Feature gate for defaulted associated type_consts with associated_type_defaults

tests/ui/const-generics/mgca/type-const-associated-default.rs

@@ -0,0 +1,9 @@
+#![feature(min_generic_const_args)]
+
+trait Trait {
+    #[type_const]
+    const N: usize = 10;
+}
+
+fn main(){
+}

tests/ui/const-generics/mgca/type-const-associated-default.stderr

@@ -0,0 +1,22 @@
+error[E0658]: associated type defaults are unstable
+  --> $DIR/type-const-associated-default.rs:6:5
+   |
+LL |     const N: usize = 10;
+   |     ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #29661 <https://github.com/rust-lang/rust/issues/29661> for more information
+   = help: add `#![feature(associated_type_defaults)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+warning: the feature `min_generic_const_args` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/type-const-associated-default.rs:2:12
+   |
+LL | #![feature(min_generic_const_args)]
+   |            ^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #132980 <https://github.com/rust-lang/rust/issues/132980> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+error: aborting due to 1 previous error; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0658`.