Auto merge of rust-lang#82898 - oli-obk:tait_🧊, r=nikomatsakis

Add a `min_type_alias_impl_trait` feature gate

This new feature gate only permits type alias impl trait to be constrained by function and trait method return types. All other possible constraining sites like const/static types, closure return types and binding types are now forbidden and gated under the `type_alias_impl_trait` and `impl_trait_in_bindings` feature gates (which are both marked as incomplete, as they have various ways to ICE the compiler or cause query cycles where they shouldn't).

r? `@nikomatsakis`

This is best reviewed commit-by-commit

Author: bors

alloc/src/lib.rs

@@ -142,7 +142,8 @@
 #![feature(alloc_layout_extra)]
 #![feature(trusted_random_access)]
 #![feature(try_trait)]
-#![feature(type_alias_impl_trait)]
+#![cfg_attr(bootstrap, feature(type_alias_impl_trait))]
+#![cfg_attr(not(bootstrap), feature(min_type_alias_impl_trait))]
 #![feature(associated_type_bounds)]
 #![feature(slice_group_by)]
 #![feature(decl_macro)]