Fix feature gating for crate/extern in paths

Author: petrochenkov

src/libsyntax/feature_gate.rs

@@ -1766,11 +1766,19 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
 
     fn visit_path(&mut self, path: &'a ast::Path, _id: NodeId) {
         for segment in &path.segments {
+            // Identifiers we are going to check could come from a legacy macro (e.g. `#[test]`).
+            // For such macros identifiers must have empty context, because this context is
+            // used during name resolution and produced names must be unhygienic for compatibility.
+            // On the other hand, we need the actual non-empty context for feature gate checking
+            // because it's hygienic even for legacy macros. As previously stated, such context
+            // cannot be kept in identifiers, so it's kept in paths instead and we take it from
+            // there while keeping location info from the ident span.
+            let span = segment.ident.span.with_ctxt(path.span.ctxt());
             if segment.ident.name == keywords::Crate.name() {
-                gate_feature_post!(&self, crate_in_paths, segment.ident.span,
+                gate_feature_post!(&self, crate_in_paths, span,
                                    "`crate` in paths is experimental");
             } else if segment.ident.name == keywords::Extern.name() {
-                gate_feature_post!(&self, extern_in_paths, segment.ident.span,
+                gate_feature_post!(&self, extern_in_paths, span,
                                    "`extern` in paths is experimental");
             }
         }