Translate zero-sized return types as void

The only way to get a value for a zero-sized type is `undef`, so
there's really no point in actually having a return type other than
void for such types. Also, while the comment in return_type_is_void
mentioned something about aiding C ABI support, @eddyb correctly
pointed out on IRC that there is no such thing as a zero-sized type in
C. And even with clang, which allows empty structs, those get
translated as void return types as well.

Fixes #28766

src/librustc_trans/trans/common.rs

@@ -119,11 +119,9 @@ pub fn type_is_zero_size<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>, ty: Ty<'tcx>) -
 }
 
 /// Identifies types which we declare to be equivalent to `void` in C for the purpose of function
-/// return types. These are `()`, bot, and uninhabited enums. Note that all such types are also
-/// zero-size, but not all zero-size types use a `void` return type (in order to aid with C ABI
-/// compatibility).
-pub fn return_type_is_void(ccx: &CrateContext, ty: Ty) -> bool {
-    ty.is_nil() || ty.is_empty(ccx.tcx())
+/// return types. These are `()`, bot, uninhabited enums and all other zero-sized types.
+pub fn return_type_is_void<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>, ty: Ty<'tcx>) -> bool {
+    ty.is_nil() || ty.is_empty(ccx.tcx()) || type_is_zero_size(ccx, ty)
 }
 
 /// Generates a unique symbol based off the name given. This is used to create

src/test/run-make/issue-28766/Makefile

@@ -0,0 +1,5 @@
+-include ../tools.mk
+
+all:
+	$(RUSTC) -O foo.rs
+	$(RUSTC) -O -L $(TMPDIR) main.rs

src/test/run-make/issue-28766/foo.rs

@@ -0,0 +1,18 @@
+// Copyright 2016 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.
+
+#![crate_type="lib"]
+pub struct Foo(());
+
+impl Foo {
+  pub fn new() -> Foo {
+    Foo(())
+  }
+}

src/test/run-make/issue-28766/main.rs

@@ -0,0 +1,17 @@
+// Copyright 2016 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.
+
+#![crate_type="lib"]
+extern crate foo;
+use foo::Foo;
+
+pub fn crash() -> Box<Foo> {
+  Box::new(Foo::new())
+}