Rollup merge of #46287 - SimonSapin:stable-constness, r=aturon

Stabilize const-calling existing const-fns in std

Fixes #46038

src/libstd/lib.rs

@@ -256,15 +256,6 @@
 #![feature(collections_range)]
 #![feature(compiler_builtins_lib)]
 #![feature(const_fn)]
-#![feature(const_max_value)]
-#![feature(const_atomic_bool_new)]
-#![feature(const_atomic_isize_new)]
-#![feature(const_atomic_usize_new)]
-#![feature(const_unsafe_cell_new)]
-#![feature(const_cell_new)]
-#![feature(const_once_new)]
-#![feature(const_ptr_null)]
-#![feature(const_ptr_null_mut)]
 #![feature(core_float)]
 #![feature(core_intrinsics)]
 #![feature(dropck_eyepatch)]
@@ -306,7 +297,6 @@
 #![feature(repr_align)]
 #![feature(repr_simd)]
 #![feature(rustc_attrs)]
-#![feature(rustc_const_unstable)]
 #![feature(shared)]
 #![feature(sip_hash_13)]
 #![feature(slice_bytes)]
@@ -331,7 +321,6 @@
 #![feature(doc_masked)]
 #![feature(doc_spotlight)]
 #![cfg_attr(test, feature(update_panic_count))]
-#![cfg_attr(windows, feature(const_atomic_ptr_new))]
 #![cfg_attr(windows, feature(used))]
 
 #![default_lib_allocator]

src/libstd/sync/once.rs

@@ -156,7 +156,6 @@ struct Finish {
 impl Once {
     /// Creates a new `Once` value.
     #[stable(feature = "once_new", since = "1.2.0")]
-    #[rustc_const_unstable(feature = "const_once_new")]
     pub const fn new() -> Once {
         Once {
             state: AtomicUsize::new(INCOMPLETE),