add UI test

src/librustc_errors/emitter.rs

@@ -795,8 +795,11 @@ impl EmitterWriter {
             if spans_updated {
                 children.push(SubDiagnostic {
                     level: Level::Note,
-                    message: vec![("this error originates in a macro outside of the current crate (run with RUST_MACRO_BACKTRACE=1 for more info)"
+                    message: vec![
-                        .to_string(), Style::NoStyle)],
+                        (["this error originates in a macro outside of the current crate",
+                          "(run with RUST_MACRO_BACKTRACE=1 for more info)"].join(" "),
+                         Style::NoStyle),
+                    ],
                     span: MultiSpan::new(),
                     render_span: None,
                 });
@@ -1242,7 +1245,7 @@ impl EmitterWriter {
         if let Some(ref cm) = self.cm {
             for trace in sp.macro_backtrace().iter().rev() {
                 let line_offset = buffer.num_lines();
- 
+
                 let mut diag_string =
                     format!("in this expansion of {}", trace.macro_decl_name);
                 if let Some(def_site_span) = trace.def_site_span {

src/test/ui/macro_backtrace/auxiliary/ping.rs

@@ -0,0 +1,20 @@
+// 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.
+
+// Test that the macro backtrace facility works (supporting file)
+
+// a non-local macro
+#[macro_export]
+macro_rules! ping {
+    () => {
+        pong!();
+    }
+}
+

src/test/ui/macro_backtrace/main.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.
+
+// Test that the macro backtrace facility works
+// aux-build:ping.rs
+// rustc-env:RUST_MACRO_BACKTRACE
+
+#[macro_use] extern crate ping;
+
+// a local macro
+macro_rules! pong {
+    () => { syntax error };
+}
+
+fn main() {
+    pong!();
+    ping!();
+}

src/test/ui/macro_backtrace/main.stderr

@@ -0,0 +1,21 @@
+error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `error`
+  --> $DIR/main.rs:19:20
+   |
+19 |     () => { syntax error };
+   |                   -^^^^^ unexpected token
+   |                   |
+   |                   expected one of 8 possible tokens here
+$DIR/main.rs:23:5: 23:13 note: in this expansion of pong! (defined in $DIR/main.rs)
+
+error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `error`
+  --> $DIR/main.rs:19:20
+   |
+19 |     () => { syntax error };
+   |                   -^^^^^ unexpected token
+   |                   |
+   |                   expected one of 8 possible tokens here
+$DIR/main.rs:24:5: 24:13 note: in this expansion of ping! (defined in <ping macros>)
+<ping macros>:1:11: 1:24 note: in this expansion of pong! (defined in $DIR/main.rs)
+
+error: aborting due to 2 previous errors
+