | 11 ++++++++
 src/test/ui/reachable/expr_type.stderr        |  5 ++++
 src/test/ui/reachable/expr_unary.stderr       |  5 ++++
 src/test/ui/reachable/expr_while.stderr       | 11 ++++++++
 .../protect-precedences.stderr                |  5 ++++
 .../ui/unreachable/unreachable-code.stderr    |  5 ++++
 .../ui/unreachable/unreachable-in-call.stderr | 11 ++++++++
 .../unreachable-try-pattern.stderr            |  5 ++++
 .../unwarned-match-on-never.stderr            | 17 +++++++++++++
 37 files changed, 328 insertions(+), 11 deletions(-)
 create mode 100644 src/test/ui/reachable/expr_return_in_macro.rs
 create mode 100644 src/test/ui/reachable/expr_return_in_macro.stderr

diff --git a/src/librustc_typeck/check/_match.rs b/src/librustc_typeck/check/_match.rs
index 308a3d8ebc2cf..a1c937b95ba8c 100644
--- a/src/librustc_typeck/check/_match.rs
+++ b/src/librustc_typeck/check/_match.rs
@@ -43,7 +43,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
         // If there are no arms, that is a diverging match; a special case.
         if arms.is_empty() {
-            self.diverges.set(self.diverges.get() | Diverges::Always);
+            self.diverges.set(self.diverges.get() | Diverges::Always(expr.span));
             return tcx.types.never;
         }
 
@@ -69,7 +69,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             // warnings).
             match all_pats_diverge {
                 Diverges::Maybe => Diverges::Maybe,
-                Diverges::Always | Diverges::WarnedAlways => Diverges::WarnedAlways,
+                Diverges::Always(..) | Diverges::WarnedAlways => Diverges::WarnedAlways,
             }
         }).collect();
 
diff --git a/src/librustc_typeck/check/expr.rs b/src/librustc_typeck/check/expr.rs
index 56bd903040ab4..3ba453748ff32 100644
--- a/src/librustc_typeck/check/expr.rs
+++ b/src/librustc_typeck/check/expr.rs
@@ -170,7 +170,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
         // Any expression that produces a value of type `!` must have diverged
         if ty.is_never() {
-            self.diverges.set(self.diverges.get() | Diverges::Always);
+            self.diverges.set(self.diverges.get() | Diverges::Always(expr.span));
         }
 
         // Record the type, which applies it effects.
diff --git a/src/librustc_typeck/check/mod.rs b/src/librustc_typeck/check/mod.rs
index 1197160fa9501..0fced37394616 100644
--- a/src/librustc_typeck/check/mod.rs
+++ b/src/librustc_typeck/check/mod.rs
@@ -450,7 +450,10 @@ pub enum Diverges {
 
     /// Definitely known to diverge and therefore
     /// not reach the next sibling or its parent.
-    Always,
+    /// The `Span` points to the expression
+    /// that caused us to diverge
+    /// (e.g. `return`, `break`, etc)
+    Always(Span),
 
     /// Same as `Always` but with a reachability
     /// warning already emitted.
@@ -487,7 +490,7 @@ impl ops::BitOrAssign for Diverges {
 
 impl Diverges {
     fn always(self) -> bool {
-        self >= Diverges::Always
+        self >= Diverges::Always(DUMMY_SP)
     }
 }
 
@@ -2307,17 +2310,23 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// Produces warning on the given node, if the current point in the
     /// function is unreachable, and there hasn't been another warning.
     fn warn_if_unreachable(&self, id: hir::HirId, span: Span, kind: &str) {
-        if self.diverges.get() == Diverges::Always &&
+        // FIXME: Combine these two 'if' expressions into one once
+        // let chains are implemented
+        if let Diverges::Always(orig_span) = self.diverges.get() {
             // If span arose from a desugaring of `if` or `while`, then it is the condition itself,
             // which diverges, that we are about to lint on. This gives suboptimal diagnostics.
             // Instead, stop here so that the `if`- or `while`-expression's block is linted instead.
-            !span.is_desugaring(DesugaringKind::CondTemporary) {
-            self.diverges.set(Diverges::WarnedAlways);
+            if !span.is_desugaring(DesugaringKind::CondTemporary) {
+                self.diverges.set(Diverges::WarnedAlways);
 
-            debug!("warn_if_unreachable: id={:?} span={:?} kind={}", id, span, kind);
+                debug!("warn_if_unreachable: id={:?} span={:?} kind={}", id, span, kind);
 
-            let msg = format!("unreachable {}", kind);
-            self.tcx().lint_hir(lint::builtin::UNREACHABLE_CODE, id, span, &msg);
+                let msg = format!("unreachable {}", kind);
+                let mut err = self.tcx().struct_span_lint_hir(lint::builtin::UNREACHABLE_CODE,
+                                                              id, span, &msg);
+                err.span_note(orig_span, "any code following this expression is unreachable");
+                err.emit();
+            }
         }
     }
 
diff --git a/src/test/ui/dead-code-ret.stderr b/src/test/ui/dead-code-ret.stderr
index 092a176f443b2..0ce31ea40ddbb 100644
--- a/src/test/ui/dead-code-ret.stderr
+++ b/src/test/ui/dead-code-ret.stderr
@@ -9,6 +9,11 @@ note: lint level defined here
    |
 LL | #![deny(unreachable_code)]
    |         ^^^^^^^^^^^^^^^^
+note: any code following this expression is unreachable
+  --> $DIR/dead-code-ret.rs:6:5
+   |
+LL |     return;
+   |     ^^^^^^
    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
 
 error: aborting due to previous error
diff --git a/src/test/ui/if-ret.stderr b/src/test/ui/if-ret.stderr
index 73402e55a4fd8..2df8f22944ee8 100644
--- a/src/test/ui/if-ret.stderr
+++ b/src/test/ui/if-ret.stderr
@@ -5,4 +5,9 @@ LL | fn foo() { if (return) { } }
    |                        ^^^
    |
    = note: `#[warn(unreachable_code)]` on by default
+note: any code following this expression is unreachable
+  --> $DIR/if-ret.rs:6:15
+   |
+LL | fn foo() { if (return) { } }
+   |               ^^^^^^^^
 
diff --git a/src/test/ui/issues/issue-2150.stderr b/src/test/ui/issues/issue-2150.stderr
index 59000f3dd5338..623f098d0b337 100644
--- a/src/test/ui/issues/issue-2150.stderr
+++ b/src/test/ui/issues/issue-2150.stderr
@@ -9,6 +9,12 @@ note: lint level defined here
    |
 LL | #![deny(unreachable_code)]
    |         ^^^^^^^^^^^^^^^^
+note: any code following this expression is unreachable
+  --> $DIR/issue-2150.rs:7:5
+   |
+LL |     panic!();
+   |     ^^^^^^^^^
+   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/issues/issue-7246.stderr b/src/test/ui/issues/issue-7246.stderr
index e049295e9136f..d1b23672dc7f4 100644
--- a/src/test/ui/issues/issue-7246.stderr
+++ b/src/test/ui/issues/issue-7246.stderr
@@ -9,6 +9,11 @@ note: lint level defined here
    |
 LL | #![deny(unreachable_code)]
    |         ^^^^^^^^^^^^^^^^
+note: any code following this expression is unreachable
+  --> $DIR/issue-7246.rs:6:5
+   |
+LL |     return;
+   |     ^^^^^^
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/lint/lint-attr-non-item-node.stderr b/src/test/ui/lint/lint-attr-non-item-node.stderr
index 6eb72c098df5c..e6c76c24c9110 100644
--- a/src/test/ui/lint/lint-attr-non-item-node.stderr
+++ b/src/test/ui/lint/lint-attr-non-item-node.stderr
@@ -9,6 +9,11 @@ note: lint level defined here
    |
 LL |     #[deny(unreachable_code)]
    |            ^^^^^^^^^^^^^^^^
+note: any code following this expression is unreachable
+  --> $DIR/lint-attr-non-item-node.rs:6:9
+   |
+LL |         break;
+   |         ^^^^^
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/liveness/liveness-unused.stderr b/src/test/ui/liveness/liveness-unused.stderr
index 40a677c08f2b0..84e9b5bab99ff 100644
--- a/src/test/ui/liveness/liveness-unused.stderr
+++ b/src/test/ui/liveness/liveness-unused.stderr
@@ -10,6 +10,11 @@ note: lint level defined here
 LL | #![warn(unused)]
    |         ^^^^^^
    = note: `#[warn(unreachable_code)]` implied by `#[warn(unused)]`
+note: any code following this expression is unreachable
+  --> $DIR/liveness-unused.rs:91:9
+   |
+LL |         continue;
+   |         ^^^^^^^^
 
 error: unused variable: `x`
   --> $DIR/liveness-unused.rs:8:7
diff --git a/src/test/ui/match/match-no-arms-unreachable-after.stderr b/src/test/ui/match/match-no-arms-unreachable-after.stderr
index 65dcc6ee46554..6c46b2473cce6 100644
--- a/src/test/ui/match/match-no-arms-unreachable-after.stderr
+++ b/src/test/ui/match/match-no-arms-unreachable-after.stderr
@@ -9,6 +9,11 @@ note: lint level defined here
    |
 LL | #![deny(unreachable_code)]
    |         ^^^^^^^^^^^^^^^^
+note: any code following this expression is unreachable
+  --> $DIR/match-no-arms-unreachable-after.rs:7:5
+   |
+LL |     match v { }
+   |     ^^^^^^^^^^^
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/never-assign-dead-code.stderr b/src/test/ui/never-assign-dead-code.stderr
index b1b5bf03fe52a..436c703e4b6b9 100644
--- a/src/test/ui/never-assign-dead-code.stderr
+++ b/src/test/ui/never-assign-dead-code.stderr
@@ -10,12 +10,24 @@ note: lint level defined here
 LL | #![warn(unused)]
    |         ^^^^^^
    = note: `#[warn(unreachable_code)]` implied by `#[warn(unused)]`
+note: any code following this expression is unreachable
+  --> $DIR/never-assign-dead-code.rs:9:16
+   |
+LL |     let x: ! = panic!("aah");
+   |                ^^^^^^^^^^^^^
+   = note: this warning originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
 
 warning: unreachable call
   --> $DIR/never-assign-dead-code.rs:10:5
    |
 LL |     drop(x);
    |     ^^^^
+   |
+note: any code following this expression is unreachable
+  --> $DIR/never-assign-dead-code.rs:10:10
+   |
+LL |     drop(x);
+   |          ^
 
 warning: unused variable: `x`
   --> $DIR/never-assign-dead-code.rs:9:9
diff --git a/src/test/ui/reachable/expr_add.stderr b/src/test/ui/reachable/expr_add.stderr
index 02b29021cb61a..47b4e467abecb 100644
--- a/src/test/ui/reachable/expr_add.stderr
+++ b/src/test/ui/reachable/expr_add.stderr
@@ -9,6 +9,11 @@ note: lint level defined here
    |
 LL | #![deny(unreachable_code)]
    |         ^^^^^^^^^^^^^^^^
+note: any code following this expression is unreachable
+  --> $DIR/expr_add.rs:17:19
+   |
+LL |     let x = Foo + return;
+   |                   ^^^^^^
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/reachable/expr_again.stderr b/src/test/ui/reachable/expr_again.stderr
index bdc3d143ea57e..8e246d940fd8d 100644
--- a/src/test/ui/reachable/expr_again.stderr
+++ b/src/test/ui/reachable/expr_again.stderr
@@ -9,6 +9,11 @@ note: lint level defined here
    |
 LL | #![deny(unreachable_code)]
    |         ^^^^^^^^^^^^^^^^
+note: any code following this expression is unreachable
+  --> $DIR/expr_again.rs:7:9
+   |
+LL |         continue;
+   |         ^^^^^^^^
    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
 
 error: aborting due to previous error
diff --git a/src/test/ui/reachable/expr_array.stderr b/src/test/ui/reachable/expr_array.stderr
index 18d7ffe74bd18..419a332e632f3 100644
--- a/src/test/ui/reachable/expr_array.stderr
+++ b/src/test/ui/reachable/expr_array.stderr
@@ -9,12 +9,23 @@ note: lint level defined here
    |
 LL | #![deny(unreachable_code)]
    |         ^^^^^^^^^^^^^^^^
+note: any code following this expression is unreachable
+  --> $DIR/expr_array.rs:9:26
+   |
+LL |     let x: [usize; 2] = [return, 22];
+   |                          ^^^^^^
 
 error: unreachable expression
   --> $DIR/expr_array.rs:14:25
    |
 LL |     let x: [usize; 2] = [22, return];
    |                         ^^^^^^^^^^^^
+   |
+note: any code following this expression is unreachable
+  --> $DIR/expr_array.rs:14:30
+   |
+LL |     let x: [usize; 2] = [22, return];
+   |                              ^^^^^^
 
 error: aborting due to 2 previous errors
 
diff --git a/src/test/ui/reachable/expr_assign.stderr b/src/test/ui/reachable/expr_assign.stderr
index def16d90a7490..7388fb4a6b9b2 100644
--- a/src/test/ui/reachable/expr_assign.stderr
+++ b/src/test/ui/reachable/expr_assign.stderr
@@ -9,18 +9,35 @@ note: lint level defined here
    |
 LL | #![deny(unreachable_code)]
    |         ^^^^^^^^^^^^^^^^
+note: any code following this expression is unreachable
+  --> $DIR/expr_assign.rs:10:9
+   |
+LL |     x = return;
+   |         ^^^^^^
 
 error: unreachable expression
   --> $DIR/expr_assign.rs:20:14
    |
 LL |         *p = return;
    |              ^^^^^^
+   |
+note: any code following this expression is unreachable
+  --> $DIR/expr_assign.rs:20:9
+   |
+LL |         *p = return;
+   |         ^^
 
 error: unreachable expression
   --> $DIR/expr_assign.rs:26:15
    |
 LL |     *{return; &mut i} = 22;
    |               ^^^^^^
+   |
+note: any code following this expression is unreachable
+  --> $DIR/expr_assign.rs:26:7
+   |
+LL |     *{return; &mut i} = 22;
+   |       ^^^^^^
 
 error: aborting due to 3 previous errors
 
diff --git a/src/test/ui/reachable/expr_block.stderr b/src/test/ui/reachable/expr_block.stderr
index a498502e6cb1a..03a6139d688bb 100644
--- a/src/test/ui/reachable/expr_block.stderr
+++ b/src/test/ui/reachable/expr_block.stderr
@@ -9,6 +9,11 @@ note: lint level defined here
    |
 LL | #![deny(unreachable_code)]
    |         ^^^^^^^^^^^^^^^^
+note: any code following this expression is unreachable
+  --> $DIR/expr_block.rs:9:9
+   |
+LL |         return;
+   |         ^^^^^^
 
 error: unreachable statement
   --> $DIR/expr_block.rs:25:9
@@ -16,6 +21,11 @@ error: unreachable statement
 LL |         println!("foo");
    |         ^^^^^^^^^^^^^^^^
    |
+note: any code following this expression is unreachable
+  --> $DIR/expr_block.rs:24:9
+   |
+LL |         return;
+   |         ^^^^^^
    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
 
 error: aborting due to 2 previous errors
diff --git a/src/test/ui/reachable/expr_box.stderr b/src/test/ui/reachable/expr_box.stderr
index 63137ce3da180..d0f666d2be44b 100644
--- a/src/test/ui/reachable/expr_box.stderr
+++ b/src/test/ui/reachable/expr_box.stderr
@@ -9,6 +9,11 @@ note: lint level defined here
    |
 LL | #![deny(unreachable_code)]
    |         ^^^^^^^^^^^^^^^^
+note: any code following this expression is unreachable
+  --> $DIR/expr_box.rs:6:17
+   |
+LL |     let x = box return;
+   |                 ^^^^^^
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/reachable/expr_call.stderr b/src/test/ui/reachable/expr_call.stderr
index f2db17e4dfe8e..3fcea90e7cd87 100644
--- a/src/test/ui/reachable/expr_call.stderr
+++ b/src/test/ui/reachable/expr_call.stderr
@@ -9,12 +9,23 @@ note: lint level defined here
    |
 LL | #![deny(unreachable_code)]
    |         ^^^^^^^^^^^^^^^^
+note: any code following this expression is unreachable
+  --> $DIR/expr_call.rs:13:9
+   |
+LL |     foo(return, 22);
+   |         ^^^^^^
 
 error: unreachable call
   --> $DIR/expr_call.rs:18:5
    |
 LL |     bar(return);
    |     ^^^
+   |
+note: any code following this expression is unreachable
+  --> $DIR/expr_call.rs:18:9
+   |
+LL |     bar(return);
+   |         ^^^^^^
 
 error: aborting due to 2 previous errors
 
diff --git a/src/test/ui/reachable/expr_cast.stderr b/src/test/ui/reachable/expr_cast.stderr
index 3086745d28ed4..d3ce0ca079f90 100644
--- a/src/test/ui/reachable/expr_cast.stderr
+++ b/src/test/ui/reachable/expr_cast.stderr
@@ -9,6 +9,11 @@ note: lint level defined here
    |
 LL | #![deny(unreachable_code)]
    |         ^^^^^^^^^^^^^^^^
+note: any code following this expression is unreachable
+  --> $DIR/expr_cast.rs:9:14
+   |
+LL |     let x = {return} as !;
+   |              ^^^^^^
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/reachable/expr_if.stderr b/src/test/ui/reachable/expr_if.stderr
index f1690e595e5d1..03284576086a7 100644
--- a/src/test/ui/reachable/expr_if.stderr
+++ b/src/test/ui/reachable/expr_if.stderr
@@ -12,6 +12,11 @@ note: lint level defined here
    |
 LL | #![deny(unreachable_code)]
    |         ^^^^^^^^^^^^^^^^
+note: any code following this expression is unreachable
+  --> $DIR/expr_if.rs:7:9
+   |
+LL |     if {return} {
+   |         ^^^^^^
 
 error: unreachable statement
   --> $DIR/expr_if.rs:27:5
@@ -19,6 +24,11 @@ error: unreachable statement
 LL |     println!("But I am.");
    |     ^^^^^^^^^^^^^^^^^^^^^^
    |
+note: any code following this expression is unreachable
+  --> $DIR/expr_if.rs:21:9
+   |
+LL |         return;
+   |         ^^^^^^
    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
 
 error: aborting due to 2 previous errors
diff --git a/src/test/ui/reachable/expr_loop.stderr b/src/test/ui/reachable/expr_loop.stderr
index 4d3e06c93a376..a4cf8cfcfd9e6 100644
--- a/src/test/ui/reachable/expr_loop.stderr
+++ b/src/test/ui/reachable/expr_loop.stderr
@@ -9,6 +9,11 @@ note: lint level defined here
    |
 LL | #![deny(unreachable_code)]
    |         ^^^^^^^^^^^^^^^^
+note: any code following this expression is unreachable
+  --> $DIR/expr_loop.rs:7:12
+   |
+LL |     loop { return; }
+   |            ^^^^^^
    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
 
 error: unreachable statement
@@ -17,6 +22,11 @@ error: unreachable statement
 LL |     println!("I am dead.");
    |     ^^^^^^^^^^^^^^^^^^^^^^^
    |
+note: any code following this expression is unreachable
+  --> $DIR/expr_loop.rs:20:12
+   |
+LL |     loop { return; }
+   |            ^^^^^^
    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
 
 error: unreachable statement
@@ -25,6 +35,11 @@ error: unreachable statement
 LL |     println!("I am dead.");
    |     ^^^^^^^^^^^^^^^^^^^^^^^
    |
+note: any code following this expression is unreachable
+  --> $DIR/expr_loop.rs:31:5
+   |
+LL |     loop { 'middle: loop { loop { break 'middle; } } }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
 
 error: aborting due to 3 previous errors
diff --git a/src/test/ui/reachable/expr_match.stderr b/src/test/ui/reachable/expr_match.stderr
index 1aef06aec5ba6..b846b92dcec8b 100644
--- a/src/test/ui/reachable/expr_match.stderr
+++ b/src/test/ui/reachable/expr_match.stderr
@@ -9,6 +9,11 @@ note: lint level defined here
    |
 LL | #![deny(unreachable_code)]
    |         ^^^^^^^^^^^^^^^^
+note: any code following this expression is unreachable
+  --> $DIR/expr_match.rs:7:22
+   |
+LL |     match () { () => return }
+   |                      ^^^^^^
    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
 
 error: unreachable statement
@@ -17,6 +22,11 @@ error: unreachable statement
 LL |     println!("I am dead");
    |     ^^^^^^^^^^^^^^^^^^^^^^
    |
+note: any code following this expression is unreachable
+  --> $DIR/expr_match.rs:18:31
+   |
+LL |     match () { () if false => return, () => return }
+   |                               ^^^^^^
    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
 
 error: aborting due to 2 previous errors
diff --git a/src/test/ui/reachable/expr_method.stderr b/src/test/ui/reachable/expr_method.stderr
index 947ea0fee889c..7ad279c9f487a 100644
--- a/src/test/ui/reachable/expr_method.stderr
+++ b/src/test/ui/reachable/expr_method.stderr
@@ -9,12 +9,23 @@ note: lint level defined here
    |
 LL | #![deny(unreachable_code)]
    |         ^^^^^^^^^^^^^^^^
+note: any code following this expression is unreachable
+  --> $DIR/expr_method.rs:16:13
+   |
+LL |     Foo.foo(return, 22);
+   |             ^^^^^^
 
 error: unreachable call
   --> $DIR/expr_method.rs:21:9
    |
 LL |     Foo.bar(return);
    |         ^^^
+   |
+note: any code following this expression is unreachable
+  --> $DIR/expr_method.rs:21:13
+   |
+LL |     Foo.bar(return);
+   |             ^^^^^^
 
 error: aborting due to 2 previous errors
 
diff --git a/src/test/ui/reachable/expr_repeat.stderr b/src/test/ui/reachable/expr_repeat.stderr
index 0536cdef72128..3ff6be76daea5 100644
--- a/src/test/ui/reachable/expr_repeat.stderr
+++ b/src/test/ui/reachable/expr_repeat.stderr
@@ -9,6 +9,11 @@ note: lint level defined here
    |
 LL | #![deny(unreachable_code)]
    |         ^^^^^^^^^^^^^^^^
+note: any code following this expression is unreachable
+  --> $DIR/expr_repeat.rs:9:26
+   |
+LL |     let x: [usize; 2] = [return; 2];
+   |                          ^^^^^^
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/reachable/expr_return.stderr b/src/test/ui/reachable/expr_return.stderr
index 3317da58aba1c..31f7ebe7618ea 100644
--- a/src/test/ui/reachable/expr_return.stderr
+++ b/src/test/ui/reachable/expr_return.stderr
@@ -9,6 +9,11 @@ note: lint level defined here
    |
 LL | #![deny(unreachable_code)]
    |         ^^^^^^^^^^^^^^^^
+note: any code following this expression is unreachable
+  --> $DIR/expr_return.rs:10:30
+   |
+LL |     let x = {return {return {return;}}};
+   |                              ^^^^^^
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/reachable/expr_return_in_macro.rs b/src/test/ui/reachable/expr_return_in_macro.rs
new file mode 100644
index 0000000000000..198ede6b14106
--- /dev/null
+++ b/src/test/ui/reachable/expr_return_in_macro.rs
@@ -0,0 +1,15 @@
+// Tests that we generate nice error messages
+// when an expression is unreachble due to control
+// flow inside of a macro expansion
+#![deny(unreachable_code)]
+
+macro_rules! early_return {
+    () => {
+        return ()
+    }
+}
+
+fn main() {
+    return early_return!();
+    //~^ ERROR unreachable expression
+}
diff --git a/src/test/ui/reachable/expr_return_in_macro.stderr b/src/test/ui/reachable/expr_return_in_macro.stderr
new file mode 100644
index 0000000000000..ff3abb5551f92
--- /dev/null
+++ b/src/test/ui/reachable/expr_return_in_macro.stderr
@@ -0,0 +1,22 @@
+error: unreachable expression
+  --> $DIR/expr_return_in_macro.rs:13:5
+   |
+LL |     return early_return!();
+   |     ^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: lint level defined here
+  --> $DIR/expr_return_in_macro.rs:4:9
+   |
+LL | #![deny(unreachable_code)]
+   |         ^^^^^^^^^^^^^^^^
+note: any code following this expression is unreachable
+  --> $DIR/expr_return_in_macro.rs:8:9
+   |
+LL |         return ()
+   |         ^^^^^^^^^
+...
+LL |     return early_return!();
+   |            --------------- in this macro invocation
+
+error: aborting due to previous error
+
diff --git a/src/test/ui/reachable/expr_struct.stderr b/src/test/ui/reachable/expr_struct.stderr
index dcccb7a4db307..d08bcc4f0d1ae 100644
--- a/src/test/ui/reachable/expr_struct.stderr
+++ b/src/test/ui/reachable/expr_struct.stderr
@@ -9,24 +9,47 @@ note: lint level defined here
    |
 LL | #![deny(unreachable_code)]
    |         ^^^^^^^^^^^^^^^^
+note: any code following this expression is unreachable
+  --> $DIR/expr_struct.rs:14:35
+   |
+LL |     let x = Foo { a: 22, b: 33, ..return };
+   |                                   ^^^^^^
 
 error: unreachable expression
   --> $DIR/expr_struct.rs:19:33
    |
 LL |     let x = Foo { a: return, b: 33, ..return };
    |                                 ^^
+   |
+note: any code following this expression is unreachable
+  --> $DIR/expr_struct.rs:19:22
+   |
+LL |     let x = Foo { a: return, b: 33, ..return };
+   |                      ^^^^^^
 
 error: unreachable expression
   --> $DIR/expr_struct.rs:24:39
    |
 LL |     let x = Foo { a: 22, b: return, ..return };
    |                                       ^^^^^^
+   |
+note: any code following this expression is unreachable
+  --> $DIR/expr_struct.rs:24:29
+   |
+LL |     let x = Foo { a: 22, b: return, ..return };
+   |                             ^^^^^^
 
 error: unreachable expression
   --> $DIR/expr_struct.rs:29:13
    |
 LL |     let x = Foo { a: 22, b: return };
    |             ^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: any code following this expression is unreachable
+  --> $DIR/expr_struct.rs:29:29
+   |
+LL |     let x = Foo { a: 22, b: return };
+   |                             ^^^^^^
 
 error: aborting due to 4 previous errors
 
diff --git a/src/test/ui/reachable/expr_tup.stderr b/src/test/ui/reachable/expr_tup.stderr
index 1837031107d59..788499533db33 100644
--- a/src/test/ui/reachable/expr_tup.stderr
+++ b/src/test/ui/reachable/expr_tup.stderr
@@ -9,12 +9,23 @@ note: lint level defined here
    |
 LL | #![deny(unreachable_code)]
    |         ^^^^^^^^^^^^^^^^
+note: any code following this expression is unreachable
+  --> $DIR/expr_tup.rs:9:30
+   |
+LL |     let x: (usize, usize) = (return, 2);
+   |                              ^^^^^^
 
 error: unreachable expression
   --> $DIR/expr_tup.rs:14:29
    |
 LL |     let x: (usize, usize) = (2, return);
    |                             ^^^^^^^^^^^
+   |
+note: any code following this expression is unreachable
+  --> $DIR/expr_tup.rs:14:33
+   |
+LL |     let x: (usize, usize) = (2, return);
+   |                                 ^^^^^^
 
 error: aborting due to 2 previous errors
 
diff --git a/src/test/ui/reachable/expr_type.stderr b/src/test/ui/reachable/expr_type.stderr
index f867c89163415..15eb735da75d8 100644
--- a/src/test/ui/reachable/expr_type.stderr
+++ b/src/test/ui/reachable/expr_type.stderr
@@ -9,6 +9,11 @@ note: lint level defined here
    |
 LL | #![deny(unreachable_code)]
    |         ^^^^^^^^^^^^^^^^
+note: any code following this expression is unreachable
+  --> $DIR/expr_type.rs:9:14
+   |
+LL |     let x = {return}: !;
+   |              ^^^^^^
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/reachable/expr_unary.stderr b/src/test/ui/reachable/expr_unary.stderr
index 61982289cdc6e..7f86519616639 100644
--- a/src/test/ui/reachable/expr_unary.stderr
+++ b/src/test/ui/reachable/expr_unary.stderr
@@ -15,6 +15,11 @@ note: lint level defined here
    |
 LL | #![deny(unreachable_code)]
    |         ^^^^^^^^^^^^^^^^
+note: any code following this expression is unreachable
+  --> $DIR/expr_unary.rs:8:20
+   |
+LL |     let x: ! = ! { return; };
+   |                    ^^^^^^
 
 error: aborting due to 2 previous errors
 
diff --git a/src/test/ui/reachable/expr_while.stderr b/src/test/ui/reachable/expr_while.stderr
index fc528926b4c97..b6d6d11ac691e 100644
--- a/src/test/ui/reachable/expr_while.stderr
+++ b/src/test/ui/reachable/expr_while.stderr
@@ -13,6 +13,11 @@ note: lint level defined here
    |
 LL | #![deny(unreachable_code)]
    |         ^^^^^^^^^^^^^^^^
+note: any code following this expression is unreachable
+  --> $DIR/expr_while.rs:7:12
+   |
+LL |     while {return} {
+   |            ^^^^^^
 
 error: unreachable block in `while` expression
   --> $DIR/expr_while.rs:22:20
@@ -23,6 +28,12 @@ LL | |
 LL | |         println!("I am dead.");
 LL | |     }
    | |_____^
+   |
+note: any code following this expression is unreachable
+  --> $DIR/expr_while.rs:22:12
+   |
+LL |     while {return} {
+   |            ^^^^^^
 
 error: aborting due to 2 previous errors
 
diff --git a/src/test/ui/rfc-2497-if-let-chains/protect-precedences.stderr b/src/test/ui/rfc-2497-if-let-chains/protect-precedences.stderr
index be7ef658411e1..ca98a3947146e 100644
--- a/src/test/ui/rfc-2497-if-let-chains/protect-precedences.stderr
+++ b/src/test/ui/rfc-2497-if-let-chains/protect-precedences.stderr
@@ -5,4 +5,9 @@ LL |         if let _ = return true && false {};
    |                                         ^^
    |
    = note: `#[warn(unreachable_code)]` on by default
+note: any code following this expression is unreachable
+  --> $DIR/protect-precedences.rs:13:20
+   |
+LL |         if let _ = return true && false {};
+   |                    ^^^^^^^^^^^^^^^^^^^^
 
diff --git a/src/test/ui/unreachable/unreachable-code.stderr b/src/test/ui/unreachable/unreachable-code.stderr
index 803bb966be8fc..226f088c63a5e 100644
--- a/src/test/ui/unreachable/unreachable-code.stderr
+++ b/src/test/ui/unreachable/unreachable-code.stderr
@@ -9,6 +9,11 @@ note: lint level defined here
    |
 LL | #![deny(unreachable_code)]
    |         ^^^^^^^^^^^^^^^^
+note: any code following this expression is unreachable
+  --> $DIR/unreachable-code.rs:5:3
+   |
+LL |   loop{}
+   |   ^^^^^^
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/unreachable/unreachable-in-call.stderr b/src/test/ui/unreachable/unreachable-in-call.stderr
index c740011c4a125..928f5634a1248 100644
--- a/src/test/ui/unreachable/unreachable-in-call.stderr
+++ b/src/test/ui/unreachable/unreachable-in-call.stderr
@@ -9,12 +9,23 @@ note: lint level defined here
    |
 LL | #![deny(unreachable_code)]
    |         ^^^^^^^^^^^^^^^^
+note: any code following this expression is unreachable
+  --> $DIR/unreachable-in-call.rs:13:10
+   |
+LL |     call(diverge(),
+   |          ^^^^^^^^^
 
 error: unreachable call
   --> $DIR/unreachable-in-call.rs:17:5
    |
 LL |     call(
    |     ^^^^
+   |
+note: any code following this expression is unreachable
+  --> $DIR/unreachable-in-call.rs:19:9
+   |
+LL |         diverge());
+   |         ^^^^^^^^^
 
 error: aborting due to 2 previous errors
 
diff --git a/src/test/ui/unreachable/unreachable-try-pattern.stderr b/src/test/ui/unreachable/unreachable-try-pattern.stderr
index 758aa5a45bc17..889df790124da 100644
--- a/src/test/ui/unreachable/unreachable-try-pattern.stderr
+++ b/src/test/ui/unreachable/unreachable-try-pattern.stderr
@@ -9,6 +9,11 @@ note: lint level defined here
    |
 LL | #![warn(unreachable_code)]
    |         ^^^^^^^^^^^^^^^^
+note: any code following this expression is unreachable
+  --> $DIR/unreachable-try-pattern.rs:19:36
+   |
+LL |     let y = (match x { Ok(n) => Ok(n as u32), Err(e) => Err(e) })?;
+   |                                    ^
 
 warning: unreachable pattern
   --> $DIR/unreachable-try-pattern.rs:19:24
diff --git a/src/test/ui/unreachable/unwarned-match-on-never.stderr b/src/test/ui/unreachable/unwarned-match-on-never.stderr
index ccb70d7431145..9ce6e3df8046e 100644
--- a/src/test/ui/unreachable/unwarned-match-on-never.stderr
+++ b/src/test/ui/unreachable/unwarned-match-on-never.stderr
@@ -9,12 +9,23 @@ note: lint level defined here
    |
 LL | #![deny(unreachable_code)]
    |         ^^^^^^^^^^^^^^^^
+note: any code following this expression is unreachable
+  --> $DIR/unwarned-match-on-never.rs:8:11
+   |
+LL |     match x {}
+   |           ^
 
 error: unreachable arm
   --> $DIR/unwarned-match-on-never.rs:15:15
    |
 LL |         () => ()
    |               ^^
+   |
+note: any code following this expression is unreachable
+  --> $DIR/unwarned-match-on-never.rs:14:11
+   |
+LL |     match (return) {
+   |           ^^^^^^^^
 
 error: unreachable expression
   --> $DIR/unwarned-match-on-never.rs:21:5
@@ -23,6 +34,12 @@ LL | /     match () {
 LL | |         () => (),
 LL | |     }
    | |_____^
+   |
+note: any code following this expression is unreachable
+  --> $DIR/unwarned-match-on-never.rs:20:5
+   |
+LL |     return;
+   |     ^^^^^^
 
 error: aborting due to 3 previous errors
 

From cd4b468e07ab27eef87aa8757220d6439defc699 Mon Sep 17 00:00:00 2001
From: Aaron Hill <aa1ronham@gmail.com>
Date: Wed, 18 Sep 2019 19:07:39 -0400
Subject: [PATCH 07/18] Make note better when all arms in a `match` diverge

---
 src/librustc_typeck/check/_match.rs     | 26 +++++++++++++++++--
 src/librustc_typeck/check/expr.rs       |  5 +++-
 src/librustc_typeck/check/mod.rs        | 33 +++++++++++++++++++------
 src/test/ui/reachable/expr_match.stderr | 12 ++++-----
 4 files changed, 60 insertions(+), 16 deletions(-)

diff --git a/src/librustc_typeck/check/_match.rs b/src/librustc_typeck/check/_match.rs
index a1c937b95ba8c..86774466ba586 100644
--- a/src/librustc_typeck/check/_match.rs
+++ b/src/librustc_typeck/check/_match.rs
@@ -43,7 +43,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
         // If there are no arms, that is a diverging match; a special case.
         if arms.is_empty() {
-            self.diverges.set(self.diverges.get() | Diverges::Always(expr.span));
+            self.diverges.set(self.diverges.get() | Diverges::Always {
+                span: expr.span,
+                custom_note: None
+            });
             return tcx.types.never;
         }
 
@@ -69,7 +72,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             // warnings).
             match all_pats_diverge {
                 Diverges::Maybe => Diverges::Maybe,
-                Diverges::Always(..) | Diverges::WarnedAlways => Diverges::WarnedAlways,
+                Diverges::Always { .. } | Diverges::WarnedAlways => Diverges::WarnedAlways,
             }
         }).collect();
 
@@ -167,6 +170,25 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             prior_arm_ty = Some(arm_ty);
         }
 
+        // If all of the arms in the 'match' diverge,
+        // and we're dealing with an actual 'match' block
+        // (as opposed to a 'match' desugared from something else'),
+        // we can emit a better note. Rather than pointing
+        // at a diverging expression in an arbitrary arm,
+        // we can point at the entire 'match' expression
+        match (all_arms_diverge, match_src) {
+            (Diverges::Always { .. }, hir::MatchSource::Normal) => {
+                all_arms_diverge = Diverges::Always {
+                    span: expr.span,
+                    custom_note: Some(
+                        "any code following this `match` expression is unreachable, \
+                        as all arms diverge"
+                    )
+                };
+            },
+            _ => {}
+        }
+
         // We won't diverge unless the discriminant or all arms diverge.
         self.diverges.set(discrim_diverges | all_arms_diverge);
 
diff --git a/src/librustc_typeck/check/expr.rs b/src/librustc_typeck/check/expr.rs
index 3ba453748ff32..5733b8d1db13e 100644
--- a/src/librustc_typeck/check/expr.rs
+++ b/src/librustc_typeck/check/expr.rs
@@ -170,7 +170,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
         // Any expression that produces a value of type `!` must have diverged
         if ty.is_never() {
-            self.diverges.set(self.diverges.get() | Diverges::Always(expr.span));
+            self.diverges.set(self.diverges.get() | Diverges::Always {
+                span: expr.span,
+                custom_note: None
+            });
         }
 
         // Record the type, which applies it effects.
diff --git a/src/librustc_typeck/check/mod.rs b/src/librustc_typeck/check/mod.rs
index 0fced37394616..c44648bb9df85 100644
--- a/src/librustc_typeck/check/mod.rs
+++ b/src/librustc_typeck/check/mod.rs
@@ -450,10 +450,20 @@ pub enum Diverges {
 
     /// Definitely known to diverge and therefore
     /// not reach the next sibling or its parent.
-    /// The `Span` points to the expression
-    /// that caused us to diverge
-    /// (e.g. `return`, `break`, etc)
-    Always(Span),
+    Always {
+        /// The `Span` points to the expression
+        /// that caused us to diverge
+        /// (e.g. `return`, `break`, etc)
+        span: Span,
+        /// In some cases (e.g. a 'match' expression
+        /// where all arms diverge), we may be
+        /// able to provide a more informative
+        /// message to the user.
+        /// If this is None, a default messsage
+        /// will be generated, which is suitable
+        /// for most cases
+        custom_note: Option<&'static str>
+    },
 
     /// Same as `Always` but with a reachability
     /// warning already emitted.
@@ -490,7 +500,13 @@ impl ops::BitOrAssign for Diverges {
 
 impl Diverges {
     fn always(self) -> bool {
-        self >= Diverges::Always(DUMMY_SP)
+        // Enum comparison ignores the
+        // contents of fields, so we just
+        // fill them in with garbage here
+        self >= Diverges::Always {
+            span: DUMMY_SP,
+            custom_note: None
+        }
     }
 }
 
@@ -2312,7 +2328,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     fn warn_if_unreachable(&self, id: hir::HirId, span: Span, kind: &str) {
         // FIXME: Combine these two 'if' expressions into one once
         // let chains are implemented
-        if let Diverges::Always(orig_span) = self.diverges.get() {
+        if let Diverges::Always { span: orig_span, custom_note } = self.diverges.get() {
             // If span arose from a desugaring of `if` or `while`, then it is the condition itself,
             // which diverges, that we are about to lint on. This gives suboptimal diagnostics.
             // Instead, stop here so that the `if`- or `while`-expression's block is linted instead.
@@ -2324,7 +2340,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 let msg = format!("unreachable {}", kind);
                 let mut err = self.tcx().struct_span_lint_hir(lint::builtin::UNREACHABLE_CODE,
                                                               id, span, &msg);
-                err.span_note(orig_span, "any code following this expression is unreachable");
+                err.span_note(
+                    orig_span,
+                    custom_note.unwrap_or("any code following this expression is unreachable")
+                );
                 err.emit();
             }
         }
diff --git a/src/test/ui/reachable/expr_match.stderr b/src/test/ui/reachable/expr_match.stderr
index b846b92dcec8b..f587e524d350d 100644
--- a/src/test/ui/reachable/expr_match.stderr
+++ b/src/test/ui/reachable/expr_match.stderr
@@ -9,11 +9,11 @@ note: lint level defined here
    |
 LL | #![deny(unreachable_code)]
    |         ^^^^^^^^^^^^^^^^
-note: any code following this expression is unreachable
-  --> $DIR/expr_match.rs:7:22
+note: any code following this `match` expression is unreachable, as all arms diverge
+  --> $DIR/expr_match.rs:7:5
    |
 LL |     match () { () => return }
-   |                      ^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
 
 error: unreachable statement
@@ -22,11 +22,11 @@ error: unreachable statement
 LL |     println!("I am dead");
    |     ^^^^^^^^^^^^^^^^^^^^^^
    |
-note: any code following this expression is unreachable
-  --> $DIR/expr_match.rs:18:31
+note: any code following this `match` expression is unreachable, as all arms diverge
+  --> $DIR/expr_match.rs:18:5
    |
 LL |     match () { () if false => return, () => return }
-   |                               ^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
 
 error: aborting due to 2 previous errors

From 9e777eb45de2bf972bbf7f2075fa491e22d2d071 Mon Sep 17 00:00:00 2001
From: Aaron Hill <aa1ronham@gmail.com>
Date: Wed, 18 Sep 2019 19:31:38 -0400
Subject: [PATCH 08/18] Some formatting cleanup

---
 src/librustc_typeck/check/mod.rs              | 13 ++++++-------
 src/test/ui/reachable/expr_return_in_macro.rs |  2 +-
 2 files changed, 7 insertions(+), 8 deletions(-)

diff --git a/src/librustc_typeck/check/mod.rs b/src/librustc_typeck/check/mod.rs
index c44648bb9df85..f6f3173e81c89 100644
--- a/src/librustc_typeck/check/mod.rs
+++ b/src/librustc_typeck/check/mod.rs
@@ -2338,13 +2338,12 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 debug!("warn_if_unreachable: id={:?} span={:?} kind={}", id, span, kind);
 
                 let msg = format!("unreachable {}", kind);
-                let mut err = self.tcx().struct_span_lint_hir(lint::builtin::UNREACHABLE_CODE,
-                                                              id, span, &msg);
-                err.span_note(
-                    orig_span,
-                    custom_note.unwrap_or("any code following this expression is unreachable")
-                );
-                err.emit();
+                self.tcx().struct_span_lint_hir(lint::builtin::UNREACHABLE_CODE, id, span, &msg)
+                    .span_note(
+                        orig_span,
+                        custom_note.unwrap_or("any code following this expression is unreachable")
+                    )
+                    .emit();
             }
         }
     }
diff --git a/src/test/ui/reachable/expr_return_in_macro.rs b/src/test/ui/reachable/expr_return_in_macro.rs
index 198ede6b14106..4e57618bf5e77 100644
--- a/src/test/ui/reachable/expr_return_in_macro.rs
+++ b/src/test/ui/reachable/expr_return_in_macro.rs
@@ -1,6 +1,6 @@
 // Tests that we generate nice error messages
 // when an expression is unreachble due to control
-// flow inside of a macro expansion
+// flow inside of a macro expansion.
 #![deny(unreachable_code)]
 
 macro_rules! early_return {

From 6edcfbe59a2067df7aadc18de990b9a86216099e Mon Sep 17 00:00:00 2001
From: Aaron Hill <aa1ronham@gmail.com>
Date: Wed, 18 Sep 2019 19:48:37 -0400
Subject: [PATCH 09/18] Apply formatting fixes

Co-Authored-By: Mazdak Farrokhzad <twingoow@gmail.com>
---
 src/librustc_typeck/check/_match.rs |  8 ++++----
 src/librustc_typeck/check/mod.rs    | 10 +++++-----
 2 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/src/librustc_typeck/check/_match.rs b/src/librustc_typeck/check/_match.rs
index 86774466ba586..034cba0f12a62 100644
--- a/src/librustc_typeck/check/_match.rs
+++ b/src/librustc_typeck/check/_match.rs
@@ -170,12 +170,12 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             prior_arm_ty = Some(arm_ty);
         }
 
-        // If all of the arms in the 'match' diverge,
-        // and we're dealing with an actual 'match' block
-        // (as opposed to a 'match' desugared from something else'),
+        // If all of the arms in the `match` diverge,
+        // and we're dealing with an actual `match` block
+        // (as opposed to a `match` desugared from something else'),
         // we can emit a better note. Rather than pointing
         // at a diverging expression in an arbitrary arm,
-        // we can point at the entire 'match' expression
+        // we can point at the entire `match` expression
         match (all_arms_diverge, match_src) {
             (Diverges::Always { .. }, hir::MatchSource::Normal) => {
                 all_arms_diverge = Diverges::Always {
diff --git a/src/librustc_typeck/check/mod.rs b/src/librustc_typeck/check/mod.rs
index f6f3173e81c89..e424aa679088d 100644
--- a/src/librustc_typeck/check/mod.rs
+++ b/src/librustc_typeck/check/mod.rs
@@ -453,15 +453,15 @@ pub enum Diverges {
     Always {
         /// The `Span` points to the expression
         /// that caused us to diverge
-        /// (e.g. `return`, `break`, etc)
+        /// (e.g. `return`, `break`, etc).
         span: Span,
-        /// In some cases (e.g. a 'match' expression
+        /// In some cases (e.g. a `match` expression
         /// where all arms diverge), we may be
         /// able to provide a more informative
         /// message to the user.
-        /// If this is None, a default messsage
+        /// If this is `None`, a default messsage
         /// will be generated, which is suitable
-        /// for most cases
+        /// for most cases.
         custom_note: Option<&'static str>
     },
 
@@ -502,7 +502,7 @@ impl Diverges {
     fn always(self) -> bool {
         // Enum comparison ignores the
         // contents of fields, so we just
-        // fill them in with garbage here
+        // fill them in with garbage here.
         self >= Diverges::Always {
             span: DUMMY_SP,
             custom_note: None

From a8ce93e13a9f1e7d7933ca76f6cae9c8f6127c34 Mon Sep 17 00:00:00 2001
From: Aaron Hill <aa1ronham@gmail.com>
Date: Wed, 18 Sep 2019 20:04:01 -0400
Subject: [PATCH 10/18] Introduce Diverges::always constructor

Rename the existing Diverges.always method to Diverges.is_always
---
 src/librustc_typeck/check/_match.rs |  7 ++-----
 src/librustc_typeck/check/expr.rs   |  5 +----
 src/librustc_typeck/check/mod.rs    | 14 ++++++++++++--
 3 files changed, 15 insertions(+), 11 deletions(-)

diff --git a/src/librustc_typeck/check/_match.rs b/src/librustc_typeck/check/_match.rs
index 034cba0f12a62..9481638fc14d8 100644
--- a/src/librustc_typeck/check/_match.rs
+++ b/src/librustc_typeck/check/_match.rs
@@ -43,10 +43,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
         // If there are no arms, that is a diverging match; a special case.
         if arms.is_empty() {
-            self.diverges.set(self.diverges.get() | Diverges::Always {
-                span: expr.span,
-                custom_note: None
-            });
+            self.diverges.set(self.diverges.get() | Diverges::always(expr.span));
             return tcx.types.never;
         }
 
@@ -198,7 +195,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// When the previously checked expression (the scrutinee) diverges,
     /// warn the user about the match arms being unreachable.
     fn warn_arms_when_scrutinee_diverges(&self, arms: &'tcx [hir::Arm], source: hir::MatchSource) {
-        if self.diverges.get().always() {
+        if self.diverges.get().is_always() {
             use hir::MatchSource::*;
             let msg = match source {
                 IfDesugar { .. } | IfLetDesugar { .. } => "block in `if` expression",
diff --git a/src/librustc_typeck/check/expr.rs b/src/librustc_typeck/check/expr.rs
index 5733b8d1db13e..049f2eb16bb00 100644
--- a/src/librustc_typeck/check/expr.rs
+++ b/src/librustc_typeck/check/expr.rs
@@ -170,10 +170,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
         // Any expression that produces a value of type `!` must have diverged
         if ty.is_never() {
-            self.diverges.set(self.diverges.get() | Diverges::Always {
-                span: expr.span,
-                custom_note: None
-            });
+            self.diverges.set(self.diverges.get() | Diverges::always(expr.span));
         }
 
         // Record the type, which applies it effects.
diff --git a/src/librustc_typeck/check/mod.rs b/src/librustc_typeck/check/mod.rs
index e424aa679088d..410eef5e092d9 100644
--- a/src/librustc_typeck/check/mod.rs
+++ b/src/librustc_typeck/check/mod.rs
@@ -470,6 +470,16 @@ pub enum Diverges {
     WarnedAlways
 }
 
+impl Diverges {
+    /// Creates a `Diverges::Always` with the provided span and the default note message
+    fn always(span: Span) -> Diverges {
+        Diverges::Always {
+            span,
+            custom_note: None
+        }
+    }
+}
+
 // Convenience impls for combinig `Diverges`.
 
 impl ops::BitAnd for Diverges {
@@ -499,7 +509,7 @@ impl ops::BitOrAssign for Diverges {
 }
 
 impl Diverges {
-    fn always(self) -> bool {
+    fn is_always(self) -> bool {
         // Enum comparison ignores the
         // contents of fields, so we just
         // fill them in with garbage here.
@@ -3852,7 +3862,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 //
                 // #41425 -- label the implicit `()` as being the
                 // "found type" here, rather than the "expected type".
-                if !self.diverges.get().always() {
+                if !self.diverges.get().is_always() {
                     // #50009 -- Do not point at the entire fn block span, point at the return type
                     // span, as it is the cause of the requirement, and
                     // `consider_hint_about_removing_semicolon` will point at the last expression

From 41e1128969adc0190c946ccbd8b6b89ad197bc34 Mon Sep 17 00:00:00 2001
From: Aaron Hill <aa1ronham@gmail.com>
Date: Wed, 18 Sep 2019 20:07:33 -0400
Subject: [PATCH 11/18] Use 'if let' instead of 'match'

---
 src/librustc_typeck/check/_match.rs | 18 +++++++-----------
 1 file changed, 7 insertions(+), 11 deletions(-)

diff --git a/src/librustc_typeck/check/_match.rs b/src/librustc_typeck/check/_match.rs
index 9481638fc14d8..50fd72f061319 100644
--- a/src/librustc_typeck/check/_match.rs
+++ b/src/librustc_typeck/check/_match.rs
@@ -173,17 +173,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         // we can emit a better note. Rather than pointing
         // at a diverging expression in an arbitrary arm,
         // we can point at the entire `match` expression
-        match (all_arms_diverge, match_src) {
-            (Diverges::Always { .. }, hir::MatchSource::Normal) => {
-                all_arms_diverge = Diverges::Always {
-                    span: expr.span,
-                    custom_note: Some(
-                        "any code following this `match` expression is unreachable, \
-                        as all arms diverge"
-                    )
-                };
-            },
-            _ => {}
+        if let (Diverges::Always { .. }, hir::MatchSource::Normal) = (all_arms_diverge, match_src) {
+            all_arms_diverge = Diverges::Always {
+                span: expr.span,
+                custom_note: Some(
+                    "any code following this `match` expression is unreachable, as all arms diverge"
+                )
+            };
         }
 
         // We won't diverge unless the discriminant or all arms diverge.

From 034a8fd1dff9bff585a6152bf5c52e882a2bb28b Mon Sep 17 00:00:00 2001
From: Aaron Hill <aa1ronham@gmail.com>
Date: Wed, 18 Sep 2019 20:14:56 -0400
Subject: [PATCH 12/18] Another formatting fix

Co-Authored-By: Mazdak Farrokhzad <twingoow@gmail.com>
---
 src/librustc_typeck/check/mod.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/librustc_typeck/check/mod.rs b/src/librustc_typeck/check/mod.rs
index 410eef5e092d9..51381447fedca 100644
--- a/src/librustc_typeck/check/mod.rs
+++ b/src/librustc_typeck/check/mod.rs
@@ -471,7 +471,7 @@ pub enum Diverges {
 }
 
 impl Diverges {
-    /// Creates a `Diverges::Always` with the provided span and the default note message
+    /// Creates a `Diverges::Always` with the provided `span` and the default note message.
     fn always(span: Span) -> Diverges {
         Diverges::Always {
             span,

From d67528ff7dbbe226fa583b9585cee2138533770e Mon Sep 17 00:00:00 2001
From: Aaron Hill <aa1ronham@gmail.com>
Date: Wed, 18 Sep 2019 21:57:50 -0400
Subject: [PATCH 13/18] Merge inherent impl blocks for `Diverges

---
 src/librustc_typeck/check/mod.rs | 18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

diff --git a/src/librustc_typeck/check/mod.rs b/src/librustc_typeck/check/mod.rs
index 51381447fedca..1d68d79db8856 100644
--- a/src/librustc_typeck/check/mod.rs
+++ b/src/librustc_typeck/check/mod.rs
@@ -470,16 +470,6 @@ pub enum Diverges {
     WarnedAlways
 }
 
-impl Diverges {
-    /// Creates a `Diverges::Always` with the provided `span` and the default note message.
-    fn always(span: Span) -> Diverges {
-        Diverges::Always {
-            span,
-            custom_note: None
-        }
-    }
-}
-
 // Convenience impls for combinig `Diverges`.
 
 impl ops::BitAnd for Diverges {
@@ -509,6 +499,14 @@ impl ops::BitOrAssign for Diverges {
 }
 
 impl Diverges {
+    /// Creates a `Diverges::Always` with the provided `span` and the default note message.
+    fn always(span: Span) -> Diverges {
+        Diverges::Always {
+            span,
+            custom_note: None
+        }
+    }
+
     fn is_always(self) -> bool {
         // Enum comparison ignores the
         // contents of fields, so we just

From 37bafeafe73cff935bb444b6736a958a9a752669 Mon Sep 17 00:00:00 2001
From: Joshua Groves <josh@joshgroves.com>
Date: Wed, 18 Sep 2019 23:17:36 -0600
Subject: [PATCH 14/18] Fix backticks in documentation

---
 src/librustc/middle/region.rs                          | 2 +-
 src/librustc/ty/query/on_disk_cache.rs                 | 2 +-
 src/libsyntax/source_map.rs                            | 4 ++--
 src/test/ui/borrowck/two-phase-surprise-no-conflict.rs | 2 +-
 4 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/src/librustc/middle/region.rs b/src/librustc/middle/region.rs
index b9d8a4ec68fad..87470140e3148 100644
--- a/src/librustc/middle/region.rs
+++ b/src/librustc/middle/region.rs
@@ -1136,7 +1136,7 @@ fn resolve_local<'tcx>(
     // Rule A. `let (ref x, ref y) = (foo().x, 44)`. The rvalue `(22, 44)`
     // would have an extended lifetime, but not `foo()`.
     //
-    // Rule B. `let x = &foo().x`. The rvalue ``foo()` would have extended
+    // Rule B. `let x = &foo().x`. The rvalue `foo()` would have extended
     // lifetime.
     //
     // In some cases, multiple rules may apply (though not to the same
diff --git a/src/librustc/ty/query/on_disk_cache.rs b/src/librustc/ty/query/on_disk_cache.rs
index 4cef6a09925ad..c20e758688959 100644
--- a/src/librustc/ty/query/on_disk_cache.rs
+++ b/src/librustc/ty/query/on_disk_cache.rs
@@ -643,7 +643,7 @@ impl<'a, 'tcx> SpecializedDecoder<DefIndex> for CacheDecoder<'a, 'tcx> {
 
 // Both the `CrateNum` and the `DefIndex` of a `DefId` can change in between two
 // compilation sessions. We use the `DefPathHash`, which is stable across
-// sessions, to map the old DefId`` to the new one.
+// sessions, to map the old `DefId` to the new one.
 impl<'a, 'tcx> SpecializedDecoder<DefId> for CacheDecoder<'a, 'tcx> {
     #[inline]
     fn specialized_decode(&mut self) -> Result<DefId, Self::Error> {
diff --git a/src/libsyntax/source_map.rs b/src/libsyntax/source_map.rs
index d7ea799e00459..7d0d2392945e5 100644
--- a/src/libsyntax/source_map.rs
+++ b/src/libsyntax/source_map.rs
@@ -3,7 +3,7 @@
 //! of source parsed during crate parsing (typically files, in-memory strings,
 //! or various bits of macro expansion) cover a continuous range of bytes in the
 //! `SourceMap` and are represented by `SourceFile`s. Byte positions are stored in
-//! `Span`` and used pervasively in the compiler. They are absolute positions
+//! `Span` and used pervasively in the compiler. They are absolute positions
 //! within the `SourceMap`, which upon request can be converted to line and column
 //! information, source code snippets, etc.
 
@@ -645,7 +645,7 @@ impl SourceMap {
     }
 
     /// Given a `Span`, tries to get a shorter span ending before the first occurrence of `char`
-    /// ``c`.
+    /// `c`.
     pub fn span_until_char(&self, sp: Span, c: char) -> Span {
         match self.span_to_snippet(sp) {
             Ok(snippet) => {
diff --git a/src/test/ui/borrowck/two-phase-surprise-no-conflict.rs b/src/test/ui/borrowck/two-phase-surprise-no-conflict.rs
index 3fd24bbf290b5..6d37d1ded6400 100644
--- a/src/test/ui/borrowck/two-phase-surprise-no-conflict.rs
+++ b/src/test/ui/borrowck/two-phase-surprise-no-conflict.rs
@@ -31,7 +31,7 @@ impl <'a> SpanlessHash<'a> {
                 //
                 // Not okay without two-phase borrows: the implicit
                 // `&mut self` of the receiver is evaluated first, and
-                // that conflicts with the `self.cx`` access during
+                // that conflicts with the `self.cx` access during
                 // argument evaluation, as demonstrated in `fn demo`
                 // above.
                 //

From 1de533ac1a3bddd6bf61791f4ee6cf52cb186fdf Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Thu, 19 Sep 2019 09:02:45 +0200
Subject: [PATCH 15/18] first determine if the variant is a niche-variant, then
 compute absolute variant

---
 src/librustc_mir/interpret/operand.rs | 29 ++++++++++++++-------------
 src/librustc_mir/interpret/place.rs   |  3 ++-
 2 files changed, 17 insertions(+), 15 deletions(-)

diff --git a/src/librustc_mir/interpret/operand.rs b/src/librustc_mir/interpret/operand.rs
index 9e7ae32f1eacf..dd214c4a031f7 100644
--- a/src/librustc_mir/interpret/operand.rs
+++ b/src/librustc_mir/interpret/operand.rs
@@ -1,7 +1,7 @@
 //! Functions concerning immediate values and operands, and reading from operands.
 //! All high-level functions to read from memory work on operands as sources.
 
-use std::convert::TryInto;
+use std::convert::{TryInto, TryFrom};
 
 use rustc::{mir, ty};
 use rustc::ty::layout::{
@@ -671,8 +671,8 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 ref niche_variants,
                 niche_start,
             } => {
-                let variants_start = niche_variants.start().as_u32() as u128;
-                let variants_end = niche_variants.end().as_u32() as u128;
+                let variants_start = niche_variants.start().as_u32();
+                let variants_end = niche_variants.end().as_u32();
                 let raw_discr = raw_discr.not_undef().map_err(|_| {
                     err_unsup!(InvalidDiscriminant(ScalarMaybeUndef::Undef))
                 })?;
@@ -687,7 +687,8 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                         (dataful_variant.as_u32() as u128, dataful_variant)
                     },
                     Ok(raw_discr) => {
-                        // We need to use machine arithmetic to get the relative variant idx.
+                        // We need to use machine arithmetic to get the relative variant idx:
+                        // variant_index_relative = discr_val - niche_start_val
                         let discr_layout = self.layout_of(discr_layout.value.to_int_ty(*self.tcx))?;
                         let discr_val = ImmTy::from_uint(raw_discr, discr_layout);
                         let niche_start_val = ImmTy::from_uint(niche_start, discr_layout);
@@ -699,21 +700,21 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                         let variant_index_relative = variant_index_relative_val
                             .to_scalar()?
                             .assert_bits(discr_val.layout.size);
-                        // Then computing the absolute variant idx should not overflow any more.
-                        let variant_index = variants_start
-                            .checked_add(variant_index_relative)
-                            .expect("oveflow computing absolute variant idx");
                         // Check if this is in the range that indicates an actual discriminant.
-                        if variants_start <= variant_index && variant_index <= variants_end {
-                            let index = variant_index as usize;
-                            assert_eq!(index as u128, variant_index);
-                            assert!(index < rval.layout.ty
+                        if variant_index_relative <= u128::from(variants_end - variants_start) {
+                            let variant_index_relative = u32::try_from(variant_index_relative)
+                                .expect("we checked that this fits into a u32");
+                            // Then computing the absolute variant idx should not overflow any more.
+                            let variant_index = variants_start
+                                .checked_add(variant_index_relative)
+                                .expect("oveflow computing absolute variant idx");
+                            assert!((variant_index as usize) < rval.layout.ty
                                 .ty_adt_def()
                                 .expect("tagged layout for non adt")
                                 .variants.len());
-                            (variant_index, VariantIdx::from_usize(index))
+                            (u128::from(variant_index), VariantIdx::from_u32(variant_index))
                         } else {
-                            (dataful_variant.as_u32() as u128, dataful_variant)
+                            (u128::from(dataful_variant.as_u32()), dataful_variant)
                         }
                     },
                 }
diff --git a/src/librustc_mir/interpret/place.rs b/src/librustc_mir/interpret/place.rs
index 5f4903d61e3b2..c3660fb7a2e28 100644
--- a/src/librustc_mir/interpret/place.rs
+++ b/src/librustc_mir/interpret/place.rs
@@ -1064,7 +1064,8 @@ where
                     let variant_index_relative = variant_index.as_u32()
                         .checked_sub(variants_start)
                         .expect("overflow computing relative variant idx");
-                    // We need to use machine arithmetic when taking into account `niche_start`.
+                    // We need to use machine arithmetic when taking into account `niche_start`:
+                    // discr_val = variant_index_relative + niche_start_val
                     let discr_layout = self.layout_of(discr_layout.value.to_int_ty(*self.tcx))?;
                     let niche_start_val = ImmTy::from_uint(niche_start, discr_layout);
                     let variant_index_relative_val =

From c23e78a38b9643f15b008393b2506a813e7ee2ab Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Adrian=20Heine=20n=C3=A9=20Lang?= <mail@adrianheine.de>
Date: Thu, 19 Sep 2019 13:36:10 +0200
Subject: [PATCH 16/18] Remove unnecessary `mut` in doc example

---
 src/libstd/process.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/libstd/process.rs b/src/libstd/process.rs
index b8d57cfafea0a..c6dc02fea2d84 100644
--- a/src/libstd/process.rs
+++ b/src/libstd/process.rs
@@ -935,7 +935,7 @@ impl Stdio {
     ///     .expect("Failed to spawn child process");
     ///
     /// {
-    ///     let mut stdin = child.stdin.as_mut().expect("Failed to open stdin");
+    ///     let stdin = child.stdin.as_mut().expect("Failed to open stdin");
     ///     stdin.write_all("Hello, world!".as_bytes()).expect("Failed to write to stdin");
     /// }
     ///

From 2fff78038cf3eb38f2bd3fecbab123bccb59c1a8 Mon Sep 17 00:00:00 2001
From: Alex Crichton <alex@alexcrichton.com>
Date: Thu, 19 Sep 2019 07:46:24 -0700
Subject: [PATCH 17/18] rustbuild: Don't package libstd twice

Looks like the packaging step for the standard library was happening
twice on CI, but it only needs to happen once! The `Analysis` packaging
step accidentally packaged `Std` instead of relying on compiling `Std`,
which meant that we ended up packaging it twice erroneously.
---
 src/bootstrap/dist.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/bootstrap/dist.rs b/src/bootstrap/dist.rs
index 076bcd878df71..e27a6bf7da0ac 100644
--- a/src/bootstrap/dist.rs
+++ b/src/bootstrap/dist.rs
@@ -762,7 +762,7 @@ impl Step for Analysis {
             return distdir(builder).join(format!("{}-{}.tar.gz", name, target));
         }
 
-        builder.ensure(Std { compiler, target });
+        builder.ensure(compile::Std { compiler, target });
 
         let image = tmpdir(builder).join(format!("{}-{}-image", name, target));
 

From d7f64749c0d32f3d63c0ee5d0999fce99f222553 Mon Sep 17 00:00:00 2001
From: Alex Crichton <alex@alexcrichton.com>
Date: Thu, 19 Sep 2019 07:48:04 -0700
Subject: [PATCH 18/18] rustbuild: Copy crate doc files fewer times

Previously when building documentation for the standard library we'd
copy all the files 5 times, and these files include libcore/libstd docs
which are huge! This commit instead only copies the files after rustdoc
has been run for each crate, reducing the number of redundant copies
we're making.
---
 src/bootstrap/doc.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/bootstrap/doc.rs b/src/bootstrap/doc.rs
index 6faedc80ad3f5..873a3c31d1535 100644
--- a/src/bootstrap/doc.rs
+++ b/src/bootstrap/doc.rs
@@ -476,11 +476,11 @@ impl Step for Std {
                  .arg("--index-page").arg(&builder.src.join("src/doc/index.md"));
 
             builder.run(&mut cargo);
-            builder.cp_r(&my_out, &out);
         };
         for krate in &["alloc", "core", "std", "proc_macro", "test"] {
             run_cargo_rustdoc_for(krate);
         }
+        builder.cp_r(&my_out, &out);
     }
 }