Suggest imports of unresolved macros

Author: hirrolot

compiler/rustc_resolve/src/diagnostics.rs

@@ -929,6 +929,10 @@ impl<'a> Resolver<'a> {
         );
         self.add_typo_suggestion(err, suggestion, ident.span);
 
+        let import_suggestions =
+            self.lookup_import_candidates(ident, Namespace::MacroNS, parent_scope, |_| true);
+        show_candidates(err, None, &import_suggestions, false, true);
+
         if macro_kind == MacroKind::Derive && (ident.name == sym::Send || ident.name == sym::Sync) {
             let msg = format!("unsafe traits like `{}` should be implemented explicitly", ident);
             err.span_note(ident.span, &msg);