Auto merge of #44383 - qmx:gh/40473/no-inline-trait-method, r=nikomatsakis

MIR: should not inline trait method

Fixes #40473.

The idea here is bailing out of inlining if we're talking about a trait method.

Author: bors

src/librustc_mir/transform/inline.rs

@@ -88,12 +88,14 @@ impl<'a, 'tcx> Inliner<'a, 'tcx> {
                 if let TerminatorKind::Call {
                     func: Operand::Constant(ref f), .. } = terminator.kind {
                     if let ty::TyFnDef(callee_def_id, substs) = f.ty.sty {
-                        callsites.push_back(CallSite {
-                            callee: callee_def_id,
-                            substs,
-                            bb,
-                            location: terminator.source_info
-                        });
+                        if self.tcx.trait_of_item(callee_def_id).is_none() {
+                            callsites.push_back(CallSite {
+                                callee: callee_def_id,
+                                substs,
+                                bb,
+                                location: terminator.source_info
+                            });
+                        }
                     }
                 }
             }

src/test/run-pass/mir-inlining/no-trait-method-issue-40473.rs

@@ -0,0 +1,25 @@
+// 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.
+
+// compile-flags:-Zmir-opt-level=2
+pub trait Foo {
+    fn bar(&self) -> usize { 2 }
+}
+
+impl Foo for () {
+    fn bar(&self) -> usize { 3 }
+}
+
+// Test a case where MIR would inline the default trait method
+// instead of bailing out. Issue #40473.
+fn main() {
+    let result = ().bar();
+    assert_eq!(result, 3);
+}