Rollup merge of #120144 - petrochenkov:unty, r=davidtwco

privacy: Stabilize lint `unnameable_types`

This is the last piece of ["RFC #2145: Type privacy and private-in-public lints"](https://github.com/rust-lang/rust/issues/48054).

Having unstable lints is not very useful because you cannot even dogfood them in the compiler/stdlib in this case (https://github.com/rust-lang/rust/pull/113284).
The worst thing that may happen when a lint is removed are some `removed_lints` warnings, but I haven't heard anyone suggesting removing this specific lint.

This lint is allow-by-default and is supposed to be enabled explicitly.
Some false positives are expected, because sometimes unnameable types are a legitimate pattern.
This lint also have some unnecessary false positives, that can be fixed - see https://github.com/rust-lang/rust/issues/120146 and https://github.com/rust-lang/rust/issues/120149.

Closes https://github.com/rust-lang/rust/issues/48054.

compiler/rustc_feature/src/unstable.rs

@@ -615,8 +615,6 @@ declare_features! (
     /// Allows creation of instances of a struct by moving fields that have
     /// not changed from prior instances of the same struct (RFC #2528)
     (unstable, type_changing_struct_update, "1.58.0", Some(86555)),
-    /// Allows using type privacy lints (`private_interfaces`, `private_bounds`, `unnameable_types`).
-    (unstable, type_privacy_lints, "1.72.0", Some(48054)),
     /// Enables rustc to generate code that instructs libstd to NOT ignore SIGPIPE.
     (unstable, unix_sigpipe, "1.65.0", Some(97889)),
     /// Allows unnamed fields of struct and union type