Rollup merge of #109914 - compiler-errors:rtn-bad-parens, r=oli-obk

Emit feature error for parenthesized generics in associated type bounds

We don't actually do AST->HIR lowering with some `-Zunpretty` flags, so it's not correct to just delay a bug instead of emitting a feature error.

Some diagnostics regressed because of the new errors, but oh well. 🤷

Fixes #109898

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -485,17 +485,10 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
             if let Some(args) = constraint.gen_args.as_ref()
                 && matches!(
                     args,
-                    ast::GenericArgs::ReturnTypeNotation(..) | ast::GenericArgs::Parenthesized(..)
+                    ast::GenericArgs::ReturnTypeNotation(..)
                 )
             {
-                // RTN is gated elsewhere, and parenthesized args will turn into
-                // another error.
-                if matches!(args, ast::GenericArgs::Parenthesized(..)) {
-                    self.sess.delay_span_bug(
-                        constraint.span,
-                        "should have emitted a parenthesized generics error",
-                    );
-                }
+                // RTN is gated below with a `gate_all`.
             } else {
                 gate_feature_post!(
                     &self,