tweak unresolved label suggestion

Only suggest label names in the same hygiene context, and use a
structured suggestion.

Author: euclio

src/librustc_resolve/lib.rs

@@ -357,7 +357,12 @@ fn resolve_struct_error<'sess, 'a>(resolver: &'sess Resolver<'_>,
                                            "use of undeclared label `{}`",
                                            name);
             if let Some(lev_candidate) = lev_candidate {
-                err.span_label(span, format!("did you mean `{}`?", lev_candidate));
+                err.span_suggestion(
+                    span,
+                    "a label with a similar name exists in this scope",
+                    lev_candidate.to_string(),
+                    Applicability::MaybeIncorrect,
+                );
             } else {
                 err.span_label(span, format!("undeclared label `{}`", name));
             }
@@ -4218,7 +4223,13 @@ impl<'a> Resolver<'a> {
                         // Picks the first label that is "close enough", which is not necessarily
                         // the closest match
                         let close_match = self.search_label(label.ident, |rib, ident| {
-                            let names = rib.bindings.iter().map(|(id, _)| &id.name);
+                            let names = rib.bindings.iter().filter_map(|(id, _)| {
+                                if id.span.ctxt() == label.ident.span.ctxt() {
+                                    Some(&id.name)
+                                } else {
+                                    None
+                                }
+                            });
                             find_best_match_for_name(names, &*ident.as_str(), None)
                         });
                         self.record_def(expr.id, err_path_resolution());

src/test/ui/hygiene/hygienic-label-1.stderr

@@ -2,7 +2,7 @@ error[E0426]: use of undeclared label `'x`
   --> $DIR/hygienic-label-1.rs:2:19
    |
 LL |     () => { break 'x; }
-   |                   ^^ did you mean `'x`?
+   |                   ^^ undeclared label `'x`
 ...
 LL |     'x: loop { foo!() }
    |                ------ in this macro invocation

src/test/ui/hygiene/hygienic-label-2.stderr

@@ -2,7 +2,7 @@ error[E0426]: use of undeclared label `'x`
   --> $DIR/hygienic-label-2.rs:6:16
    |
 LL |     foo!(break 'x);
-   |                ^^ did you mean `'x`?
+   |                ^^ undeclared label `'x`
 
 error: aborting due to previous error
 

src/test/ui/hygiene/hygienic-label-3.stderr

@@ -2,7 +2,7 @@ error[E0426]: use of undeclared label `'x`
   --> $DIR/hygienic-label-3.rs:2:19
    |
 LL |     () => { break 'x; }
-   |                   ^^ did you mean `'x`?
+   |                   ^^ undeclared label `'x`
 ...
 LL |         foo!()
    |         ------ in this macro invocation

src/test/ui/hygiene/hygienic-label-4.stderr

@@ -2,7 +2,7 @@ error[E0426]: use of undeclared label `'x`
   --> $DIR/hygienic-label-4.rs:6:16
    |
 LL |     foo!(break 'x);
-   |                ^^ did you mean `'x`?
+   |                ^^ undeclared label `'x`
 
 error: aborting due to previous error
 

src/test/ui/suggestions/suggest-labels.stderr

@@ -2,19 +2,31 @@ error[E0426]: use of undeclared label `'fo`
   --> $DIR/suggest-labels.rs:4:15
    |
 LL |         break 'fo;
-   |               ^^^ did you mean `'foo`?
+   |               ^^^
+help: a label with a similar name exists in this scope
+   |
+LL |         break 'foo;
+   |               ^^^^
 
 error[E0426]: use of undeclared label `'bor`
   --> $DIR/suggest-labels.rs:8:18
    |
 LL |         continue 'bor;
-   |                  ^^^^ did you mean `'bar`?
+   |                  ^^^^
+help: a label with a similar name exists in this scope
+   |
+LL |         continue 'bar;
+   |                  ^^^^
 
 error[E0426]: use of undeclared label `'longlable`
   --> $DIR/suggest-labels.rs:13:19
    |
 LL |             break 'longlable;
-   |                   ^^^^^^^^^^ did you mean `'longlabel1`?
+   |                   ^^^^^^^^^^
+help: a label with a similar name exists in this scope
+   |
+LL |             break 'longlabel1;
+   |                   ^^^^^^^^^^^
 
 error: aborting due to 3 previous errors