Auto merge of #28651 - dotdash:exhaustive_match, r=eddyb

By putting an "unreachable" instruction into the default arm of a switch
instruction we can let LLVM know that the match is exhaustive, allowing
for better optimizations.

For example, this match:
```rust
pub enum Enum {
    One,
    Two,
    Three,
}

impl Enum {
    pub fn get_disc(self) -> u8 {
        match self {
            Enum::One => 0,
            Enum::Two => 1,
            Enum::Three => 2,
        }
    }
}
```

Currently compiles to this on x86_64:
```asm
  .cfi_startproc
  movzbl  %dil, %ecx
  cmpl  $1, %ecx
  setne %al
  testb %cl, %cl
  je  .LBB0_2
  incb  %al
  movb  %al, %dil
.LBB0_2:
  movb  %dil, %al
  retq
.Lfunc_end0:
```

But with this change we get:
```asm
  .cfi_startproc
  movb  %dil, %al
  retq
.Lfunc_end0:
```

Author: bors

src/librustc_trans/trans/_match.rs

@@ -1283,6 +1283,10 @@ fn compile_submatch_continue<'a, 'p, 'blk, 'tcx>(mut bcx: Block<'blk, 'tcx>,
     let exhaustive = chk.is_infallible() && defaults.is_empty();
     let len = opts.len();
 
+    if exhaustive && kind == Switch {
+        build::Unreachable(else_cx);
+    }
+
     // Compile subtrees for each option
     for (i, opt) in opts.iter().enumerate() {
         // In some cases of range and vector pattern matching, we need to
@@ -1293,7 +1297,7 @@ fn compile_submatch_continue<'a, 'p, 'blk, 'tcx>(mut bcx: Block<'blk, 'tcx>,
         let mut opt_cx = else_cx;
         let debug_loc = opt.debug_loc();
 
-        if !exhaustive || i + 1 < len {
+        if kind == Switch || !exhaustive || i + 1 < len {
             opt_cx = bcx.fcx.new_temp_block("match_case");
             match kind {
                 Single => Br(bcx, opt_cx.llbb, debug_loc),

src/test/codegen/match.rs

@@ -0,0 +1,30 @@
+// Copyright 2015 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.
+
+// compile-flags: -C no-prepopulate-passes
+
+#![crate_type = "lib"]
+
+pub enum E {
+    A,
+    B,
+}
+
+// CHECK-LABEL: @exhaustive_match
+#[no_mangle]
+pub fn exhaustive_match(e: E) {
+// CHECK: switch{{.*}}, label %[[DEFAULT:[a-zA-Z0-9_]+]]
+// CHECK: [[DEFAULT]]:
+// CHECK-NEXT: unreachable
+    match e {
+        E::A => (),
+        E::B => (),
+    }
+}