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

privacy: Stabilize lint `unnameable_types`

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

Having unstable lints is not very useful because you cannot even dogfood them in the compiler/stdlib in this case (rust-lang#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 rust-lang#120146 and rust-lang#120149.

Closes rust-lang#48054.

compiler/rustc_feature/src/accepted.rs

@@ -355,6 +355,8 @@ declare_features! (
     (accepted, type_alias_enum_variants, "1.37.0", Some(49683)),
     /// Allows macros to appear in the type position.
     (accepted, type_macros, "1.13.0", Some(27245)),
+    /// Allows using type privacy lints (`private_interfaces`, `private_bounds`, `unnameable_types`).
+    (accepted, type_privacy_lints, "CURRENT_RUSTC_VERSION", Some(48054)),
     /// Allows `const _: TYPE = VALUE`.
     (accepted, underscore_const_names, "1.37.0", Some(54912)),
     /// Allows `use path as _;` and `extern crate c as _;`.

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

compiler/rustc_lint/src/builtin.rs

@@ -1337,8 +1337,9 @@ impl<'tcx> LateLintPass<'tcx> for UngatedAsyncFnTrackCaller {
 }
 
 declare_lint! {
-    /// The `unreachable_pub` lint triggers for `pub` items not reachable from
-    /// the crate root.
+    /// The `unreachable_pub` lint triggers for `pub` items not reachable from other crates - that
+    /// means neither directly accessible, nor reexported, nor leaked through things like return
+    /// types.
     ///
     /// ### Example
     ///

compiler/rustc_lint_defs/src/builtin.rs

@@ -4311,7 +4311,6 @@ declare_lint! {
     /// ### Example
     ///
     /// ```rust,compile_fail
-    /// # #![feature(type_privacy_lints)]
     /// # #![allow(unused)]
     /// #![deny(unnameable_types)]
     /// mod m {
@@ -4328,10 +4327,14 @@ declare_lint! {
     ///
     /// It is often expected that if you can obtain an object of type `T`, then
     /// you can name the type `T` as well, this lint attempts to enforce this rule.
+    /// The recommended action is to either reexport the type properly to make it nameable,
+    /// or document that users are not supposed to be able to name it for one reason or another.
+    ///
+    /// Besides types, this lint applies to traits because traits can also leak through signatures,
+    /// and you may obtain objects of their `dyn Trait` or `impl Trait` types.
     pub UNNAMEABLE_TYPES,
     Allow,
     "effective visibility of a type is larger than the area in which it can be named",
-    @feature_gate = sym::type_privacy_lints;
 }
 
 declare_lint! {

tests/ui/privacy/unnameable_types.rs

@@ -1,4 +1,3 @@
-#![feature(type_privacy_lints)]
 #![deny(unnameable_types)]
 
 mod m {

tests/ui/privacy/unnameable_types.stderr

@@ -1,23 +1,23 @@
 error: struct `PubStruct` is reachable but cannot be named
-  --> $DIR/unnameable_types.rs:5:5
+  --> $DIR/unnameable_types.rs:4:5
    |
 LL |     pub struct PubStruct(pub i32);
    |     ^^^^^^^^^^^^^^^^^^^^ reachable at visibility `pub`, but can only be named at visibility `pub(crate)`
    |
 note: the lint level is defined here
-  --> $DIR/unnameable_types.rs:2:9
+  --> $DIR/unnameable_types.rs:1:9
    |
 LL | #![deny(unnameable_types)]
    |         ^^^^^^^^^^^^^^^^
 
 error: enum `PubE` is reachable but cannot be named
-  --> $DIR/unnameable_types.rs:7:5
+  --> $DIR/unnameable_types.rs:6:5
    |
 LL |     pub enum PubE {
    |     ^^^^^^^^^^^^^ reachable at visibility `pub`, but can only be named at visibility `pub(crate)`
 
 error: trait `PubTr` is reachable but cannot be named
-  --> $DIR/unnameable_types.rs:11:5
+  --> $DIR/unnameable_types.rs:10:5
    |
 LL |     pub trait PubTr {
    |     ^^^^^^^^^^^^^^^ reachable at visibility `pub`, but can only be named at visibility `pub(crate)`