Disallow ambiguous attributes on expressions

compiler/rustc_parse/src/errors.rs

@@ -495,6 +495,15 @@ pub(crate) struct OuterAttributeNotAllowedOnIfElse {
     pub attributes: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(parse_outer_attr_ambiguous)]
+pub(crate) struct AmbiguousOuterAttributes {
+    #[primary_span]
+    pub span: Span,
+    #[subdiagnostic]
+    pub sugg: WrapInParentheses,
+}
+
 #[derive(Diagnostic)]
 #[diag(parse_missing_in_in_for_loop)]
 pub(crate) struct MissingInInForLoop {