Rollup merge of #129401 - workingjubilee:partial-initialization-of-stabilization, r=dtolnay,joboet

Partially stabilize `feature(new_uninit)`

Finished comment period: rust-lang/rust#63291 (comment)

The following API has been stabilized from rust-lang/rust#63291

```rust
impl<T> Box<T> { pub fn new_uninit() -> Box<MaybeUninit<T>> {…} }
impl<T> Rc<T> { pub fn new_uninit() -> Rc<MaybeUninit<T>> {…} }
impl<T> Arc<T> { pub fn new_uninit() -> Arc<MaybeUninit<T>> {…} }

impl<T> Box<[T]> { pub fn new_uninit_slice(len: usize) -> Box<[MaybeUninit<T>]> {…} }
impl<T> Rc<[T]> { pub fn new_uninit_slice(len: usize) -> Rc<[MaybeUninit<T>]> {…} }
impl<T> Arc<[T]> { pub fn new_uninit_slice(len: usize) -> Arc<[MaybeUninit<T>]> {…} }

impl<T> Box<MaybeUninit<T>> { pub unsafe fn assume_init(self) -> Box<T> {…} }
impl<T> Box<[MaybeUninit<T>]> { pub unsafe fn assume_init(self) -> Box<[T]> {…} }
impl<T> Rc<MaybeUninit<T>> { pub unsafe fn assume_init(self) -> Rc<T> {…} }
impl<T> Rc<[MaybeUninit<T>]> { pub unsafe fn assume_init(self) -> Rc<[T]> {…} }
impl<T> Arc<MaybeUninit<T>> { pub unsafe fn assume_init(self) -> Arc<T> {…} }
impl<T> Arc<[MaybeUninit<T>]> { pub unsafe fn assume_init(self) -> Arc<[T]> {…} }
```

The remaining API is split between new issues
- `new_zeroed_alloc`: rust-lang/rust#129396
- `box_uninit_write`: rust-lang/rust#129397

All relevant code is thus either stabilized or split out of that issue, so this closes #63291 as, with the FCP concluded, that issue has served its purpose.

try-job: x86_64-rust-for-linux

Author: workingjubilee

tests/fail/data_race/alloc_read_race.rs

@@ -1,7 +1,6 @@
 //@compile-flags: -Zmiri-disable-weak-memory-emulation -Zmiri-preemption-rate=0 -Zmiri-disable-stacked-borrows
 // Avoid accidental synchronization via address reuse inside `thread::spawn`.
 //@compile-flags: -Zmiri-address-reuse-cross-thread-rate=0
-#![feature(new_uninit)]
 
 use std::mem::MaybeUninit;
 use std::ptr::null_mut;

tests/fail/data_race/alloc_write_race.rs

@@ -1,7 +1,6 @@
 //@compile-flags: -Zmiri-disable-weak-memory-emulation -Zmiri-preemption-rate=0 -Zmiri-disable-stacked-borrows
 // Avoid accidental synchronization via address reuse inside `thread::spawn`.
 //@compile-flags: -Zmiri-address-reuse-cross-thread-rate=0
-#![feature(new_uninit)]
 
 use std::ptr::null_mut;
 use std::sync::atomic::{AtomicPtr, Ordering};

tests/fail/weak_memory/weak_uninit.rs

@@ -6,7 +6,6 @@
 // run multiple times until one try returns true.
 // Spurious failure is possible, if you are really unlucky with
 // the RNG and always read the latest value from the store buffer.
-#![feature(new_uninit)]
 
 use std::sync::atomic::*;
 use std::thread::spawn;

tests/pass/rc.rs

@@ -1,7 +1,6 @@
 //@revisions: stack tree
 //@[tree]compile-flags: -Zmiri-tree-borrows
 //@compile-flags: -Zmiri-strict-provenance
-#![feature(new_uninit)]
 #![feature(get_mut_unchecked)]
 #![allow(ambiguous_wide_pointer_comparisons)]
 

tests/pass/slices.rs

@@ -1,7 +1,6 @@
 //@revisions: stack tree
 //@[tree]compile-flags: -Zmiri-tree-borrows
 //@compile-flags: -Zmiri-strict-provenance
-#![feature(new_uninit)]
 #![feature(slice_as_chunks)]
 #![feature(slice_partition_dedup)]
 #![feature(layout_for_ptr)]