rustc: Always emit the uwtable attribute on Windows

This commit alters the translation layer to unconditionally emit the `uwtable`
LLVM attribute on Windows regardless of the `no_landing_pads` setting.
Previously I believe we omitted this attribute as an optimization when the
`-Cpanic=abort` flag was passed, but this unfortunately caused problems for
Gecko.

It [was discovered] that there was trouble unwinding through Rust functions due
to foreign exceptions such as illegal instructions or otherwise in-practice
methods used to abort a process. In testing it looked like the major difference
between a working binary and a non-working binary is indeed this `uwtable`
attribute, but this PR has unfortunately not been thoroughly tested in terms of
compiling Gecko with `-C panic=abort` *and* this PR to see whether it works, so
this is still somewhat working on just suspicion.

[was discovered]: https://bugzilla.mozilla.org/show_bug.cgi?id=1302078

src/librustc_trans/base.rs

@@ -583,7 +583,24 @@ pub fn trans_instance<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>, instance: Instance
 
     ccx.stats().n_closures.set(ccx.stats().n_closures.get() + 1);
 
-    if !ccx.sess().no_landing_pads() {
+    // The `uwtable` attribute according to LLVM is:
+    //
+    //     This attribute indicates that the ABI being targeted requires that an
+    //     unwind table entry be produced for this function even if we can show
+    //     that no exceptions passes by it. This is normally the case for the
+    //     ELF x86-64 abi, but it can be disabled for some compilation units.
+    //
+    // Typically when we're compiling with `-C panic=abort` (which implies this
+    // `no_landing_pads` check) we don't need `uwtable` because we can't
+    // generate any exceptions! On Windows, however, exceptions include other
+    // events such as illegal instructions, segfaults, etc. This means that on
+    // Windows we end up still needing the `uwtable` attribute even if the `-C
+    // panic=abort` flag is passed.
+    //
+    // You can also find more info on why Windows is whitelisted here in:
+    //      https://bugzilla.mozilla.org/show_bug.cgi?id=1302078
+    if !ccx.sess().no_landing_pads() ||
+       ccx.sess().target.target.options.is_like_windows {
         attributes::emit_uwtable(lldecl, true);
     }
 

src/test/codegen/panic-abort-windows.rs

@@ -0,0 +1,41 @@
+// 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.
+
+// ignore-tidy-linelength
+
+// This test is for *-windows-msvc only.
+// ignore-android
+// ignore-bitrig
+// ignore-macos
+// ignore-dragonfly
+// ignore-freebsd
+// ignore-haiku
+// ignore-ios
+// ignore-linux
+// ignore-netbsd
+// ignore-openbsd
+// ignore-solaris
+// ignore-emscripten
+
+// compile-flags: -C no-prepopulate-passes -C panic=abort -O
+
+#![crate_type = "lib"]
+
+// CHECK: Function Attrs: uwtable
+// CHECK-NEXT: define void @normal_uwtable()
+#[no_mangle]
+pub fn normal_uwtable() {
+}
+
+// CHECK: Function Attrs: nounwind uwtable
+// CHECK-NEXT: define void @extern_uwtable()
+#[no_mangle]
+pub extern fn extern_uwtable() {
+}