Use feature(const_fn_trait_bound) instead of feature(const_fn)

    error[E0658]: trait bounds other than `Sized` on const fn parameters are unstable
      --> test_suite/tests/test.rs:105:30
        |
    105 |     const fn const_generics4<T: T1<A1 = S1<1, { 2 + 1 }>>, const C: usize>() -> S1<C, { C }> {
        |                              ^
        |
        = note: see issue #57563 <rust-lang/rust#57563> for more information
        = help: add `#![feature(const_fn_trait_bound)]` to the crate attributes to enable

test_suite/tests/test.rs

@@ -1,4 +1,4 @@
-#![cfg_attr(const_unstable, feature(const_fn, const_extern_fn))]
+#![cfg_attr(const_unstable, feature(const_extern_fn, const_fn_trait_bound))]
 #![warn(rust_2018_idioms, single_use_lifetimes)]
 #![allow(clippy::missing_safety_doc)] // this is test