From 95318b47831365e0fac4ea8af62e218cde9d1e3a Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Sat, 9 Mar 2024 23:17:25 +0100
Subject: [PATCH 01/11] MIR: build up a list of required_fns

---
 compiler/rustc_middle/src/mir/mod.rs          |  7 ++-
 .../rustc_mir_build/src/build/custom/mod.rs   |  1 +
 compiler/rustc_mir_transform/src/lib.rs       |  5 +-
 .../src/required_consts.rs                    | 46 +++++++++++++++++--
 4 files changed, 52 insertions(+), 7 deletions(-)

diff --git a/compiler/rustc_middle/src/mir/mod.rs b/compiler/rustc_middle/src/mir/mod.rs
index 0221bc6c36383..f5e70b683ad35 100644
--- a/compiler/rustc_middle/src/mir/mod.rs
+++ b/compiler/rustc_middle/src/mir/mod.rs
@@ -9,7 +9,7 @@ use crate::ty::fold::{FallibleTypeFolder, TypeFoldable};
 use crate::ty::print::{pretty_print_const, with_no_trimmed_paths};
 use crate::ty::print::{FmtPrinter, Printer};
 use crate::ty::visit::TypeVisitableExt;
-use crate::ty::{self, List, Ty, TyCtxt};
+use crate::ty::{self, Instance, List, Ty, TyCtxt};
 use crate::ty::{AdtDef, InstanceDef, UserTypeAnnotationIndex};
 use crate::ty::{GenericArg, GenericArgsRef};
 
@@ -375,6 +375,9 @@ pub struct Body<'tcx> {
     /// We hold in this field all the constants we are not able to evaluate yet.
     pub required_consts: Vec<ConstOperand<'tcx>>,
 
+    /// Functions that need to monomorphize successfully for this MIR to be well-formed.
+    pub required_fns: Vec<Instance<'tcx>>,
+
     /// Does this body use generic parameters. This is used for the `ConstEvaluatable` check.
     ///
     /// Note that this does not actually mean that this body is not computable right now.
@@ -445,6 +448,7 @@ impl<'tcx> Body<'tcx> {
             var_debug_info,
             span,
             required_consts: Vec::new(),
+            required_fns: Vec::new(),
             is_polymorphic: false,
             injection_phase: None,
             tainted_by_errors,
@@ -473,6 +477,7 @@ impl<'tcx> Body<'tcx> {
             spread_arg: None,
             span: DUMMY_SP,
             required_consts: Vec::new(),
+            required_fns: Vec::new(),
             var_debug_info: Vec::new(),
             is_polymorphic: false,
             injection_phase: None,
diff --git a/compiler/rustc_mir_build/src/build/custom/mod.rs b/compiler/rustc_mir_build/src/build/custom/mod.rs
index c2bff9084c67e..3132083d0e9a5 100644
--- a/compiler/rustc_mir_build/src/build/custom/mod.rs
+++ b/compiler/rustc_mir_build/src/build/custom/mod.rs
@@ -56,6 +56,7 @@ pub(super) fn build_custom_mir<'tcx>(
         var_debug_info: Vec::new(),
         span,
         required_consts: Vec::new(),
+        required_fns: Vec::new(),
         is_polymorphic: false,
         tainted_by_errors: None,
         injection_phase: None,
diff --git a/compiler/rustc_mir_transform/src/lib.rs b/compiler/rustc_mir_transform/src/lib.rs
index cd9b98e4f32cd..6f6921fc0bd3f 100644
--- a/compiler/rustc_mir_transform/src/lib.rs
+++ b/compiler/rustc_mir_transform/src/lib.rs
@@ -344,11 +344,14 @@ fn mir_promoted(
     }
 
     let mut required_consts = Vec::new();
-    let mut required_consts_visitor = RequiredConstsVisitor::new(&mut required_consts);
+    let mut required_fns = Vec::new();
+    let mut required_consts_visitor =
+        RequiredConstsVisitor::new(tcx, &body, &mut required_consts, &mut required_fns);
     for (bb, bb_data) in traversal::reverse_postorder(&body) {
         required_consts_visitor.visit_basic_block_data(bb, bb_data);
     }
     body.required_consts = required_consts;
+    body.required_fns = required_fns;
 
     // What we need to run borrowck etc.
     let promote_pass = promote_consts::PromoteTemps::default();
diff --git a/compiler/rustc_mir_transform/src/required_consts.rs b/compiler/rustc_mir_transform/src/required_consts.rs
index abde6a47e83aa..cd215d7659548 100644
--- a/compiler/rustc_mir_transform/src/required_consts.rs
+++ b/compiler/rustc_mir_transform/src/required_consts.rs
@@ -1,14 +1,23 @@
+use rustc_hir::LangItem;
 use rustc_middle::mir::visit::Visitor;
-use rustc_middle::mir::{Const, ConstOperand, Location};
-use rustc_middle::ty::ConstKind;
+use rustc_middle::mir::{self, Const, ConstOperand, Location};
+use rustc_middle::ty::{self, ConstKind, Instance, InstanceDef, TyCtxt};
 
 pub struct RequiredConstsVisitor<'a, 'tcx> {
+    tcx: TyCtxt<'tcx>,
+    body: &'a mir::Body<'tcx>,
     required_consts: &'a mut Vec<ConstOperand<'tcx>>,
+    required_fns: &'a mut Vec<Instance<'tcx>>,
 }
 
 impl<'a, 'tcx> RequiredConstsVisitor<'a, 'tcx> {
-    pub fn new(required_consts: &'a mut Vec<ConstOperand<'tcx>>) -> Self {
-        RequiredConstsVisitor { required_consts }
+    pub fn new(
+        tcx: TyCtxt<'tcx>,
+        body: &'a mir::Body<'tcx>,
+        required_consts: &'a mut Vec<ConstOperand<'tcx>>,
+        required_fns: &'a mut Vec<Instance<'tcx>>,
+    ) -> Self {
+        RequiredConstsVisitor { tcx, body, required_consts, required_fns }
     }
 }
 
@@ -21,7 +30,34 @@ impl<'tcx> Visitor<'tcx> for RequiredConstsVisitor<'_, 'tcx> {
                 _ => bug!("only ConstKind::Param/Value should be encountered here, got {:#?}", c),
             },
             Const::Unevaluated(..) => self.required_consts.push(*constant),
-            Const::Val(..) => {}
+            Const::Val(_val, ty) => {
+                // This is how function items get referenced: via zero-sized constants of `FnDef` type
+                if let ty::FnDef(def_id, args) = ty.kind() {
+                    debug!("adding to required_fns: {def_id:?}");
+                    // FIXME maybe we shouldn't use `Instance`? We can't use `Instance::new`, it is
+                    // for codegen. But `Instance` feels like the right representation... Check what
+                    // the regular collector does.
+                    self.required_fns.push(Instance { def: InstanceDef::Item(*def_id), args });
+                }
+            }
+        }
+    }
+
+    fn visit_terminator(&mut self, terminator: &mir::Terminator<'tcx>, location: Location) {
+        self.super_terminator(terminator, location);
+
+        match terminator.kind {
+            // We don't need to handle `Call` as we already handled all function type operands in
+            // `visit_constant`. But we do need to handle `Drop`.
+            mir::TerminatorKind::Drop { place, .. } => {
+                let ty = place.ty(self.body, self.tcx).ty;
+                let def_id = self.tcx.require_lang_item(LangItem::DropInPlace, None);
+                let args = self.tcx.mk_args(&[ty.into()]);
+                // FIXME: same as above (we cannot use `Instance::resolve_drop_in_place` as this is
+                // still generic).
+                self.required_fns.push(Instance { def: InstanceDef::Item(def_id), args });
+            }
+            _ => {}
         }
     }
 }

From 937b3a42a2acc1723a52615b001af1bf80b656ab Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Sun, 10 Mar 2024 11:19:55 +0100
Subject: [PATCH 02/11] introduce new type for required items of a MIR body

---
 compiler/rustc_middle/src/mir/mod.rs          | 17 +++++++++----
 .../rustc_mir_build/src/build/custom/mod.rs   |  2 +-
 compiler/rustc_mir_transform/src/lib.rs       |  6 ++---
 .../src/required_consts.rs                    | 24 +++++++------------
 4 files changed, 24 insertions(+), 25 deletions(-)

diff --git a/compiler/rustc_middle/src/mir/mod.rs b/compiler/rustc_middle/src/mir/mod.rs
index f5e70b683ad35..9c141e3f7033a 100644
--- a/compiler/rustc_middle/src/mir/mod.rs
+++ b/compiler/rustc_middle/src/mir/mod.rs
@@ -9,7 +9,7 @@ use crate::ty::fold::{FallibleTypeFolder, TypeFoldable};
 use crate::ty::print::{pretty_print_const, with_no_trimmed_paths};
 use crate::ty::print::{FmtPrinter, Printer};
 use crate::ty::visit::TypeVisitableExt;
-use crate::ty::{self, Instance, List, Ty, TyCtxt};
+use crate::ty::{self, List, Ty, TyCtxt};
 use crate::ty::{AdtDef, InstanceDef, UserTypeAnnotationIndex};
 use crate::ty::{GenericArg, GenericArgsRef};
 
@@ -310,6 +310,13 @@ impl<'tcx> CoroutineInfo<'tcx> {
     }
 }
 
+/// Some item that needs to monomorphize successfully for a MIR body to be considered well-formed.
+#[derive(Clone, TyEncodable, TyDecodable, Debug, HashStable, TypeFoldable, TypeVisitable)]
+pub enum RequiredItem<'tcx> {
+    Fn(DefId, GenericArgsRef<'tcx>),
+    Drop(Ty<'tcx>),
+}
+
 /// The lowered representation of a single function.
 #[derive(Clone, TyEncodable, TyDecodable, Debug, HashStable, TypeFoldable, TypeVisitable)]
 pub struct Body<'tcx> {
@@ -375,8 +382,8 @@ pub struct Body<'tcx> {
     /// We hold in this field all the constants we are not able to evaluate yet.
     pub required_consts: Vec<ConstOperand<'tcx>>,
 
-    /// Functions that need to monomorphize successfully for this MIR to be well-formed.
-    pub required_fns: Vec<Instance<'tcx>>,
+    /// Further items that need to monomorphize successfully for this MIR to be well-formed.
+    pub required_items: Vec<RequiredItem<'tcx>>,
 
     /// Does this body use generic parameters. This is used for the `ConstEvaluatable` check.
     ///
@@ -448,7 +455,7 @@ impl<'tcx> Body<'tcx> {
             var_debug_info,
             span,
             required_consts: Vec::new(),
-            required_fns: Vec::new(),
+            required_items: Vec::new(),
             is_polymorphic: false,
             injection_phase: None,
             tainted_by_errors,
@@ -477,7 +484,7 @@ impl<'tcx> Body<'tcx> {
             spread_arg: None,
             span: DUMMY_SP,
             required_consts: Vec::new(),
-            required_fns: Vec::new(),
+            required_items: Vec::new(),
             var_debug_info: Vec::new(),
             is_polymorphic: false,
             injection_phase: None,
diff --git a/compiler/rustc_mir_build/src/build/custom/mod.rs b/compiler/rustc_mir_build/src/build/custom/mod.rs
index 3132083d0e9a5..0ad6b22ee5405 100644
--- a/compiler/rustc_mir_build/src/build/custom/mod.rs
+++ b/compiler/rustc_mir_build/src/build/custom/mod.rs
@@ -56,7 +56,7 @@ pub(super) fn build_custom_mir<'tcx>(
         var_debug_info: Vec::new(),
         span,
         required_consts: Vec::new(),
-        required_fns: Vec::new(),
+        required_items: Vec::new(),
         is_polymorphic: false,
         tainted_by_errors: None,
         injection_phase: None,
diff --git a/compiler/rustc_mir_transform/src/lib.rs b/compiler/rustc_mir_transform/src/lib.rs
index 6f6921fc0bd3f..b76ee39b1ba97 100644
--- a/compiler/rustc_mir_transform/src/lib.rs
+++ b/compiler/rustc_mir_transform/src/lib.rs
@@ -344,14 +344,14 @@ fn mir_promoted(
     }
 
     let mut required_consts = Vec::new();
-    let mut required_fns = Vec::new();
+    let mut required_items = Vec::new();
     let mut required_consts_visitor =
-        RequiredConstsVisitor::new(tcx, &body, &mut required_consts, &mut required_fns);
+        RequiredConstsVisitor::new(tcx, &body, &mut required_consts, &mut required_items);
     for (bb, bb_data) in traversal::reverse_postorder(&body) {
         required_consts_visitor.visit_basic_block_data(bb, bb_data);
     }
     body.required_consts = required_consts;
-    body.required_fns = required_fns;
+    body.required_items = required_items;
 
     // What we need to run borrowck etc.
     let promote_pass = promote_consts::PromoteTemps::default();
diff --git a/compiler/rustc_mir_transform/src/required_consts.rs b/compiler/rustc_mir_transform/src/required_consts.rs
index cd215d7659548..52dc9353fdc8b 100644
--- a/compiler/rustc_mir_transform/src/required_consts.rs
+++ b/compiler/rustc_mir_transform/src/required_consts.rs
@@ -1,13 +1,12 @@
-use rustc_hir::LangItem;
 use rustc_middle::mir::visit::Visitor;
-use rustc_middle::mir::{self, Const, ConstOperand, Location};
-use rustc_middle::ty::{self, ConstKind, Instance, InstanceDef, TyCtxt};
+use rustc_middle::mir::{self, Const, ConstOperand, Location, RequiredItem};
+use rustc_middle::ty::{self, ConstKind, TyCtxt};
 
 pub struct RequiredConstsVisitor<'a, 'tcx> {
     tcx: TyCtxt<'tcx>,
     body: &'a mir::Body<'tcx>,
     required_consts: &'a mut Vec<ConstOperand<'tcx>>,
-    required_fns: &'a mut Vec<Instance<'tcx>>,
+    required_items: &'a mut Vec<RequiredItem<'tcx>>,
 }
 
 impl<'a, 'tcx> RequiredConstsVisitor<'a, 'tcx> {
@@ -15,9 +14,9 @@ impl<'a, 'tcx> RequiredConstsVisitor<'a, 'tcx> {
         tcx: TyCtxt<'tcx>,
         body: &'a mir::Body<'tcx>,
         required_consts: &'a mut Vec<ConstOperand<'tcx>>,
-        required_fns: &'a mut Vec<Instance<'tcx>>,
+        required_items: &'a mut Vec<RequiredItem<'tcx>>,
     ) -> Self {
-        RequiredConstsVisitor { tcx, body, required_consts, required_fns }
+        RequiredConstsVisitor { tcx, body, required_consts, required_items }
     }
 }
 
@@ -33,11 +32,8 @@ impl<'tcx> Visitor<'tcx> for RequiredConstsVisitor<'_, 'tcx> {
             Const::Val(_val, ty) => {
                 // This is how function items get referenced: via zero-sized constants of `FnDef` type
                 if let ty::FnDef(def_id, args) = ty.kind() {
-                    debug!("adding to required_fns: {def_id:?}");
-                    // FIXME maybe we shouldn't use `Instance`? We can't use `Instance::new`, it is
-                    // for codegen. But `Instance` feels like the right representation... Check what
-                    // the regular collector does.
-                    self.required_fns.push(Instance { def: InstanceDef::Item(*def_id), args });
+                    debug!("adding to required_items: {def_id:?}");
+                    self.required_items.push(RequiredItem::Fn(*def_id, args));
                 }
             }
         }
@@ -51,11 +47,7 @@ impl<'tcx> Visitor<'tcx> for RequiredConstsVisitor<'_, 'tcx> {
             // `visit_constant`. But we do need to handle `Drop`.
             mir::TerminatorKind::Drop { place, .. } => {
                 let ty = place.ty(self.body, self.tcx).ty;
-                let def_id = self.tcx.require_lang_item(LangItem::DropInPlace, None);
-                let args = self.tcx.mk_args(&[ty.into()]);
-                // FIXME: same as above (we cannot use `Instance::resolve_drop_in_place` as this is
-                // still generic).
-                self.required_fns.push(Instance { def: InstanceDef::Item(def_id), args });
+                self.required_items.push(RequiredItem::Drop(ty));
             }
             _ => {}
         }

From 3c7d61a2b66eb045440bf2a586dd672bec3effde Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Sun, 10 Mar 2024 11:33:43 +0100
Subject: [PATCH 03/11] collect and extend our tests for
 monomorphization-related const errors

---
 .../consts/const-eval/erroneous-const.stderr  | 15 ----------
 .../consts/const-eval/erroneous-const2.stderr | 15 ----------
 .../dead-code-in-called-fn.no-opt.stderr      | 17 +++++++++++
 .../dead-code-in-called-fn.opt.stderr         | 17 +++++++++++
 .../dead-code-in-called-fn.rs}                | 17 ++++++-----
 ...dead-code-in-const-called-fn.no-opt.stderr | 17 +++++++++++
 .../dead-code-in-const-called-fn.opt.stderr   | 17 +++++++++++
 .../dead-code-in-const-called-fn.rs}          | 11 +++++---
 .../dead-code-in-dead-fn.no-opt.stderr        | 17 +++++++++++
 .../dead-code-in-dead-fn.opt.stderr}          |  8 +++---
 .../monomorphization/dead-code-in-dead-fn.rs  | 28 +++++++++++++++++++
 .../dead-code-in-static.no-opt.stderr         | 17 +++++++++++
 .../dead-code-in-static.opt.stderr            | 17 +++++++++++
 .../dead-code-in-static.rs}                   | 10 ++++---
 14 files changed, 174 insertions(+), 49 deletions(-)
 delete mode 100644 tests/ui/consts/const-eval/erroneous-const.stderr
 delete mode 100644 tests/ui/consts/const-eval/erroneous-const2.stderr
 create mode 100644 tests/ui/consts/monomorphization/dead-code-in-called-fn.no-opt.stderr
 create mode 100644 tests/ui/consts/monomorphization/dead-code-in-called-fn.opt.stderr
 rename tests/ui/consts/{const-eval/unused-broken-const-late.rs => monomorphization/dead-code-in-called-fn.rs} (62%)
 create mode 100644 tests/ui/consts/monomorphization/dead-code-in-const-called-fn.no-opt.stderr
 create mode 100644 tests/ui/consts/monomorphization/dead-code-in-const-called-fn.opt.stderr
 rename tests/ui/consts/{const-eval/erroneous-const.rs => monomorphization/dead-code-in-const-called-fn.rs} (62%)
 create mode 100644 tests/ui/consts/monomorphization/dead-code-in-dead-fn.no-opt.stderr
 rename tests/ui/consts/{const-eval/unused-broken-const-late.stderr => monomorphization/dead-code-in-dead-fn.opt.stderr} (53%)
 create mode 100644 tests/ui/consts/monomorphization/dead-code-in-dead-fn.rs
 create mode 100644 tests/ui/consts/monomorphization/dead-code-in-static.no-opt.stderr
 create mode 100644 tests/ui/consts/monomorphization/dead-code-in-static.opt.stderr
 rename tests/ui/consts/{const-eval/erroneous-const2.rs => monomorphization/dead-code-in-static.rs} (61%)

diff --git a/tests/ui/consts/const-eval/erroneous-const.stderr b/tests/ui/consts/const-eval/erroneous-const.stderr
deleted file mode 100644
index bd25e96c2cf13..0000000000000
--- a/tests/ui/consts/const-eval/erroneous-const.stderr
+++ /dev/null
@@ -1,15 +0,0 @@
-error[E0080]: evaluation of `PrintName::<i32>::VOID` failed
-  --> $DIR/erroneous-const.rs:6:22
-   |
-LL |     const VOID: () = [()][2];
-   |                      ^^^^^^^ index out of bounds: the length is 1 but the index is 2
-
-note: erroneous constant encountered
-  --> $DIR/erroneous-const.rs:13:13
-   |
-LL |             PrintName::<T>::VOID;
-   |             ^^^^^^^^^^^^^^^^^^^^
-
-error: aborting due to 1 previous error
-
-For more information about this error, try `rustc --explain E0080`.
diff --git a/tests/ui/consts/const-eval/erroneous-const2.stderr b/tests/ui/consts/const-eval/erroneous-const2.stderr
deleted file mode 100644
index 6a5839e3dfb41..0000000000000
--- a/tests/ui/consts/const-eval/erroneous-const2.stderr
+++ /dev/null
@@ -1,15 +0,0 @@
-error[E0080]: evaluation of `PrintName::<i32>::VOID` failed
-  --> $DIR/erroneous-const2.rs:6:22
-   |
-LL |     const VOID: () = [()][2];
-   |                      ^^^^^^^ index out of bounds: the length is 1 but the index is 2
-
-note: erroneous constant encountered
-  --> $DIR/erroneous-const2.rs:13:9
-   |
-LL |         PrintName::<i32>::VOID;
-   |         ^^^^^^^^^^^^^^^^^^^^^^
-
-error: aborting due to 1 previous error
-
-For more information about this error, try `rustc --explain E0080`.
diff --git a/tests/ui/consts/monomorphization/dead-code-in-called-fn.no-opt.stderr b/tests/ui/consts/monomorphization/dead-code-in-called-fn.no-opt.stderr
new file mode 100644
index 0000000000000..b69d10d322e86
--- /dev/null
+++ b/tests/ui/consts/monomorphization/dead-code-in-called-fn.no-opt.stderr
@@ -0,0 +1,17 @@
+error[E0080]: evaluation of `Fail::<i32>::C` failed
+  --> $DIR/dead-code-in-called-fn.rs:9:19
+   |
+LL |     const C: () = panic!();
+   |                   ^^^^^^^^ the evaluated program panicked at 'explicit panic', $DIR/dead-code-in-called-fn.rs:9:19
+   |
+   = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+note: the above error was encountered while instantiating `fn called::<i32>`
+  --> $DIR/dead-code-in-called-fn.rs:22:5
+   |
+LL |     called::<i32>();
+   |     ^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0080`.
diff --git a/tests/ui/consts/monomorphization/dead-code-in-called-fn.opt.stderr b/tests/ui/consts/monomorphization/dead-code-in-called-fn.opt.stderr
new file mode 100644
index 0000000000000..b69d10d322e86
--- /dev/null
+++ b/tests/ui/consts/monomorphization/dead-code-in-called-fn.opt.stderr
@@ -0,0 +1,17 @@
+error[E0080]: evaluation of `Fail::<i32>::C` failed
+  --> $DIR/dead-code-in-called-fn.rs:9:19
+   |
+LL |     const C: () = panic!();
+   |                   ^^^^^^^^ the evaluated program panicked at 'explicit panic', $DIR/dead-code-in-called-fn.rs:9:19
+   |
+   = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+note: the above error was encountered while instantiating `fn called::<i32>`
+  --> $DIR/dead-code-in-called-fn.rs:22:5
+   |
+LL |     called::<i32>();
+   |     ^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0080`.
diff --git a/tests/ui/consts/const-eval/unused-broken-const-late.rs b/tests/ui/consts/monomorphization/dead-code-in-called-fn.rs
similarity index 62%
rename from tests/ui/consts/const-eval/unused-broken-const-late.rs
rename to tests/ui/consts/monomorphization/dead-code-in-called-fn.rs
index c4916061f9e5a..cc7bafa7adb85 100644
--- a/tests/ui/consts/const-eval/unused-broken-const-late.rs
+++ b/tests/ui/consts/monomorphization/dead-code-in-called-fn.rs
@@ -1,20 +1,23 @@
+//@revisions: opt no-opt
 //@ build-fail
-//@ compile-flags: -O
+//@[opt] compile-flags: -O
 //! Make sure we detect erroneous constants post-monomorphization even when they are unused. This is
 //! crucial, people rely on it for soundness. (https://github.com/rust-lang/rust/issues/112090)
 
-struct PrintName<T>(T);
-impl<T> PrintName<T> {
-    const VOID: () = panic!(); //~ERROR evaluation of `PrintName::<i32>::VOID` failed
+struct Fail<T>(T);
+impl<T> Fail<T> {
+    const C: () = panic!(); //~ERROR evaluation of `Fail::<i32>::C` failed
 }
 
-fn no_codegen<T>() {
+#[inline(never)]
+fn called<T>() {
     // Any function that is called is guaranteed to have all consts that syntactically
     // appear in its body evaluated, even if they only appear in dead code.
     if false {
-        let _ = PrintName::<T>::VOID;
+        let _ = Fail::<T>::C;
     }
 }
+
 pub fn main() {
-    no_codegen::<i32>();
+    called::<i32>();
 }
diff --git a/tests/ui/consts/monomorphization/dead-code-in-const-called-fn.no-opt.stderr b/tests/ui/consts/monomorphization/dead-code-in-const-called-fn.no-opt.stderr
new file mode 100644
index 0000000000000..6448aef3ef501
--- /dev/null
+++ b/tests/ui/consts/monomorphization/dead-code-in-const-called-fn.no-opt.stderr
@@ -0,0 +1,17 @@
+error[E0080]: evaluation of `Fail::<i32>::C` failed
+  --> $DIR/dead-code-in-const-called-fn.rs:8:19
+   |
+LL |     const C: () = panic!();
+   |                   ^^^^^^^^ the evaluated program panicked at 'explicit panic', $DIR/dead-code-in-const-called-fn.rs:8:19
+   |
+   = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+note: erroneous constant encountered
+  --> $DIR/dead-code-in-const-called-fn.rs:16:9
+   |
+LL |         Fail::<T>::C;
+   |         ^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0080`.
diff --git a/tests/ui/consts/monomorphization/dead-code-in-const-called-fn.opt.stderr b/tests/ui/consts/monomorphization/dead-code-in-const-called-fn.opt.stderr
new file mode 100644
index 0000000000000..6448aef3ef501
--- /dev/null
+++ b/tests/ui/consts/monomorphization/dead-code-in-const-called-fn.opt.stderr
@@ -0,0 +1,17 @@
+error[E0080]: evaluation of `Fail::<i32>::C` failed
+  --> $DIR/dead-code-in-const-called-fn.rs:8:19
+   |
+LL |     const C: () = panic!();
+   |                   ^^^^^^^^ the evaluated program panicked at 'explicit panic', $DIR/dead-code-in-const-called-fn.rs:8:19
+   |
+   = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+note: erroneous constant encountered
+  --> $DIR/dead-code-in-const-called-fn.rs:16:9
+   |
+LL |         Fail::<T>::C;
+   |         ^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0080`.
diff --git a/tests/ui/consts/const-eval/erroneous-const.rs b/tests/ui/consts/monomorphization/dead-code-in-const-called-fn.rs
similarity index 62%
rename from tests/ui/consts/const-eval/erroneous-const.rs
rename to tests/ui/consts/monomorphization/dead-code-in-const-called-fn.rs
index 74d44c5259a3e..9a5737831cdf0 100644
--- a/tests/ui/consts/const-eval/erroneous-const.rs
+++ b/tests/ui/consts/monomorphization/dead-code-in-const-called-fn.rs
@@ -1,16 +1,19 @@
+//@revisions: opt no-opt
+//@[opt] compile-flags: -O
 //! Make sure we error on erroneous consts even if they are unused.
 #![allow(unconditional_panic)]
 
-struct PrintName<T>(T);
-impl<T> PrintName<T> {
-    const VOID: () = [()][2]; //~ERROR evaluation of `PrintName::<i32>::VOID` failed
+struct Fail<T>(T);
+impl<T> Fail<T> {
+    const C: () = panic!(); //~ERROR evaluation of `Fail::<i32>::C` failed
 }
 
+#[inline(never)]
 const fn no_codegen<T>() {
     if false {
         // This bad constant is only used in dead code in a no-codegen function... and yet we still
         // must make sure that the build fails.
-            PrintName::<T>::VOID; //~ constant
+        Fail::<T>::C; //~ constant
     }
 }
 
diff --git a/tests/ui/consts/monomorphization/dead-code-in-dead-fn.no-opt.stderr b/tests/ui/consts/monomorphization/dead-code-in-dead-fn.no-opt.stderr
new file mode 100644
index 0000000000000..884fcb4e415ca
--- /dev/null
+++ b/tests/ui/consts/monomorphization/dead-code-in-dead-fn.no-opt.stderr
@@ -0,0 +1,17 @@
+error[E0080]: evaluation of `Fail::<i32>::C` failed
+  --> $DIR/dead-code-in-dead-fn.rs:9:19
+   |
+LL |     const C: () = panic!();
+   |                   ^^^^^^^^ the evaluated program panicked at 'explicit panic', $DIR/dead-code-in-dead-fn.rs:9:19
+   |
+   = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+note: the above error was encountered while instantiating `fn not_called::<i32>`
+  --> $DIR/dead-code-in-dead-fn.rs:22:9
+   |
+LL |         not_called::<T>();
+   |         ^^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0080`.
diff --git a/tests/ui/consts/const-eval/unused-broken-const-late.stderr b/tests/ui/consts/monomorphization/dead-code-in-dead-fn.opt.stderr
similarity index 53%
rename from tests/ui/consts/const-eval/unused-broken-const-late.stderr
rename to tests/ui/consts/monomorphization/dead-code-in-dead-fn.opt.stderr
index c2cf2f3813c5e..302bcae18331c 100644
--- a/tests/ui/consts/const-eval/unused-broken-const-late.stderr
+++ b/tests/ui/consts/monomorphization/dead-code-in-dead-fn.opt.stderr
@@ -1,8 +1,8 @@
-error[E0080]: evaluation of `PrintName::<i32>::VOID` failed
-  --> $DIR/unused-broken-const-late.rs:8:22
+error[E0080]: evaluation of `Fail::<i32>::C` failed
+  --> $DIR/dead-code-in-dead-fn.rs:9:19
    |
-LL |     const VOID: () = panic!();
-   |                      ^^^^^^^^ the evaluated program panicked at 'explicit panic', $DIR/unused-broken-const-late.rs:8:22
+LL |     const C: () = panic!();
+   |                   ^^^^^^^^ the evaluated program panicked at 'explicit panic', $DIR/dead-code-in-dead-fn.rs:9:19
    |
    = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
 
diff --git a/tests/ui/consts/monomorphization/dead-code-in-dead-fn.rs b/tests/ui/consts/monomorphization/dead-code-in-dead-fn.rs
new file mode 100644
index 0000000000000..9fa335d4038f1
--- /dev/null
+++ b/tests/ui/consts/monomorphization/dead-code-in-dead-fn.rs
@@ -0,0 +1,28 @@
+//@revisions: opt no-opt
+//@ build-fail
+//@[opt] compile-flags: -O
+//! Make sure we detect erroneous constants post-monomorphization even when they are unused. This is
+//! crucial, people rely on it for soundness. (https://github.com/rust-lang/rust/issues/112090)
+
+struct Fail<T>(T);
+impl<T> Fail<T> {
+    const C: () = panic!(); //~ERROR evaluation of `Fail::<i32>::C` failed
+}
+
+// This function is not actually called, but it is mentioned in a function that is called.
+// Make sure we still find this error.
+#[inline(never)]
+fn not_called<T>() {
+    let _ = Fail::<T>::C;
+}
+
+#[inline(never)]
+fn called<T>() {
+    if false {
+        not_called::<T>();
+    }
+}
+
+pub fn main() {
+    called::<i32>();
+}
diff --git a/tests/ui/consts/monomorphization/dead-code-in-static.no-opt.stderr b/tests/ui/consts/monomorphization/dead-code-in-static.no-opt.stderr
new file mode 100644
index 0000000000000..abd1456d5549f
--- /dev/null
+++ b/tests/ui/consts/monomorphization/dead-code-in-static.no-opt.stderr
@@ -0,0 +1,17 @@
+error[E0080]: evaluation of `Fail::<i32>::C` failed
+  --> $DIR/dead-code-in-static.rs:8:19
+   |
+LL |     const C: () = panic!();
+   |                   ^^^^^^^^ the evaluated program panicked at 'explicit panic', $DIR/dead-code-in-static.rs:8:19
+   |
+   = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+note: erroneous constant encountered
+  --> $DIR/dead-code-in-static.rs:15:9
+   |
+LL |         Fail::<i32>::C;
+   |         ^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0080`.
diff --git a/tests/ui/consts/monomorphization/dead-code-in-static.opt.stderr b/tests/ui/consts/monomorphization/dead-code-in-static.opt.stderr
new file mode 100644
index 0000000000000..abd1456d5549f
--- /dev/null
+++ b/tests/ui/consts/monomorphization/dead-code-in-static.opt.stderr
@@ -0,0 +1,17 @@
+error[E0080]: evaluation of `Fail::<i32>::C` failed
+  --> $DIR/dead-code-in-static.rs:8:19
+   |
+LL |     const C: () = panic!();
+   |                   ^^^^^^^^ the evaluated program panicked at 'explicit panic', $DIR/dead-code-in-static.rs:8:19
+   |
+   = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+note: erroneous constant encountered
+  --> $DIR/dead-code-in-static.rs:15:9
+   |
+LL |         Fail::<i32>::C;
+   |         ^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0080`.
diff --git a/tests/ui/consts/const-eval/erroneous-const2.rs b/tests/ui/consts/monomorphization/dead-code-in-static.rs
similarity index 61%
rename from tests/ui/consts/const-eval/erroneous-const2.rs
rename to tests/ui/consts/monomorphization/dead-code-in-static.rs
index 61f2955f2d822..e8c69e7d74ea4 100644
--- a/tests/ui/consts/const-eval/erroneous-const2.rs
+++ b/tests/ui/consts/monomorphization/dead-code-in-static.rs
@@ -1,16 +1,18 @@
+//@revisions: opt no-opt
+//@[opt] compile-flags: -O
 //! Make sure we error on erroneous consts even if they are unused.
 #![allow(unconditional_panic)]
 
-struct PrintName<T>(T);
-impl<T> PrintName<T> {
-    const VOID: () = [()][2]; //~ERROR evaluation of `PrintName::<i32>::VOID` failed
+struct Fail<T>(T);
+impl<T> Fail<T> {
+    const C: () = panic!(); //~ERROR evaluation of `Fail::<i32>::C` failed
 }
 
 pub static FOO: () = {
     if false {
         // This bad constant is only used in dead code in a static initializer... and yet we still
         // must make sure that the build fails.
-        PrintName::<i32>::VOID; //~ constant
+        Fail::<i32>::C; //~ constant
     }
 };
 

From 39a1d18ced86ef791391c048c7fe2e77201437f8 Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Sun, 10 Mar 2024 11:56:00 +0100
Subject: [PATCH 04/11] monormophize required_itms

---
 compiler/rustc_middle/src/mir/mod.rs          |  2 +-
 compiler/rustc_monomorphize/src/collector.rs  | 32 ++++++++++++++++++-
 .../dead-code-in-called-fn.no-opt.stderr      |  4 ---
 .../dead-code-in-called-fn.opt.stderr         |  4 ---
 .../dead-code-in-dead-fn.no-opt.stderr        |  4 ---
 .../dead-code-in-dead-fn.opt.stderr           |  2 ++
 ...st_monomorphization_error_backtrace.stderr |  8 -----
 7 files changed, 34 insertions(+), 22 deletions(-)

diff --git a/compiler/rustc_middle/src/mir/mod.rs b/compiler/rustc_middle/src/mir/mod.rs
index 9c141e3f7033a..a234a9d400226 100644
--- a/compiler/rustc_middle/src/mir/mod.rs
+++ b/compiler/rustc_middle/src/mir/mod.rs
@@ -311,7 +311,7 @@ impl<'tcx> CoroutineInfo<'tcx> {
 }
 
 /// Some item that needs to monomorphize successfully for a MIR body to be considered well-formed.
-#[derive(Clone, TyEncodable, TyDecodable, Debug, HashStable, TypeFoldable, TypeVisitable)]
+#[derive(Copy, Clone, TyEncodable, TyDecodable, Debug, HashStable, TypeFoldable, TypeVisitable)]
 pub enum RequiredItem<'tcx> {
     Fn(DefId, GenericArgsRef<'tcx>),
     Drop(Ty<'tcx>),
diff --git a/compiler/rustc_monomorphize/src/collector.rs b/compiler/rustc_monomorphize/src/collector.rs
index 32f823a5ac68f..84d170bb4c8b8 100644
--- a/compiler/rustc_monomorphize/src/collector.rs
+++ b/compiler/rustc_monomorphize/src/collector.rs
@@ -345,7 +345,7 @@ fn collect_items_rec<'tcx>(
         return;
     }
 
-    let mut used_items = Vec::new();
+    let mut used_items = MonoItems::new();
     let recursion_depth_reset;
 
     // Post-monomorphization errors MVP
@@ -734,6 +734,17 @@ impl<'a, 'tcx> MirUsedCollector<'a, 'tcx> {
 }
 
 impl<'a, 'tcx> MirVisitor<'tcx> for MirUsedCollector<'a, 'tcx> {
+    fn visit_body(&mut self, body: &mir::Body<'tcx>) {
+        for item in &body.required_items {
+            // All these also need monomorphization to ensure that if that leads to error, we find
+            // those errors.
+            let item = self.monomorphize(*item);
+            visit_required_item(self.tcx, item, self.output);
+        }
+
+        self.super_body(body);
+    }
+
     fn visit_rvalue(&mut self, rvalue: &mir::Rvalue<'tcx>, location: Location) {
         debug!("visiting rvalue {:?}", *rvalue);
 
@@ -915,6 +926,25 @@ impl<'a, 'tcx> MirVisitor<'tcx> for MirUsedCollector<'a, 'tcx> {
     }
 }
 
+fn visit_required_item<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    item: mir::RequiredItem<'tcx>,
+    output: &mut MonoItems<'tcx>,
+) {
+    let instance = match item {
+        mir::RequiredItem::Fn(def_id, args) => {
+            Instance::expect_resolve(tcx, ty::ParamEnv::reveal_all(), def_id, args)
+        }
+        mir::RequiredItem::Drop(ty) => Instance::resolve_drop_in_place(tcx, ty),
+    };
+    // We pretend this is a direct call, just to make sure this is visited at all.
+    // "indirect" would mean we also generate some shims, but we don't care about the
+    // generated code, just about the side-effect of code generation causing errors, so we
+    // can skip the shims.
+    // FIXME: track the span so that we can show it here.
+    visit_instance_use(tcx, instance, /*is_direct_call*/ true, DUMMY_SP, output);
+}
+
 fn visit_drop_use<'tcx>(
     tcx: TyCtxt<'tcx>,
     ty: Ty<'tcx>,
diff --git a/tests/ui/consts/monomorphization/dead-code-in-called-fn.no-opt.stderr b/tests/ui/consts/monomorphization/dead-code-in-called-fn.no-opt.stderr
index b69d10d322e86..6dbe2c8eeee35 100644
--- a/tests/ui/consts/monomorphization/dead-code-in-called-fn.no-opt.stderr
+++ b/tests/ui/consts/monomorphization/dead-code-in-called-fn.no-opt.stderr
@@ -7,10 +7,6 @@ LL |     const C: () = panic!();
    = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 note: the above error was encountered while instantiating `fn called::<i32>`
-  --> $DIR/dead-code-in-called-fn.rs:22:5
-   |
-LL |     called::<i32>();
-   |     ^^^^^^^^^^^^^^^
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/consts/monomorphization/dead-code-in-called-fn.opt.stderr b/tests/ui/consts/monomorphization/dead-code-in-called-fn.opt.stderr
index b69d10d322e86..6dbe2c8eeee35 100644
--- a/tests/ui/consts/monomorphization/dead-code-in-called-fn.opt.stderr
+++ b/tests/ui/consts/monomorphization/dead-code-in-called-fn.opt.stderr
@@ -7,10 +7,6 @@ LL |     const C: () = panic!();
    = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 note: the above error was encountered while instantiating `fn called::<i32>`
-  --> $DIR/dead-code-in-called-fn.rs:22:5
-   |
-LL |     called::<i32>();
-   |     ^^^^^^^^^^^^^^^
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/consts/monomorphization/dead-code-in-dead-fn.no-opt.stderr b/tests/ui/consts/monomorphization/dead-code-in-dead-fn.no-opt.stderr
index 884fcb4e415ca..4c4c5f85f697e 100644
--- a/tests/ui/consts/monomorphization/dead-code-in-dead-fn.no-opt.stderr
+++ b/tests/ui/consts/monomorphization/dead-code-in-dead-fn.no-opt.stderr
@@ -7,10 +7,6 @@ LL |     const C: () = panic!();
    = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 note: the above error was encountered while instantiating `fn not_called::<i32>`
-  --> $DIR/dead-code-in-dead-fn.rs:22:9
-   |
-LL |         not_called::<T>();
-   |         ^^^^^^^^^^^^^^^^^
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/consts/monomorphization/dead-code-in-dead-fn.opt.stderr b/tests/ui/consts/monomorphization/dead-code-in-dead-fn.opt.stderr
index 302bcae18331c..4c4c5f85f697e 100644
--- a/tests/ui/consts/monomorphization/dead-code-in-dead-fn.opt.stderr
+++ b/tests/ui/consts/monomorphization/dead-code-in-dead-fn.opt.stderr
@@ -6,6 +6,8 @@ LL |     const C: () = panic!();
    |
    = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
 
+note: the above error was encountered while instantiating `fn not_called::<i32>`
+
 error: aborting due to 1 previous error
 
 For more information about this error, try `rustc --explain E0080`.
diff --git a/tests/ui/generics/post_monomorphization_error_backtrace.stderr b/tests/ui/generics/post_monomorphization_error_backtrace.stderr
index 0d707d83d2660..ed44c07630d14 100644
--- a/tests/ui/generics/post_monomorphization_error_backtrace.stderr
+++ b/tests/ui/generics/post_monomorphization_error_backtrace.stderr
@@ -7,10 +7,6 @@ LL |         const V: () = assert!(std::mem::size_of::<T>() == 0);
    = note: this error originates in the macro `assert` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 note: the above error was encountered while instantiating `fn assert_zst::<u32>`
-  --> $DIR/post_monomorphization_error_backtrace.rs:18:5
-   |
-LL |     assert_zst::<U>()
-   |     ^^^^^^^^^^^^^^^^^
 
 error[E0080]: evaluation of `assert_zst::F::<i32>::V` failed
   --> $DIR/post_monomorphization_error_backtrace.rs:6:23
@@ -21,10 +17,6 @@ LL |         const V: () = assert!(std::mem::size_of::<T>() == 0);
    = note: this error originates in the macro `assert` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 note: the above error was encountered while instantiating `fn assert_zst::<i32>`
-  --> $DIR/post_monomorphization_error_backtrace.rs:18:5
-   |
-LL |     assert_zst::<U>()
-   |     ^^^^^^^^^^^^^^^^^
 
 error: aborting due to 2 previous errors
 

From e7f4399c86a5dd5fe0c8b535d94b19ada706bc46 Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Sun, 10 Mar 2024 12:02:50 +0100
Subject: [PATCH 05/11] more extensive dead cases for more kinds of dead code

---
 .../dead-code-in-dead-drop.no-opt.stderr      | 14 ++++++++
 .../dead-code-in-dead-drop.opt.stderr         | 14 ++++++++
 .../dead-code-in-dead-drop.rs                 | 30 ++++++++++++++++
 .../monomorphization/dead-code-in-dead-fn.rs  |  4 +--
 .../dead-code-in-dead-vtable.no-opt.stderr    | 17 ++++++++++
 .../dead-code-in-dead-vtable.rs               | 34 +++++++++++++++++++
 6 files changed, 111 insertions(+), 2 deletions(-)
 create mode 100644 tests/ui/consts/monomorphization/dead-code-in-dead-drop.no-opt.stderr
 create mode 100644 tests/ui/consts/monomorphization/dead-code-in-dead-drop.opt.stderr
 create mode 100644 tests/ui/consts/monomorphization/dead-code-in-dead-drop.rs
 create mode 100644 tests/ui/consts/monomorphization/dead-code-in-dead-vtable.no-opt.stderr
 create mode 100644 tests/ui/consts/monomorphization/dead-code-in-dead-vtable.rs

diff --git a/tests/ui/consts/monomorphization/dead-code-in-dead-drop.no-opt.stderr b/tests/ui/consts/monomorphization/dead-code-in-dead-drop.no-opt.stderr
new file mode 100644
index 0000000000000..e3ef78842f77c
--- /dev/null
+++ b/tests/ui/consts/monomorphization/dead-code-in-dead-drop.no-opt.stderr
@@ -0,0 +1,14 @@
+error[E0080]: evaluation of `Fail::<i32>::C` failed
+  --> $DIR/dead-code-in-dead-drop.rs:9:19
+   |
+LL |     const C: () = panic!();
+   |                   ^^^^^^^^ the evaluated program panicked at 'explicit panic', $DIR/dead-code-in-dead-drop.rs:9:19
+   |
+   = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+note: the above error was encountered while instantiating `fn <Fail<i32> as std::ops::Drop>::drop`
+  --> $SRC_DIR/core/src/ptr/mod.rs:LL:COL
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0080`.
diff --git a/tests/ui/consts/monomorphization/dead-code-in-dead-drop.opt.stderr b/tests/ui/consts/monomorphization/dead-code-in-dead-drop.opt.stderr
new file mode 100644
index 0000000000000..e3ef78842f77c
--- /dev/null
+++ b/tests/ui/consts/monomorphization/dead-code-in-dead-drop.opt.stderr
@@ -0,0 +1,14 @@
+error[E0080]: evaluation of `Fail::<i32>::C` failed
+  --> $DIR/dead-code-in-dead-drop.rs:9:19
+   |
+LL |     const C: () = panic!();
+   |                   ^^^^^^^^ the evaluated program panicked at 'explicit panic', $DIR/dead-code-in-dead-drop.rs:9:19
+   |
+   = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+note: the above error was encountered while instantiating `fn <Fail<i32> as std::ops::Drop>::drop`
+  --> $SRC_DIR/core/src/ptr/mod.rs:LL:COL
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0080`.
diff --git a/tests/ui/consts/monomorphization/dead-code-in-dead-drop.rs b/tests/ui/consts/monomorphization/dead-code-in-dead-drop.rs
new file mode 100644
index 0000000000000..fc1052858b64c
--- /dev/null
+++ b/tests/ui/consts/monomorphization/dead-code-in-dead-drop.rs
@@ -0,0 +1,30 @@
+//@revisions: opt no-opt
+//@ build-fail
+//@[opt] compile-flags: -O
+//! Make sure we detect erroneous constants post-monomorphization even when they are unused. This is
+//! crucial, people rely on it for soundness. (https://github.com/rust-lang/rust/issues/112090)
+
+struct Fail<T>(T);
+impl<T> Fail<T> {
+    const C: () = panic!(); //~ERROR evaluation of `Fail::<i32>::C` failed
+}
+
+// This function is not actually called, but is mentioned implicitly as destructor in dead code in a
+// function that is called. Make sure we still find this error.
+impl<T> Drop for Fail<T> {
+    fn drop(&mut self) {
+        let _ = Fail::<T>::C;
+    }
+}
+
+#[inline(never)]
+fn called<T>(x: T) {
+    if false {
+        let v = Fail(x);
+        drop(v);
+    }
+}
+
+pub fn main() {
+    called::<i32>(0);
+}
diff --git a/tests/ui/consts/monomorphization/dead-code-in-dead-fn.rs b/tests/ui/consts/monomorphization/dead-code-in-dead-fn.rs
index 9fa335d4038f1..79cb47f4b75ee 100644
--- a/tests/ui/consts/monomorphization/dead-code-in-dead-fn.rs
+++ b/tests/ui/consts/monomorphization/dead-code-in-dead-fn.rs
@@ -9,8 +9,8 @@ impl<T> Fail<T> {
     const C: () = panic!(); //~ERROR evaluation of `Fail::<i32>::C` failed
 }
 
-// This function is not actually called, but it is mentioned in a function that is called.
-// Make sure we still find this error.
+// This function is not actually called, but it is mentioned in dead code in a function that is
+// called. Make sure we still find this error.
 #[inline(never)]
 fn not_called<T>() {
     let _ = Fail::<T>::C;
diff --git a/tests/ui/consts/monomorphization/dead-code-in-dead-vtable.no-opt.stderr b/tests/ui/consts/monomorphization/dead-code-in-dead-vtable.no-opt.stderr
new file mode 100644
index 0000000000000..eb8dd41cd567b
--- /dev/null
+++ b/tests/ui/consts/monomorphization/dead-code-in-dead-vtable.no-opt.stderr
@@ -0,0 +1,17 @@
+error[E0080]: evaluation of `Fail::<i32>::C` failed
+  --> $DIR/dead-code-in-dead-vtable.rs:9:19
+   |
+LL |     const C: () = panic!();
+   |                   ^^^^^^^^ the evaluated program panicked at 'explicit panic', $DIR/dead-code-in-dead-vtable.rs:9:19
+   |
+   = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+note: the above error was encountered while instantiating `fn <std::vec::Vec<i32> as MyTrait>::not_called`
+  --> $DIR/dead-code-in-dead-vtable.rs:28:40
+   |
+LL |         let gen_vtable: &dyn MyTrait = &v;
+   |                                        ^^
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0080`.
diff --git a/tests/ui/consts/monomorphization/dead-code-in-dead-vtable.rs b/tests/ui/consts/monomorphization/dead-code-in-dead-vtable.rs
new file mode 100644
index 0000000000000..70a9444125cb5
--- /dev/null
+++ b/tests/ui/consts/monomorphization/dead-code-in-dead-vtable.rs
@@ -0,0 +1,34 @@
+//@revisions: opt no-opt
+//@ build-fail
+//@[opt] compile-flags: -O
+//! Make sure we detect erroneous constants post-monomorphization even when they are unused. This is
+//! crucial, people rely on it for soundness. (https://github.com/rust-lang/rust/issues/112090)
+
+struct Fail<T>(T);
+impl<T> Fail<T> {
+    const C: () = panic!(); //~ERROR evaluation of `Fail::<i32>::C` failed
+}
+
+trait MyTrait {
+    fn not_called(&self);
+}
+
+// This function is not actually called, but it is mentioned in a vtable in a function that is
+// called. Make sure we still find this error.
+impl<T> MyTrait for Vec<T> {
+    fn not_called(&self) {
+        let _ = Fail::<T>::C;
+    }
+}
+
+#[inline(never)]
+fn called<T>() {
+    if false {
+        let v: Vec<T> = Vec::new();
+        let gen_vtable: &dyn MyTrait = &v;
+    }
+}
+
+pub fn main() {
+    called::<i32>();
+}

From 70612b7b4373a67121be0c39a65490577a662790 Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Tue, 12 Mar 2024 14:07:17 +0100
Subject: [PATCH 06/11] reorganize tests a bit

---
 .../dead-code-in-called-fn.no-opt.stderr                  | 0
 .../dead-code-in-called-fn.opt.stderr                     | 0
 .../dead-code-in-called-fn.rs                             | 1 +
 .../dead-code-in-const-called-fn.no-opt.stderr            | 2 +-
 .../dead-code-in-const-called-fn.opt.stderr               | 2 +-
 .../dead-code-in-const-called-fn.rs                       | 1 +
 .../dead-code-in-dead-drop.no-opt.stderr                  | 0
 .../dead-code-in-dead-drop.opt.stderr                     | 0
 .../dead-code-in-dead-drop.rs                             | 0
 .../dead-code-in-dead-fn.no-opt.stderr                    | 0
 .../dead-code-in-dead-fn.opt.stderr                       | 0
 .../dead-code-in-dead-fn.rs                               | 6 +++++-
 .../dead-code-in-dead-vtable.no-opt.stderr                | 4 ++--
 .../dead-code-in-dead-vtable.rs                           | 8 ++++++--
 .../dead-code-in-static.no-opt.stderr                     | 2 +-
 .../dead-code-in-static.opt.stderr                        | 2 +-
 .../dead-code-in-static.rs                                | 1 +
 17 files changed, 20 insertions(+), 9 deletions(-)
 rename tests/ui/consts/{monomorphization => required-consts}/dead-code-in-called-fn.no-opt.stderr (100%)
 rename tests/ui/consts/{monomorphization => required-consts}/dead-code-in-called-fn.opt.stderr (100%)
 rename tests/ui/consts/{monomorphization => required-consts}/dead-code-in-called-fn.rs (87%)
 rename tests/ui/consts/{monomorphization => required-consts}/dead-code-in-const-called-fn.no-opt.stderr (93%)
 rename tests/ui/consts/{monomorphization => required-consts}/dead-code-in-const-called-fn.opt.stderr (93%)
 rename tests/ui/consts/{monomorphization => required-consts}/dead-code-in-const-called-fn.rs (87%)
 rename tests/ui/consts/{monomorphization => required-consts}/dead-code-in-dead-drop.no-opt.stderr (100%)
 rename tests/ui/consts/{monomorphization => required-consts}/dead-code-in-dead-drop.opt.stderr (100%)
 rename tests/ui/consts/{monomorphization => required-consts}/dead-code-in-dead-drop.rs (100%)
 rename tests/ui/consts/{monomorphization => required-consts}/dead-code-in-dead-fn.no-opt.stderr (100%)
 rename tests/ui/consts/{monomorphization => required-consts}/dead-code-in-dead-fn.opt.stderr (100%)
 rename tests/ui/consts/{monomorphization => required-consts}/dead-code-in-dead-fn.rs (76%)
 rename tests/ui/consts/{monomorphization => required-consts}/dead-code-in-dead-vtable.no-opt.stderr (85%)
 rename tests/ui/consts/{monomorphization => required-consts}/dead-code-in-dead-vtable.rs (70%)
 rename tests/ui/consts/{monomorphization => required-consts}/dead-code-in-static.no-opt.stderr (94%)
 rename tests/ui/consts/{monomorphization => required-consts}/dead-code-in-static.opt.stderr (94%)
 rename tests/ui/consts/{monomorphization => required-consts}/dead-code-in-static.rs (84%)

diff --git a/tests/ui/consts/monomorphization/dead-code-in-called-fn.no-opt.stderr b/tests/ui/consts/required-consts/dead-code-in-called-fn.no-opt.stderr
similarity index 100%
rename from tests/ui/consts/monomorphization/dead-code-in-called-fn.no-opt.stderr
rename to tests/ui/consts/required-consts/dead-code-in-called-fn.no-opt.stderr
diff --git a/tests/ui/consts/monomorphization/dead-code-in-called-fn.opt.stderr b/tests/ui/consts/required-consts/dead-code-in-called-fn.opt.stderr
similarity index 100%
rename from tests/ui/consts/monomorphization/dead-code-in-called-fn.opt.stderr
rename to tests/ui/consts/required-consts/dead-code-in-called-fn.opt.stderr
diff --git a/tests/ui/consts/monomorphization/dead-code-in-called-fn.rs b/tests/ui/consts/required-consts/dead-code-in-called-fn.rs
similarity index 87%
rename from tests/ui/consts/monomorphization/dead-code-in-called-fn.rs
rename to tests/ui/consts/required-consts/dead-code-in-called-fn.rs
index cc7bafa7adb85..2dd58b9e71d75 100644
--- a/tests/ui/consts/monomorphization/dead-code-in-called-fn.rs
+++ b/tests/ui/consts/required-consts/dead-code-in-called-fn.rs
@@ -13,6 +13,7 @@ impl<T> Fail<T> {
 fn called<T>() {
     // Any function that is called is guaranteed to have all consts that syntactically
     // appear in its body evaluated, even if they only appear in dead code.
+    // This relies on mono-item collection checking `required_consts` in collected functions.
     if false {
         let _ = Fail::<T>::C;
     }
diff --git a/tests/ui/consts/monomorphization/dead-code-in-const-called-fn.no-opt.stderr b/tests/ui/consts/required-consts/dead-code-in-const-called-fn.no-opt.stderr
similarity index 93%
rename from tests/ui/consts/monomorphization/dead-code-in-const-called-fn.no-opt.stderr
rename to tests/ui/consts/required-consts/dead-code-in-const-called-fn.no-opt.stderr
index 6448aef3ef501..feacf51cfca0e 100644
--- a/tests/ui/consts/monomorphization/dead-code-in-const-called-fn.no-opt.stderr
+++ b/tests/ui/consts/required-consts/dead-code-in-const-called-fn.no-opt.stderr
@@ -7,7 +7,7 @@ LL |     const C: () = panic!();
    = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 note: erroneous constant encountered
-  --> $DIR/dead-code-in-const-called-fn.rs:16:9
+  --> $DIR/dead-code-in-const-called-fn.rs:17:9
    |
 LL |         Fail::<T>::C;
    |         ^^^^^^^^^^^^
diff --git a/tests/ui/consts/monomorphization/dead-code-in-const-called-fn.opt.stderr b/tests/ui/consts/required-consts/dead-code-in-const-called-fn.opt.stderr
similarity index 93%
rename from tests/ui/consts/monomorphization/dead-code-in-const-called-fn.opt.stderr
rename to tests/ui/consts/required-consts/dead-code-in-const-called-fn.opt.stderr
index 6448aef3ef501..feacf51cfca0e 100644
--- a/tests/ui/consts/monomorphization/dead-code-in-const-called-fn.opt.stderr
+++ b/tests/ui/consts/required-consts/dead-code-in-const-called-fn.opt.stderr
@@ -7,7 +7,7 @@ LL |     const C: () = panic!();
    = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 note: erroneous constant encountered
-  --> $DIR/dead-code-in-const-called-fn.rs:16:9
+  --> $DIR/dead-code-in-const-called-fn.rs:17:9
    |
 LL |         Fail::<T>::C;
    |         ^^^^^^^^^^^^
diff --git a/tests/ui/consts/monomorphization/dead-code-in-const-called-fn.rs b/tests/ui/consts/required-consts/dead-code-in-const-called-fn.rs
similarity index 87%
rename from tests/ui/consts/monomorphization/dead-code-in-const-called-fn.rs
rename to tests/ui/consts/required-consts/dead-code-in-const-called-fn.rs
index 9a5737831cdf0..03af0da78a3e3 100644
--- a/tests/ui/consts/monomorphization/dead-code-in-const-called-fn.rs
+++ b/tests/ui/consts/required-consts/dead-code-in-const-called-fn.rs
@@ -13,6 +13,7 @@ const fn no_codegen<T>() {
     if false {
         // This bad constant is only used in dead code in a no-codegen function... and yet we still
         // must make sure that the build fails.
+        // This relies on const-eval evaluating all `required_consts` of `const fn`.
         Fail::<T>::C; //~ constant
     }
 }
diff --git a/tests/ui/consts/monomorphization/dead-code-in-dead-drop.no-opt.stderr b/tests/ui/consts/required-consts/dead-code-in-dead-drop.no-opt.stderr
similarity index 100%
rename from tests/ui/consts/monomorphization/dead-code-in-dead-drop.no-opt.stderr
rename to tests/ui/consts/required-consts/dead-code-in-dead-drop.no-opt.stderr
diff --git a/tests/ui/consts/monomorphization/dead-code-in-dead-drop.opt.stderr b/tests/ui/consts/required-consts/dead-code-in-dead-drop.opt.stderr
similarity index 100%
rename from tests/ui/consts/monomorphization/dead-code-in-dead-drop.opt.stderr
rename to tests/ui/consts/required-consts/dead-code-in-dead-drop.opt.stderr
diff --git a/tests/ui/consts/monomorphization/dead-code-in-dead-drop.rs b/tests/ui/consts/required-consts/dead-code-in-dead-drop.rs
similarity index 100%
rename from tests/ui/consts/monomorphization/dead-code-in-dead-drop.rs
rename to tests/ui/consts/required-consts/dead-code-in-dead-drop.rs
diff --git a/tests/ui/consts/monomorphization/dead-code-in-dead-fn.no-opt.stderr b/tests/ui/consts/required-consts/dead-code-in-dead-fn.no-opt.stderr
similarity index 100%
rename from tests/ui/consts/monomorphization/dead-code-in-dead-fn.no-opt.stderr
rename to tests/ui/consts/required-consts/dead-code-in-dead-fn.no-opt.stderr
diff --git a/tests/ui/consts/monomorphization/dead-code-in-dead-fn.opt.stderr b/tests/ui/consts/required-consts/dead-code-in-dead-fn.opt.stderr
similarity index 100%
rename from tests/ui/consts/monomorphization/dead-code-in-dead-fn.opt.stderr
rename to tests/ui/consts/required-consts/dead-code-in-dead-fn.opt.stderr
diff --git a/tests/ui/consts/monomorphization/dead-code-in-dead-fn.rs b/tests/ui/consts/required-consts/dead-code-in-dead-fn.rs
similarity index 76%
rename from tests/ui/consts/monomorphization/dead-code-in-dead-fn.rs
rename to tests/ui/consts/required-consts/dead-code-in-dead-fn.rs
index 79cb47f4b75ee..826149ca2a81f 100644
--- a/tests/ui/consts/monomorphization/dead-code-in-dead-fn.rs
+++ b/tests/ui/consts/required-consts/dead-code-in-dead-fn.rs
@@ -11,9 +11,13 @@ impl<T> Fail<T> {
 
 // This function is not actually called, but it is mentioned in dead code in a function that is
 // called. Make sure we still find this error.
+// This relies on mono-item collection checking `required_consts` in functions that syntactically
+// are called in collected functions (even inside dead code).
 #[inline(never)]
 fn not_called<T>() {
-    let _ = Fail::<T>::C;
+    if false {
+        let _ = Fail::<T>::C;
+    }
 }
 
 #[inline(never)]
diff --git a/tests/ui/consts/monomorphization/dead-code-in-dead-vtable.no-opt.stderr b/tests/ui/consts/required-consts/dead-code-in-dead-vtable.no-opt.stderr
similarity index 85%
rename from tests/ui/consts/monomorphization/dead-code-in-dead-vtable.no-opt.stderr
rename to tests/ui/consts/required-consts/dead-code-in-dead-vtable.no-opt.stderr
index eb8dd41cd567b..b70d675459236 100644
--- a/tests/ui/consts/monomorphization/dead-code-in-dead-vtable.no-opt.stderr
+++ b/tests/ui/consts/required-consts/dead-code-in-dead-vtable.no-opt.stderr
@@ -7,9 +7,9 @@ LL |     const C: () = panic!();
    = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 note: the above error was encountered while instantiating `fn <std::vec::Vec<i32> as MyTrait>::not_called`
-  --> $DIR/dead-code-in-dead-vtable.rs:28:40
+  --> $DIR/dead-code-in-dead-vtable.rs:30:40
    |
-LL |         let gen_vtable: &dyn MyTrait = &v;
+LL |         let gen_vtable: &dyn MyTrait = &v; // vtable "appears" here
    |                                        ^^
 
 error: aborting due to 1 previous error
diff --git a/tests/ui/consts/monomorphization/dead-code-in-dead-vtable.rs b/tests/ui/consts/required-consts/dead-code-in-dead-vtable.rs
similarity index 70%
rename from tests/ui/consts/monomorphization/dead-code-in-dead-vtable.rs
rename to tests/ui/consts/required-consts/dead-code-in-dead-vtable.rs
index 70a9444125cb5..642e12d5bfaac 100644
--- a/tests/ui/consts/monomorphization/dead-code-in-dead-vtable.rs
+++ b/tests/ui/consts/required-consts/dead-code-in-dead-vtable.rs
@@ -15,9 +15,13 @@ trait MyTrait {
 
 // This function is not actually called, but it is mentioned in a vtable in a function that is
 // called. Make sure we still find this error.
+// This relies on mono-item collection checking `required_consts` in functions that are referenced
+// in vtables that syntactically appear in collected functions (even inside dead code).
 impl<T> MyTrait for Vec<T> {
     fn not_called(&self) {
-        let _ = Fail::<T>::C;
+        if false {
+            let _ = Fail::<T>::C;
+        }
     }
 }
 
@@ -25,7 +29,7 @@ impl<T> MyTrait for Vec<T> {
 fn called<T>() {
     if false {
         let v: Vec<T> = Vec::new();
-        let gen_vtable: &dyn MyTrait = &v;
+        let gen_vtable: &dyn MyTrait = &v; // vtable "appears" here
     }
 }
 
diff --git a/tests/ui/consts/monomorphization/dead-code-in-static.no-opt.stderr b/tests/ui/consts/required-consts/dead-code-in-static.no-opt.stderr
similarity index 94%
rename from tests/ui/consts/monomorphization/dead-code-in-static.no-opt.stderr
rename to tests/ui/consts/required-consts/dead-code-in-static.no-opt.stderr
index abd1456d5549f..b64e2fc45f430 100644
--- a/tests/ui/consts/monomorphization/dead-code-in-static.no-opt.stderr
+++ b/tests/ui/consts/required-consts/dead-code-in-static.no-opt.stderr
@@ -7,7 +7,7 @@ LL |     const C: () = panic!();
    = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 note: erroneous constant encountered
-  --> $DIR/dead-code-in-static.rs:15:9
+  --> $DIR/dead-code-in-static.rs:16:9
    |
 LL |         Fail::<i32>::C;
    |         ^^^^^^^^^^^^^^
diff --git a/tests/ui/consts/monomorphization/dead-code-in-static.opt.stderr b/tests/ui/consts/required-consts/dead-code-in-static.opt.stderr
similarity index 94%
rename from tests/ui/consts/monomorphization/dead-code-in-static.opt.stderr
rename to tests/ui/consts/required-consts/dead-code-in-static.opt.stderr
index abd1456d5549f..b64e2fc45f430 100644
--- a/tests/ui/consts/monomorphization/dead-code-in-static.opt.stderr
+++ b/tests/ui/consts/required-consts/dead-code-in-static.opt.stderr
@@ -7,7 +7,7 @@ LL |     const C: () = panic!();
    = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 note: erroneous constant encountered
-  --> $DIR/dead-code-in-static.rs:15:9
+  --> $DIR/dead-code-in-static.rs:16:9
    |
 LL |         Fail::<i32>::C;
    |         ^^^^^^^^^^^^^^
diff --git a/tests/ui/consts/monomorphization/dead-code-in-static.rs b/tests/ui/consts/required-consts/dead-code-in-static.rs
similarity index 84%
rename from tests/ui/consts/monomorphization/dead-code-in-static.rs
rename to tests/ui/consts/required-consts/dead-code-in-static.rs
index e8c69e7d74ea4..b1da8153e944b 100644
--- a/tests/ui/consts/monomorphization/dead-code-in-static.rs
+++ b/tests/ui/consts/required-consts/dead-code-in-static.rs
@@ -12,6 +12,7 @@ pub static FOO: () = {
     if false {
         // This bad constant is only used in dead code in a static initializer... and yet we still
         // must make sure that the build fails.
+        // This relies on const-eval evaluating all `required_consts` of the `static` MIR body.
         Fail::<i32>::C; //~ constant
     }
 };

From 2f88ecd7b8c18e75b682b87743aa62da899bed71 Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Wed, 13 Mar 2024 17:18:34 +0100
Subject: [PATCH 07/11] inliner: absorb callees required_items

---
 compiler/rustc_middle/src/mir/mod.rs         |  3 ++-
 compiler/rustc_mir_transform/src/inline.rs   | 15 ++++++++++++++-
 compiler/rustc_monomorphize/src/collector.rs |  2 +-
 3 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/compiler/rustc_middle/src/mir/mod.rs b/compiler/rustc_middle/src/mir/mod.rs
index a234a9d400226..02333f4575344 100644
--- a/compiler/rustc_middle/src/mir/mod.rs
+++ b/compiler/rustc_middle/src/mir/mod.rs
@@ -311,7 +311,8 @@ impl<'tcx> CoroutineInfo<'tcx> {
 }
 
 /// Some item that needs to monomorphize successfully for a MIR body to be considered well-formed.
-#[derive(Copy, Clone, TyEncodable, TyDecodable, Debug, HashStable, TypeFoldable, TypeVisitable)]
+#[derive(Copy, Clone, PartialEq, Debug, HashStable, TyEncodable, TyDecodable)]
+#[derive(TypeFoldable, TypeVisitable)]
 pub enum RequiredItem<'tcx> {
     Fn(DefId, GenericArgsRef<'tcx>),
     Drop(Ty<'tcx>),
diff --git a/compiler/rustc_mir_transform/src/inline.rs b/compiler/rustc_mir_transform/src/inline.rs
index f6a0945c22297..22911d1ac82c1 100644
--- a/compiler/rustc_mir_transform/src/inline.rs
+++ b/compiler/rustc_mir_transform/src/inline.rs
@@ -565,7 +565,8 @@ impl<'tcx> Inliner<'tcx> {
         mut callee_body: Body<'tcx>,
     ) {
         let terminator = caller_body[callsite.block].terminator.take().unwrap();
-        let TerminatorKind::Call { args, destination, unwind, target, .. } = terminator.kind else {
+        let TerminatorKind::Call { func, args, destination, unwind, target, .. } = terminator.kind
+        else {
             bug!("unexpected terminator kind {:?}", terminator.kind);
         };
 
@@ -717,6 +718,18 @@ impl<'tcx> Inliner<'tcx> {
                 Const::Val(..) | Const::Unevaluated(..) => true,
             },
         ));
+        //  Now that we incorporated the callee's `required_consts`, we can remove the callee from
+        //  `required_items` -- but we have to take their `required_consts` in return.
+        let callee_item = {
+            // We need to reconstruct the `required_item` for the callee.
+            let func_ty = func.ty(caller_body, self.tcx);
+            match func_ty.kind() {
+                ty::FnDef(def_id, args) => RequiredItem::Fn(*def_id, args),
+                _ => bug!(),
+            }
+        };
+        caller_body.required_items.retain(|item| *item != callee_item);
+        caller_body.required_items.extend(callee_body.required_items);
     }
 
     fn make_call_args(
diff --git a/compiler/rustc_monomorphize/src/collector.rs b/compiler/rustc_monomorphize/src/collector.rs
index 84d170bb4c8b8..0e838f132a2dc 100644
--- a/compiler/rustc_monomorphize/src/collector.rs
+++ b/compiler/rustc_monomorphize/src/collector.rs
@@ -983,7 +983,7 @@ fn visit_instance_use<'tcx>(
     source: Span,
     output: &mut MonoItems<'tcx>,
 ) {
-    debug!("visit_item_use({:?}, is_direct_call={:?})", instance, is_direct_call);
+    debug!("visit_instance_use({:?}, is_direct_call={:?})", instance, is_direct_call);
     if !should_codegen_locally(tcx, &instance) {
         return;
     }

From f2b88663af5fe2923ebf3da0a22143ecefe23084 Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Wed, 13 Mar 2024 17:27:38 +0100
Subject: [PATCH 08/11] required_items: skip Drop for now

---
 compiler/rustc_mir_transform/src/required_consts.rs | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/compiler/rustc_mir_transform/src/required_consts.rs b/compiler/rustc_mir_transform/src/required_consts.rs
index 52dc9353fdc8b..a888956b39a90 100644
--- a/compiler/rustc_mir_transform/src/required_consts.rs
+++ b/compiler/rustc_mir_transform/src/required_consts.rs
@@ -2,6 +2,7 @@ use rustc_middle::mir::visit::Visitor;
 use rustc_middle::mir::{self, Const, ConstOperand, Location, RequiredItem};
 use rustc_middle::ty::{self, ConstKind, TyCtxt};
 
+#[allow(unused)]
 pub struct RequiredConstsVisitor<'a, 'tcx> {
     tcx: TyCtxt<'tcx>,
     body: &'a mir::Body<'tcx>,
@@ -39,7 +40,7 @@ impl<'tcx> Visitor<'tcx> for RequiredConstsVisitor<'_, 'tcx> {
         }
     }
 
-    fn visit_terminator(&mut self, terminator: &mir::Terminator<'tcx>, location: Location) {
+    /*fn visit_terminator(&mut self, terminator: &mir::Terminator<'tcx>, location: Location) {
         self.super_terminator(terminator, location);
 
         match terminator.kind {
@@ -51,5 +52,5 @@ impl<'tcx> Visitor<'tcx> for RequiredConstsVisitor<'_, 'tcx> {
             }
             _ => {}
         }
-    }
+    }*/
 }

From 003214eff3766e92e30f8f788b4e28f029aa2286 Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Wed, 13 Mar 2024 22:38:43 +0100
Subject: [PATCH 09/11] adjust tests to reality

---
 ....stderr => dead-code-in-dead-drop.noopt.stderr} |  4 ++--
 .../dead-code-in-dead-drop.opt.stderr              | 14 --------------
 .../required-consts/dead-code-in-dead-drop.rs      |  9 ++++++---
 ...tderr => dead-code-in-dead-vtable.noopt.stderr} |  6 +++---
 .../required-consts/dead-code-in-dead-vtable.rs    |  9 ++++++---
 5 files changed, 17 insertions(+), 25 deletions(-)
 rename tests/ui/consts/required-consts/{dead-code-in-dead-drop.no-opt.stderr => dead-code-in-dead-drop.noopt.stderr} (85%)
 delete mode 100644 tests/ui/consts/required-consts/dead-code-in-dead-drop.opt.stderr
 rename tests/ui/consts/required-consts/{dead-code-in-dead-vtable.no-opt.stderr => dead-code-in-dead-vtable.noopt.stderr} (82%)

diff --git a/tests/ui/consts/required-consts/dead-code-in-dead-drop.no-opt.stderr b/tests/ui/consts/required-consts/dead-code-in-dead-drop.noopt.stderr
similarity index 85%
rename from tests/ui/consts/required-consts/dead-code-in-dead-drop.no-opt.stderr
rename to tests/ui/consts/required-consts/dead-code-in-dead-drop.noopt.stderr
index e3ef78842f77c..14fe323272f0d 100644
--- a/tests/ui/consts/required-consts/dead-code-in-dead-drop.no-opt.stderr
+++ b/tests/ui/consts/required-consts/dead-code-in-dead-drop.noopt.stderr
@@ -1,8 +1,8 @@
 error[E0080]: evaluation of `Fail::<i32>::C` failed
-  --> $DIR/dead-code-in-dead-drop.rs:9:19
+  --> $DIR/dead-code-in-dead-drop.rs:12:19
    |
 LL |     const C: () = panic!();
-   |                   ^^^^^^^^ the evaluated program panicked at 'explicit panic', $DIR/dead-code-in-dead-drop.rs:9:19
+   |                   ^^^^^^^^ the evaluated program panicked at 'explicit panic', $DIR/dead-code-in-dead-drop.rs:12:19
    |
    = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
 
diff --git a/tests/ui/consts/required-consts/dead-code-in-dead-drop.opt.stderr b/tests/ui/consts/required-consts/dead-code-in-dead-drop.opt.stderr
deleted file mode 100644
index e3ef78842f77c..0000000000000
--- a/tests/ui/consts/required-consts/dead-code-in-dead-drop.opt.stderr
+++ /dev/null
@@ -1,14 +0,0 @@
-error[E0080]: evaluation of `Fail::<i32>::C` failed
-  --> $DIR/dead-code-in-dead-drop.rs:9:19
-   |
-LL |     const C: () = panic!();
-   |                   ^^^^^^^^ the evaluated program panicked at 'explicit panic', $DIR/dead-code-in-dead-drop.rs:9:19
-   |
-   = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
-
-note: the above error was encountered while instantiating `fn <Fail<i32> as std::ops::Drop>::drop`
-  --> $SRC_DIR/core/src/ptr/mod.rs:LL:COL
-
-error: aborting due to 1 previous error
-
-For more information about this error, try `rustc --explain E0080`.
diff --git a/tests/ui/consts/required-consts/dead-code-in-dead-drop.rs b/tests/ui/consts/required-consts/dead-code-in-dead-drop.rs
index fc1052858b64c..d6ec873e99df1 100644
--- a/tests/ui/consts/required-consts/dead-code-in-dead-drop.rs
+++ b/tests/ui/consts/required-consts/dead-code-in-dead-drop.rs
@@ -1,12 +1,15 @@
-//@revisions: opt no-opt
-//@ build-fail
+//@revisions: noopt opt
+//@[noopt] build-fail
 //@[opt] compile-flags: -O
+//FIXME: `opt` revision currently does not stop with an error due to
+//<https://github.com/rust-lang/rust/issues/107503>.
+//@[opt] build-pass
 //! Make sure we detect erroneous constants post-monomorphization even when they are unused. This is
 //! crucial, people rely on it for soundness. (https://github.com/rust-lang/rust/issues/112090)
 
 struct Fail<T>(T);
 impl<T> Fail<T> {
-    const C: () = panic!(); //~ERROR evaluation of `Fail::<i32>::C` failed
+    const C: () = panic!(); //[noopt]~ERROR evaluation of `Fail::<i32>::C` failed
 }
 
 // This function is not actually called, but is mentioned implicitly as destructor in dead code in a
diff --git a/tests/ui/consts/required-consts/dead-code-in-dead-vtable.no-opt.stderr b/tests/ui/consts/required-consts/dead-code-in-dead-vtable.noopt.stderr
similarity index 82%
rename from tests/ui/consts/required-consts/dead-code-in-dead-vtable.no-opt.stderr
rename to tests/ui/consts/required-consts/dead-code-in-dead-vtable.noopt.stderr
index b70d675459236..ab60450049546 100644
--- a/tests/ui/consts/required-consts/dead-code-in-dead-vtable.no-opt.stderr
+++ b/tests/ui/consts/required-consts/dead-code-in-dead-vtable.noopt.stderr
@@ -1,13 +1,13 @@
 error[E0080]: evaluation of `Fail::<i32>::C` failed
-  --> $DIR/dead-code-in-dead-vtable.rs:9:19
+  --> $DIR/dead-code-in-dead-vtable.rs:12:19
    |
 LL |     const C: () = panic!();
-   |                   ^^^^^^^^ the evaluated program panicked at 'explicit panic', $DIR/dead-code-in-dead-vtable.rs:9:19
+   |                   ^^^^^^^^ the evaluated program panicked at 'explicit panic', $DIR/dead-code-in-dead-vtable.rs:12:19
    |
    = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 note: the above error was encountered while instantiating `fn <std::vec::Vec<i32> as MyTrait>::not_called`
-  --> $DIR/dead-code-in-dead-vtable.rs:30:40
+  --> $DIR/dead-code-in-dead-vtable.rs:35:40
    |
 LL |         let gen_vtable: &dyn MyTrait = &v; // vtable "appears" here
    |                                        ^^
diff --git a/tests/ui/consts/required-consts/dead-code-in-dead-vtable.rs b/tests/ui/consts/required-consts/dead-code-in-dead-vtable.rs
index 642e12d5bfaac..f21a1cc1fc2fd 100644
--- a/tests/ui/consts/required-consts/dead-code-in-dead-vtable.rs
+++ b/tests/ui/consts/required-consts/dead-code-in-dead-vtable.rs
@@ -1,12 +1,15 @@
-//@revisions: opt no-opt
-//@ build-fail
+//@revisions: noopt opt
+//@[noopt] build-fail
 //@[opt] compile-flags: -O
+//FIXME: `opt` revision currently does not stop with an error due to
+//<https://github.com/rust-lang/rust/issues/107503>.
+//@[opt] build-pass
 //! Make sure we detect erroneous constants post-monomorphization even when they are unused. This is
 //! crucial, people rely on it for soundness. (https://github.com/rust-lang/rust/issues/112090)
 
 struct Fail<T>(T);
 impl<T> Fail<T> {
-    const C: () = panic!(); //~ERROR evaluation of `Fail::<i32>::C` failed
+    const C: () = panic!(); //[noopt]~ERROR evaluation of `Fail::<i32>::C` failed
 }
 
 trait MyTrait {

From fd02409b064612d59898dc01ca0cc1b97248dd4e Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Wed, 13 Mar 2024 22:44:41 +0100
Subject: [PATCH 10/11] nicer errors

---
 compiler/rustc_monomorphize/src/collector.rs              | 6 ++++--
 .../required-consts/dead-code-in-called-fn.no-opt.stderr  | 4 ++++
 .../required-consts/dead-code-in-called-fn.opt.stderr     | 4 ++++
 .../required-consts/dead-code-in-dead-fn.no-opt.stderr    | 4 ++++
 .../generics/post_monomorphization_error_backtrace.stderr | 8 ++++++++
 5 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/compiler/rustc_monomorphize/src/collector.rs b/compiler/rustc_monomorphize/src/collector.rs
index 0e838f132a2dc..e62861d42edbf 100644
--- a/compiler/rustc_monomorphize/src/collector.rs
+++ b/compiler/rustc_monomorphize/src/collector.rs
@@ -735,14 +735,16 @@ impl<'a, 'tcx> MirUsedCollector<'a, 'tcx> {
 
 impl<'a, 'tcx> MirVisitor<'tcx> for MirUsedCollector<'a, 'tcx> {
     fn visit_body(&mut self, body: &mir::Body<'tcx>) {
+        self.super_body(body);
+
+        // Go over `required_items` *after* body, so if an item appears in both we reach it the
+        // "normal" way, via the body.
         for item in &body.required_items {
             // All these also need monomorphization to ensure that if that leads to error, we find
             // those errors.
             let item = self.monomorphize(*item);
             visit_required_item(self.tcx, item, self.output);
         }
-
-        self.super_body(body);
     }
 
     fn visit_rvalue(&mut self, rvalue: &mir::Rvalue<'tcx>, location: Location) {
diff --git a/tests/ui/consts/required-consts/dead-code-in-called-fn.no-opt.stderr b/tests/ui/consts/required-consts/dead-code-in-called-fn.no-opt.stderr
index 6dbe2c8eeee35..9a072ac1df697 100644
--- a/tests/ui/consts/required-consts/dead-code-in-called-fn.no-opt.stderr
+++ b/tests/ui/consts/required-consts/dead-code-in-called-fn.no-opt.stderr
@@ -7,6 +7,10 @@ LL |     const C: () = panic!();
    = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 note: the above error was encountered while instantiating `fn called::<i32>`
+  --> $DIR/dead-code-in-called-fn.rs:23:5
+   |
+LL |     called::<i32>();
+   |     ^^^^^^^^^^^^^^^
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/consts/required-consts/dead-code-in-called-fn.opt.stderr b/tests/ui/consts/required-consts/dead-code-in-called-fn.opt.stderr
index 6dbe2c8eeee35..9a072ac1df697 100644
--- a/tests/ui/consts/required-consts/dead-code-in-called-fn.opt.stderr
+++ b/tests/ui/consts/required-consts/dead-code-in-called-fn.opt.stderr
@@ -7,6 +7,10 @@ LL |     const C: () = panic!();
    = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 note: the above error was encountered while instantiating `fn called::<i32>`
+  --> $DIR/dead-code-in-called-fn.rs:23:5
+   |
+LL |     called::<i32>();
+   |     ^^^^^^^^^^^^^^^
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/consts/required-consts/dead-code-in-dead-fn.no-opt.stderr b/tests/ui/consts/required-consts/dead-code-in-dead-fn.no-opt.stderr
index 4c4c5f85f697e..5c410fc7c4570 100644
--- a/tests/ui/consts/required-consts/dead-code-in-dead-fn.no-opt.stderr
+++ b/tests/ui/consts/required-consts/dead-code-in-dead-fn.no-opt.stderr
@@ -7,6 +7,10 @@ LL |     const C: () = panic!();
    = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 note: the above error was encountered while instantiating `fn not_called::<i32>`
+  --> $DIR/dead-code-in-dead-fn.rs:26:9
+   |
+LL |         not_called::<T>();
+   |         ^^^^^^^^^^^^^^^^^
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/generics/post_monomorphization_error_backtrace.stderr b/tests/ui/generics/post_monomorphization_error_backtrace.stderr
index ed44c07630d14..0d707d83d2660 100644
--- a/tests/ui/generics/post_monomorphization_error_backtrace.stderr
+++ b/tests/ui/generics/post_monomorphization_error_backtrace.stderr
@@ -7,6 +7,10 @@ LL |         const V: () = assert!(std::mem::size_of::<T>() == 0);
    = note: this error originates in the macro `assert` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 note: the above error was encountered while instantiating `fn assert_zst::<u32>`
+  --> $DIR/post_monomorphization_error_backtrace.rs:18:5
+   |
+LL |     assert_zst::<U>()
+   |     ^^^^^^^^^^^^^^^^^
 
 error[E0080]: evaluation of `assert_zst::F::<i32>::V` failed
   --> $DIR/post_monomorphization_error_backtrace.rs:6:23
@@ -17,6 +21,10 @@ LL |         const V: () = assert!(std::mem::size_of::<T>() == 0);
    = note: this error originates in the macro `assert` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 note: the above error was encountered while instantiating `fn assert_zst::<i32>`
+  --> $DIR/post_monomorphization_error_backtrace.rs:18:5
+   |
+LL |     assert_zst::<U>()
+   |     ^^^^^^^^^^^^^^^^^
 
 error: aborting due to 2 previous errors
 

From 28e616d647cff6481f487a2f3282cccb0df9fd44 Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Thu, 14 Mar 2024 08:59:51 +0100
Subject: [PATCH 11/11] bless coverage tests... no idea why they change

---
 tests/coverage/async.cov-map   | 24 ++++++++++++------------
 tests/coverage/closure.cov-map | 18 +++++++++++-------
 2 files changed, 23 insertions(+), 19 deletions(-)

diff --git a/tests/coverage/async.cov-map b/tests/coverage/async.cov-map
index e06d1676358fb..67d149076837a 100644
--- a/tests/coverage/async.cov-map
+++ b/tests/coverage/async.cov-map
@@ -37,21 +37,21 @@ Number of expressions: 0
 Number of file 0 mappings: 1
 - Code(Counter(0)) at (prev + 17, 20) to (start + 0, 25)
 
-Function name: async::e (unused)
-Raw bytes (9): 0x[01, 01, 00, 01, 00, 13, 01, 00, 14]
+Function name: async::e
+Raw bytes (9): 0x[01, 01, 00, 01, 01, 13, 01, 00, 14]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 0
 Number of file 0 mappings: 1
-- Code(Zero) at (prev + 19, 1) to (start + 0, 20)
+- Code(Counter(0)) at (prev + 19, 1) to (start + 0, 20)
 
-Function name: async::e::{closure#0} (unused)
-Raw bytes (9): 0x[01, 01, 00, 01, 00, 13, 14, 00, 19]
+Function name: async::e::{closure#0}
+Raw bytes (9): 0x[01, 01, 00, 01, 01, 13, 14, 00, 19]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 0
 Number of file 0 mappings: 1
-- Code(Zero) at (prev + 19, 20) to (start + 0, 25)
+- Code(Counter(0)) at (prev + 19, 20) to (start + 0, 25)
 
 Function name: async::f
 Raw bytes (9): 0x[01, 01, 00, 01, 01, 15, 01, 00, 14]
@@ -69,21 +69,21 @@ Number of expressions: 0
 Number of file 0 mappings: 1
 - Code(Counter(0)) at (prev + 21, 20) to (start + 0, 25)
 
-Function name: async::foo (unused)
-Raw bytes (9): 0x[01, 01, 00, 01, 00, 17, 01, 00, 1e]
+Function name: async::foo
+Raw bytes (9): 0x[01, 01, 00, 01, 01, 17, 01, 00, 1e]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 0
 Number of file 0 mappings: 1
-- Code(Zero) at (prev + 23, 1) to (start + 0, 30)
+- Code(Counter(0)) at (prev + 23, 1) to (start + 0, 30)
 
-Function name: async::foo::{closure#0} (unused)
-Raw bytes (9): 0x[01, 01, 00, 01, 00, 17, 1e, 00, 2d]
+Function name: async::foo::{closure#0}
+Raw bytes (9): 0x[01, 01, 00, 01, 01, 17, 1e, 00, 2d]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 0
 Number of file 0 mappings: 1
-- Code(Zero) at (prev + 23, 30) to (start + 0, 45)
+- Code(Counter(0)) at (prev + 23, 30) to (start + 0, 45)
 
 Function name: async::g
 Raw bytes (9): 0x[01, 01, 00, 01, 01, 19, 01, 00, 17]
diff --git a/tests/coverage/closure.cov-map b/tests/coverage/closure.cov-map
index 9f0d33745bcb5..2f2b75b8f3dd6 100644
--- a/tests/coverage/closure.cov-map
+++ b/tests/coverage/closure.cov-map
@@ -33,16 +33,20 @@ Number of file 0 mappings: 24
 - Code(Expression(1, Add)) at (prev + 1, 5) to (start + 3, 2)
     = (c1 + (c0 - c1))
 
-Function name: closure::main::{closure#0} (unused)
-Raw bytes (24): 0x[01, 01, 00, 04, 00, 28, 05, 02, 14, 00, 02, 15, 02, 0a, 00, 02, 0a, 00, 0b, 00, 01, 09, 01, 06]
+Function name: closure::main::{closure#0}
+Raw bytes (28): 0x[01, 01, 02, 01, 05, 05, 02, 04, 01, 28, 05, 02, 14, 05, 02, 15, 02, 0a, 02, 02, 0a, 00, 0b, 07, 01, 09, 01, 06]
 Number of files: 1
 - file 0 => global file 1
-Number of expressions: 0
+Number of expressions: 2
+- expression 0 operands: lhs = Counter(0), rhs = Counter(1)
+- expression 1 operands: lhs = Counter(1), rhs = Expression(0, Sub)
 Number of file 0 mappings: 4
-- Code(Zero) at (prev + 40, 5) to (start + 2, 20)
-- Code(Zero) at (prev + 2, 21) to (start + 2, 10)
-- Code(Zero) at (prev + 2, 10) to (start + 0, 11)
-- Code(Zero) at (prev + 1, 9) to (start + 1, 6)
+- Code(Counter(0)) at (prev + 40, 5) to (start + 2, 20)
+- Code(Counter(1)) at (prev + 2, 21) to (start + 2, 10)
+- Code(Expression(0, Sub)) at (prev + 2, 10) to (start + 0, 11)
+    = (c0 - c1)
+- Code(Expression(1, Add)) at (prev + 1, 9) to (start + 1, 6)
+    = (c1 + (c0 - c1))
 
 Function name: closure::main::{closure#10} (unused)
 Raw bytes (10): 0x[01, 01, 00, 01, 00, 9b, 01, 07, 00, 21]