Add inherent versions of MaybeUninit methods for slices

[clarfonthey]

This is my attempt to un-stall #63569 and #79995, by creating methods that mirror the existing MaybeUninit API:

impl<T> MaybeUninit<T> {
    pub fn write(&mut self, value: T) -> &mut T;
    pub unsafe fn assume_init_drop(&mut self);
    pub unsafe fn assume_init_ref(&self) -> &T;
    pub unsafe fn assume_init_mut(&mut self) -> &mut T;
}

Offering these new methods:

impl<T> MaybeUninit<T> {
    pub fn write_copy_of_slice(&mut self, value: &[T]) -> &mut [T] where T: Copy;
    pub fn write_clone_of_slice(&mut self, value: &[T]) -> &mut [T] where T: Clone;
    pub unsafe fn assume_init_drop(&mut self);
    pub unsafe fn assume_init_ref(&self) -> &T;
    pub unsafe fn assume_init_mut(&mut self) -> &mut T;
}

Since the assume_init methods are identical to those on non-slices, they feel pretty natural. The main issue with the write methods is naming, as discussed in #79995 among other places. My rationale:

• The term "write" should be in them somewhere, to mirror the other API, and this pretty much automatically makes them not collide with any other inherent slice methods.
• I chose write_clone_of_slice and write_copy_of_slice since clone and copy are being used as nouns here, whereas they're being used as verbs in clone_from_slice and copy_from_slice.

I'm hoping these don't require an ACP since they're just refining upon existing APIs, but I can write one up if you prefer.

[rustbot]

r? @tgross35

rustbot has assigned @tgross35.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[tgross35]

A test for panic during clone would probably be good to verify expected behavior

[tgross35]

The assume_init_* methods could probably use examples.

[tgross35]

The implementation looks pretty reasonable to me, but as this is an API decision:

r? libs-api

Is the intent here to replace the existing copy_from_slice and clone_from_slice? If so, those should be removed.

[clarfonthey]

I decided to not delete them since so many people are using these methods, but I can deprecate them depending on what people think is best.

[rust-log-analyzer]

The job x86_64-gnu-llvm-17 failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

------
 > importing cache manifest from ghcr.io/rust-lang/rust-ci-cache:20d3b4d4a2629cbf7865cdbf92fe47512a7c96658c24253a045ff38e8075cd7fb37ca6fcadfa6e6d093333943ad24f6fc4f163ec5b74fd940de9d5bb03eb4d3b:
------
##[endgroup]
Setting extra environment values for docker:  --env ENABLE_GCC_CODEGEN=1 --env GCC_EXEC_PREFIX=/usr/lib/gcc/
[CI_JOB_NAME=x86_64-gnu-llvm-17]
---
sccache: Starting the server...
##[group]Configure the build
configure: processing command line
configure: 
configure: build.configure-args := ['--build=x86_64-unknown-linux-gnu', '--llvm-root=/usr/lib/llvm-17', '--enable-llvm-link-shared', '--set', 'rust.thin-lto-import-instr-limit=10', '--set', 'change-id=99999999', '--enable-verbose-configure', '--enable-sccache', '--disable-manage-submodules', '--enable-locked-deps', '--enable-cargo-native-static', '--set', 'rust.codegen-units-std=1', '--set', 'dist.compression-profile=balanced', '--dist-compression-formats=xz', '--set', 'rust.lld=false', '--disable-dist-src', '--release-channel=nightly', '--enable-debug-assertions', '--enable-overflow-checks', '--enable-llvm-assertions', '--set', 'rust.verify-llvm-ir', '--set', 'rust.codegen-backends=llvm,cranelift,gcc', '--set', 'llvm.static-libstdcpp', '--enable-new-symbol-mangling']
configure: target.x86_64-unknown-linux-gnu.llvm-config := /usr/lib/llvm-17/bin/llvm-config
configure: llvm.link-shared     := True
configure: rust.thin-lto-import-instr-limit := 10
configure: change-id            := 99999999
---
  Downloaded boml v0.3.1
   Compiling boml v0.3.1
   Compiling y v0.1.0 (/checkout/compiler/rustc_codegen_gcc/build_system)
    Finished `release` profile [optimized] target(s) in 3.75s
     Running `/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-codegen/x86_64-unknown-linux-gnu/release/y test --use-system-gcc --use-backend gcc --out-dir /checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools/cg_gcc --release --mini-tests --std-tests`
Using system GCC
[BUILD] example
[AOT] mini_core_hello_world
/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools/cg_gcc/mini_core_hello_world
abc
---
##[group]Building stage0 tool linkchecker (x86_64-unknown-linux-gnu)
    Finished `release` profile [optimized] target(s) in 0.13s
##[endgroup]
##[group]Testing stage0 Linkcheck (x86_64-unknown-linux-gnu)
core/primitive.slice.html:132: broken link - `/checkout/obj/build/x86_64-unknown-linux-gnu/std/vec/struct.Vec.html`
std/primitive.slice.html:276: broken link - `/checkout/obj/build/x86_64-unknown-linux-gnu/std/vec/struct.Vec.html`
number of HTML files scanned: 43652
number of HTML redirects found: 13723
number of links checked: 3237221
number of links ignored due to external: 78841
number of links ignored due to external: 78841
number of links ignored due to exceptions: 17
number of intra doc links ignored: 8
errors found: 2
found some broken links
Command has failed. Rerun with -v to see more details.
  local time: Sun Sep  1 20:45:56 UTC 2024
  network time: Sun, 01 Sep 2024 20:45:56 GMT
##[error]Process completed with exit code 1.
Post job cleanup.