Rollup merge of #134583 - Enselic:maybe-uninit-transmute, r=workingjubilee

docs: `transmute<&mut T, &mut MaybeUninit<T>>` is unsound when exposed to safe code

Closes #66699

On my system (Edit: And also in the [playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=90529e2a9900599cb759e4bfaa5b5efe)) the example program terminates with an unpredictable exit code:
```console
$ cargo +nightly build && target/debug/bin ; echo $?
255
$ cargo +nightly build && target/debug/bin ; echo $?
253
```

And miri considers the code to have undefined behavior:
```console
$ cargo +nightly miri run
error: Undefined Behavior: using uninitialized data, but this operation requires initialized memory
  --> src/main.rs:12:24
   |
12 |     std::process::exit(*code); // UB! Accessing uninitialized memory
   |                        ^^^^^ using uninitialized data, but this operation requires initialized memory
   |
error: aborting due to 1 previous error
```

0 files changed, 0 insertions, 0 deletions