rustc_bulltin_macros: tweak span_labels

Author: Centril

src/librustc_builtin_macros/test.rs

@@ -377,13 +377,13 @@ fn has_test_signature(cx: &ExtCtxt<'_>, i: &ast::Item) -> bool {
     if let ast::ItemKind::Fn(ref sig, ref generics, _) = i.kind {
         if let ast::Unsafe::Yes(span) = sig.header.unsafety {
             sd.struct_span_err(i.span, "unsafe functions cannot be used for tests")
-                .span_label(span, "unsafe because of this")
+                .span_label(span, "`unsafe` because of this")
                 .emit();
             return false;
         }
         if let ast::Async::Yes { span, .. } = sig.header.asyncness {
             sd.struct_span_err(i.span, "async functions cannot be used for tests")
-                .span_label(span, "async because of this")
+                .span_label(span, "`async` because of this")
                 .emit();
             return false;
         }