std: Stabilize the thread_local_const_init feature

This commit is intended to follow the stabilization disposition of the
FCP that has now finished in rust-lang#84223. This stabilizes the ability to flag
thread local initializers as `const` expressions which enables the macro
to generate more efficient code for accessing it, notably removing
runtime checks for initialization.

More information can also be found in rust-lang#84223 as well as the tests where
the feature usage was removed in this PR.

Closes rust-lang#84223

Author: alexcrichton

std/src/lib.rs

@@ -216,10 +216,7 @@
 // std may use features in a platform-specific way
 #![allow(unused_features)]
 #![feature(rustc_allow_const_fn_unstable)]
-#![cfg_attr(
-    test,
-    feature(internal_output_capture, print_internals, update_panic_count, thread_local_const_init)
-)]
+#![cfg_attr(test, feature(internal_output_capture, print_internals, update_panic_count))]
 #![cfg_attr(
     all(target_vendor = "fortanix", target_env = "sgx"),
     feature(slice_index_methods, coerce_unsized, sgx_platform)

std/src/thread/local.rs

@@ -164,7 +164,6 @@ macro_rules! __thread_local_inner {
     (@key $t:ty, const $init:expr) => {{
         #[cfg_attr(not(windows), inline)] // see comments below
         unsafe fn __getit() -> $crate::option::Option<&'static $t> {
-            const _REQUIRE_UNSTABLE: () = $crate::thread::require_unstable_const_init_thread_local();
             const INIT_EXPR: $t = $init;
 
             // wasm without atomics maps directly to `static mut`, and dtors

std/src/thread/mod.rs

@@ -204,13 +204,6 @@ pub use self::local::os::Key as __OsLocalKeyInner;
 #[doc(hidden)]
 pub use self::local::statik::Key as __StaticLocalKeyInner;
 
-// This is only used to make thread locals with `const { .. }` initialization
-// expressions unstable. If and/or when that syntax is stabilized with thread
-// locals this will simply be removed.
-#[doc(hidden)]
-#[unstable(feature = "thread_local_const_init", issue = "84223")]
-pub const fn require_unstable_const_init_thread_local() {}
-
 ////////////////////////////////////////////////////////////////////////////////
 // Builder
 ////////////////////////////////////////////////////////////////////////////////