Emit feature error for parenthesized generics in associated type bounds

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,

tests/ui/associated-type-bounds/return-type-notation/bad-inputs-and-output.rs

@@ -10,11 +10,13 @@ trait Trait {
 
 fn foo<T: Trait<method(i32): Send>>() {}
 //~^ ERROR argument types not allowed with return type notation
+//~| ERROR associated type bounds are unstable
 
 fn bar<T: Trait<method(..) -> (): Send>>() {}
 //~^ ERROR return type not allowed with return type notation
 
 fn baz<T: Trait<method(): Send>>() {}
 //~^ ERROR return type notation arguments must be elided with `..`
+//~| ERROR associated type bounds are unstable
 
 fn main() {}

tests/ui/associated-type-bounds/return-type-notation/bad-inputs-and-output.stderr

@@ -1,9 +1,27 @@
 error: return type not allowed with return type notation
-  --> $DIR/bad-inputs-and-output.rs:14:28
+  --> $DIR/bad-inputs-and-output.rs:15:28
    |
 LL | fn bar<T: Trait<method(..) -> (): Send>>() {}
    |                            ^^^^^ help: remove the return type
 
+error[E0658]: associated type bounds are unstable
+  --> $DIR/bad-inputs-and-output.rs:11:17
+   |
+LL | fn foo<T: Trait<method(i32): Send>>() {}
+   |                 ^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #52662 <https://github.com/rust-lang/rust/issues/52662> for more information
+   = help: add `#![feature(associated_type_bounds)]` to the crate attributes to enable
+
+error[E0658]: associated type bounds are unstable
+  --> $DIR/bad-inputs-and-output.rs:18:17
+   |
+LL | fn baz<T: Trait<method(): Send>>() {}
+   |                 ^^^^^^^^^^^^^^
+   |
+   = note: see issue #52662 <https://github.com/rust-lang/rust/issues/52662> for more information
+   = help: add `#![feature(associated_type_bounds)]` to the crate attributes to enable
+
 warning: the feature `return_type_notation` is incomplete and may not be safe to use and/or cause compiler crashes
   --> $DIR/bad-inputs-and-output.rs:3:12
    |
@@ -28,10 +46,11 @@ LL | fn foo<T: Trait<method(i32): Send>>() {}
    |                       ^^^^^ help: remove the input types: `(..)`
 
 error: return type notation arguments must be elided with `..`
-  --> $DIR/bad-inputs-and-output.rs:17:23
+  --> $DIR/bad-inputs-and-output.rs:18:23
    |
 LL | fn baz<T: Trait<method(): Send>>() {}
    |                       ^^ help: add `..`: `(..)`
 
-error: aborting due to 3 previous errors; 2 warnings emitted
+error: aborting due to 5 previous errors; 2 warnings emitted
 
+For more information about this error, try `rustc --explain E0658`.

tests/ui/associated-type-bounds/return-type-notation/unpretty-parenthesized.rs

@@ -0,0 +1,11 @@
+// edition: 2021
+// compile-flags: -Zunpretty=expanded
+
+trait Trait {
+    async fn method() {}
+}
+
+fn foo<T: Trait<method(i32): Send>>() {}
+//~^ ERROR associated type bounds are unstable
+
+fn main() {}

tests/ui/associated-type-bounds/return-type-notation/unpretty-parenthesized.stderr

@@ -0,0 +1,12 @@
+error[E0658]: associated type bounds are unstable
+  --> $DIR/unpretty-parenthesized.rs:8:17
+   |
+LL | fn foo<T: Trait<method(i32): Send>>() {}
+   |                 ^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #52662 <https://github.com/rust-lang/rust/issues/52662> for more information
+   = help: add `#![feature(associated_type_bounds)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.

tests/ui/associated-type-bounds/return-type-notation/unpretty-parenthesized.stdout

@@ -0,0 +1,15 @@
+#![feature(prelude_import)]
+#[prelude_import]
+use std::prelude::rust_2021::*;
+#[macro_use]
+extern crate std;
+// edition: 2021
+// compile-flags: -Zunpretty=expanded
+
+trait Trait {
+    async fn method() {}
+}
+
+fn foo<T: Trait<method(i32) : Send>>() {}
+
+fn main() {}