Add parsing for builtin # in expression and item context

compiler/rustc_parse/src/errors.rs

@@ -2644,3 +2644,18 @@ pub(crate) struct MalformedCfgAttr {
     pub span: Span,
     pub sugg: &'static str,
 }
+
+#[derive(Diagnostic)]
+#[diag(parse_unknown_builtin_construct)]
+pub(crate) struct UnknownBuiltinConstruct {
+    #[primary_span]
+    pub span: Span,
+    pub name: Symbol,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_expected_builtin_ident)]
+pub(crate) struct ExpectedBuiltinIdent {
+    #[primary_span]
+    pub span: Span,
+}