Auto merge of #47291 - varkor:unreachable-never-patterns, r=<try>

Eager unreachable blocks for `!` type

This change optimises the generation of MIR in the presence of `!`. Blocks are now immediately marked as unreachable if:
- A function argument is derived from the value of type `!`.
- A match arm binds a value whose type is derived from `!`.
- An rvalue is created whose type is derived from `!`.

This avoids unnecessary MIR generation and also fixes #43061 and #41446.

r? @nikomatsakis

Author: bors

src/librustc/ty/sty.rs

@@ -1283,6 +1283,16 @@ impl<'a, 'gcx, 'tcx> TyS<'tcx> {
         }
     }
 
+    pub fn conservative_is_uninhabited(&self) -> bool {
+        // "rustc-1.0-style" uncontentious uninhabitableness check
+        match self.sty {
+            ty::TyNever => true,
+            ty::TyAdt(def, _) => def.variants.is_empty(),
+            ty::TyTuple(tys) => tys.iter().any(|ty| ty.conservative_is_uninhabited()),
+            _ => false
+        }
+    }
+
     pub fn is_primitive(&self) -> bool {
         match self.sty {
             TyBool | TyChar | TyInt(_) | TyUint(_) | TyFloat(_) => true,

src/librustc_mir/build/expr/into.rs

@@ -27,8 +27,8 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                      expr: Expr<'tcx>)
                      -> BlockAnd<()>
     {
-        debug!("into_expr(destination={:?}, block={:?}, expr={:?})",
-               destination, block, expr);
+        debug!("into_expr(destination={:?}, block={:?}, expr_kind={:?}, expr={:?})",
+               destination, block, expr.kind, expr);
 
         // since we frequently have to reference `self` from within a
         // closure, where `self` would be shadowed, it's easier to
@@ -317,9 +317,19 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                     _ => true,
                 });
 
+                let ty = expr.ty;
                 let rvalue = unpack!(block = this.as_local_rvalue(block, expr));
-                this.cfg.push_assign(block, source_info, destination, rvalue);
-                block.unit()
+
+                if ty.conservative_is_uninhabited() {
+                    // It's impossible to have an rvalue of type `!`, so if we encounter one,
+                    // we can terminate the block as unreachable immediately.
+                    this.cfg.terminate(block, source_info, TerminatorKind::Unreachable);
+                    let end_block = this.cfg.start_new_block();
+                    end_block.unit()
+                } else {
+                    this.cfg.push_assign(block, source_info, destination, rvalue);
+                    block.unit()
+                }
             }
         }
     }

src/librustc_mir/build/matches/mod.rs

@@ -54,6 +54,12 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
         let dummy_temp = self.temp(dummy_ty, dummy_source_info.span);
         self.cfg.push_assign(block, dummy_source_info, &dummy_temp, dummy_access);
 
+        let arms: Vec<Arm<'tcx>> = arms.into_iter().filter_map(|mut arm| {
+            arm.patterns =
+                arm.patterns.iter().filter(|pat| !pat.is_unreachable()).cloned().collect();
+            if !arm.patterns.is_empty() { Some(arm) } else { None }
+        }).collect();
+
         let mut arm_blocks = ArmBlocks {
             blocks: arms.iter()
                         .map(|_| self.cfg.start_new_block())

src/librustc_mir/build/mod.rs

@@ -623,6 +623,7 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
         }
 
         let mut scope = None;
+        let mut unreachable_arguments = false;
         // Bind the argument patterns
         for (index, &(ty, pattern)) in arguments.iter().enumerate() {
             // Function arguments always get the first Local indices after the return place
@@ -632,6 +633,11 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
             if let Some(pattern) = pattern {
                 let pattern = self.hir.pattern_from_hir(pattern);
 
+                if pattern.is_unreachable() {
+                    unreachable_arguments = true;
+                    break;
+                }
+
                 match *pattern.kind {
                     // Don't introduce extra copies for simple bindings
                     PatternKind::Binding { mutability, var, mode: BindingMode::ByValue, .. } => {
@@ -649,7 +655,6 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
             // Make sure we drop (parts of) the argument even when not matched on.
             self.schedule_drop(pattern.as_ref().map_or(ast_body.span, |pat| pat.span),
                                argument_scope, &place, ty);
-
         }
 
         // Enter the argument pattern bindings visibility scope, if it exists.
@@ -658,6 +663,12 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
         }
 
         let body = self.hir.mirror(ast_body);
+        if unreachable_arguments {
+            let source_info = self.source_info(body.span);
+            self.cfg.terminate(block, source_info, TerminatorKind::Unreachable);
+            let end_block = self.cfg.start_new_block();
+            return end_block.unit();
+        }
         self.into(&Place::Local(RETURN_PLACE), block, body)
     }
 

src/librustc_mir/hair/pattern/check_match.rs

@@ -225,7 +225,7 @@ impl<'a, 'tcx> MatchVisitor<'a, 'tcx> {
                 let scrutinee_is_uninhabited = if self.tcx.features().exhaustive_patterns {
                     self.tcx.is_ty_uninhabited_from(module, pat_ty)
                 } else {
-                    self.conservative_is_uninhabited(pat_ty)
+                    pat_ty.conservative_is_uninhabited()
                 };
                 if !scrutinee_is_uninhabited {
                     // We know the type is inhabited, so this must be wrong
@@ -253,15 +253,6 @@ impl<'a, 'tcx> MatchVisitor<'a, 'tcx> {
         })
     }
 
-    fn conservative_is_uninhabited(&self, scrutinee_ty: Ty<'tcx>) -> bool {
-        // "rustc-1.0-style" uncontentious uninhabitableness check
-        match scrutinee_ty.sty {
-            ty::TyNever => true,
-            ty::TyAdt(def, _) => def.variants.is_empty(),
-            _ => false
-        }
-    }
-
     fn check_irrefutable(&self, pat: &'tcx Pat, origin: &str) {
         let module = self.tcx.hir.get_module_parent(pat.id);
         MatchCheckCtxt::create_and_enter(self.tcx, module, |ref mut cx| {

src/librustc_mir/hair/pattern/mod.rs

@@ -65,6 +65,7 @@ pub struct Pattern<'tcx> {
 
 #[derive(Clone, Debug)]
 pub enum PatternKind<'tcx> {
+    /// Wildcard `_`
     Wild,
 
     /// x, ref x, x @ P, etc
@@ -306,6 +307,39 @@ impl<'a, 'tcx> Pattern<'tcx> {
         debug!("Pattern::from_hir({:?}) = {:?}", pat, result);
         result
     }
+
+    // Returns true if the pattern cannot bind, as it would require a value of type `!` to have
+    // been constructed. This check is conservative.
+    pub fn is_unreachable(&self) -> bool {
+        if self.ty.conservative_is_uninhabited() {
+            return true;
+        }
+        match *self.kind {
+            PatternKind::Binding { ty, ref subpattern, .. } => {
+                if ty.conservative_is_uninhabited() {
+                    return true;
+                }
+                if let &Some(ref subpattern) = subpattern {
+                    subpattern.is_unreachable()
+                } else { false }
+            },
+            PatternKind::Variant { ref subpatterns, .. } |
+            PatternKind::Leaf { ref subpatterns } => {
+                subpatterns.iter().any(|field_pattern|
+                    field_pattern.pattern.is_unreachable())
+            },
+            PatternKind::Deref { ref subpattern } => {
+                subpattern.is_unreachable()
+            },
+            PatternKind::Slice { ref prefix, ref slice, ref suffix } |
+            PatternKind::Array { ref prefix, ref slice, ref suffix } => {
+                prefix.iter().any(|pat| pat.is_unreachable()) ||
+                slice.iter().any(|pat| pat.is_unreachable()) ||
+                suffix.iter().any(|pat| pat.is_unreachable())
+            },
+            _ => false
+        }
+    }
 }
 
 impl<'a, 'tcx> PatternContext<'a, 'tcx> {

src/test/compile-fail/uninhabited-matches-feature-gated.rs

@@ -19,9 +19,6 @@ fn main() {
     let x: &Void = unsafe { std::mem::uninitialized() };
     let _ = match x {}; //~ ERROR non-exhaustive
 
-    let x: (Void,) = unsafe { std::mem::uninitialized() };
-    let _ = match x {}; //~ ERROR non-exhaustive
-
     let x: [Void; 1] = unsafe { std::mem::uninitialized() };
     let _ = match x {}; //~ ERROR non-exhaustive
 
@@ -30,9 +27,6 @@ fn main() {
         &[] => (),
     };
 
-    let x: Void = unsafe { std::mem::uninitialized() };
-    let _ = match x {}; // okay
-
     let x: Result<u32, Void> = Ok(23);
     let _ = match x {   //~ ERROR non-exhaustive
         Ok(x) => x,

src/test/mir-opt/never_type_unreachable.rs

@@ -0,0 +1,120 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(never_type)]
+#![allow(dead_code)]
+#![allow(path_statements)]
+#![allow(unreachable_patterns)]
+
+fn never_direct(x: !) {
+    x;
+}
+
+fn never_ref_pat(ref x: !) {
+    *x;
+}
+
+fn never_ref(x: &!) {
+    let &y = x;
+    y;
+}
+
+fn never_slice(x: &[!]) {
+    x[0];
+}
+
+fn never_match(x: Result<(), !>) {
+    match x {
+        Ok(_) => {},
+        Err(_) => {},
+    }
+}
+
+fn never_match_disj_patterns() {
+    let x: Option<!> = None;
+    match x {
+        Some(_) | None => {}
+    }
+}
+
+pub fn main() { }
+
+// END RUST SOURCE
+
+// START rustc.never_direct.SimplifyCfg-initial.after.mir
+// bb0: {
+//     unreachable;
+// }
+// END rustc.never_direct.SimplifyCfg-initial.after.mir
+
+// START rustc.never_ref_pat.SimplifyCfg-initial.after.mir
+// bb0: {
+//     unreachable;
+// }
+// END rustc.never_ref_pat.SimplifyCfg-initial.after.mir
+
+// START rustc.never_ref.SimplifyCfg-initial.after.mir
+// bb0: {
+//     ...
+//     unreachable;
+// }
+// END rustc.never_ref.SimplifyCfg-initial.after.mir
+
+// START rustc.never_slice.SimplifyCfg-initial.after.mir
+// bb1: {
+//     ...
+//     unreachable;
+// }
+// END rustc.never_slice.SimplifyCfg-initial.after.mir
+
+// START rustc.never_match.SimplifyCfg-initial.after.mir
+// fn never_match(_1: std::result::Result<(), !>) -> () {
+//     ...
+//     bb0: {
+//         ...
+//     }
+//     bb1: {
+//         _0 = ();
+//         return;
+//     }
+//     bb2: {
+//         ...
+//     }
+//     bb3: {
+//         unreachable;
+//     }
+//     bb4: {
+//         unreachable;
+//     }
+// }
+// END rustc.never_match.SimplifyCfg-initial.after.mir
+
+// START rustc.never_match_disj_patterns.SimplifyCfg-initial.after.mir
+// fn never_match_disj_patterns() -> () {
+//     ...
+//     bb0: {
+//         ...
+//     }
+//     bb1: {
+//         _0 = ();
+//         StorageDead(_1);
+//         return;
+//     }
+//     bb2: {
+//         ...
+//     }
+//     bb3: {
+//         unreachable;
+//     }
+//     bb4: {
+//         unreachable;
+//     }
+// }
+// END rustc.never_match_disj_patterns.SimplifyCfg-initial.after.mir