Use diagnostics for trace_macro instead of println

src/librustc_errors/lib.rs

@@ -388,6 +388,12 @@ impl Handler {
     pub fn span_note_without_error<S: Into<MultiSpan>>(&self, sp: S, msg: &str) {
         self.emit(&sp.into(), msg, Note);
     }
+    pub fn span_label_without_error(&self, sp: Span, msg: &str, lbl: &str) {
+        let mut db = DiagnosticBuilder::new(self, Note, msg);
+        db.set_span(sp);
+        db.span_label(sp, &lbl);
+        db.emit();
+    }
     pub fn span_unimpl<S: Into<MultiSpan>>(&self, sp: S, msg: &str) -> ! {
         self.span_bug(sp, &format!("unimplemented {}", msg));
     }

src/libsyntax/ext/base.rs

@@ -765,6 +765,9 @@ impl<'a> ExtCtxt<'a> {
     pub fn span_bug(&self, sp: Span, msg: &str) -> ! {
         self.parse_sess.span_diagnostic.span_bug(sp, msg);
     }
+    pub fn span_label_without_error(&self, sp: Span, msg: &str, label: &str) {
+        self.parse_sess.span_diagnostic.span_label_without_error(sp, msg, label);
+    }
     pub fn bug(&self, msg: &str) -> ! {
         self.parse_sess.span_diagnostic.bug(msg);
     }

src/libsyntax/ext/tt/macro_rules.rs

@@ -93,7 +93,9 @@ fn generic_extension<'cx>(cx: &'cx ExtCtxt,
                           rhses: &[quoted::TokenTree])
                           -> Box<MacResult+'cx> {
     if cx.trace_macros() {
-        println!("{}! {{ {} }}", name, arg);
+        cx.span_label_without_error(sp,
+                                    &"trace_macro",
+                                    &format!("expands to `{}! {{ {} }}`", name, arg));
     }
 
     // Which arm's failure should we report? (the one furthest along)

src/test/ui/macros/trace-macro.stderr

@@ -0,0 +1,14 @@
+note: trace_macro
+  --> $DIR/trace-macro.rs:12:5
+   |
+12 |     println!("Hello, World!");
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ expands to `println! { "Hello, World!" }`
+
+note: trace_macro
+  --> $DIR/trace-macro.rs:12:5
+   |
+12 |     println!("Hello, World!");
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ expands to `print! { concat ! ( "Hello, World!" , "\n" ) }`
+   |
+   = note: this error originates in a macro outside of the current crate
+