Rollup merge of rust-lang#119801 - zachs18:zachs18-patch-1, r=steffah…

…n,Nilstrieb

Fix deallocation with wrong allocator in (A)Rc::from_box_in

Deallocate the `Box` with the original allocator (via `&A`), not `Global`.

Fixes rust-lang#119749

<details> <summary>Example code with error and Miri output</summary>

(Note that this UB is not observable on stable, because the only usable allocator on stable is `Global` anyway.)

Code ([playground link](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=96193c2c6a1912d7f669fbbe39174b09)):

```rs
#![feature(allocator_api)]
use std::alloc::System;

// uncomment one of these
use std::rc::Rc;
//use std::sync::Arc as Rc;

fn main() {
    let x: Box<[u32], System> = Box::new_in([1,2,3], System);
    let _: Rc<[u32], System> = Rc::from(x);
}
```

Miri output:

```rs
error: Undefined Behavior: deallocating alloc904, which is C heap memory, using Rust heap deallocation operation
   --> /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/alloc.rs:117:14
    |
117 |     unsafe { __rust_dealloc(ptr, layout.size(), layout.align()) }
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ deallocating alloc904, which is C heap memory, using Rust heap deallocation operation
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
    = note: BACKTRACE:
    = note: inside `std::alloc::dealloc` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/alloc.rs:117:14: 117:64
    = note: inside `<std::alloc::Global as std::alloc::Allocator>::deallocate` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/alloc.rs:254:22: 254:51
    = note: inside `<std::boxed::Box<std::mem::ManuallyDrop<[u32]>> as std::ops::Drop>::drop` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/boxed.rs:1244:17: 1244:66
    = note: inside `std::ptr::drop_in_place::<std::boxed::Box<std::mem::ManuallyDrop<[u32]>>> - shim(Some(std::boxed::Box<std::mem::ManuallyDrop<[u32]>>))` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/mod.rs:507:1: 507:56
    = note: inside `std::mem::drop::<std::boxed::Box<std::mem::ManuallyDrop<[u32]>>>` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/mem/mod.rs:992:24: 992:25
    = note: inside `std::rc::Rc::<[u32], std::alloc::System>::from_box_in` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/rc.rs:1928:13: 1928:22
    = note: inside `<std::rc::Rc<[u32], std::alloc::System> as std::convert::From<std::boxed::Box<[u32], std::alloc::System>>>::from` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/rc.rs:2504:9: 2504:27
note: inside `main`
   --> src/main.rs:10:32
    |
10  |     let _: Rc<[u32], System> = Rc::from(x);
    |                                ^^^^^^^^^^^

note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace

error: aborting due to 1 previous error
```

</details>

library/alloc/src/rc.rs

@@ -1924,7 +1924,7 @@ impl<T: ?Sized, A: Allocator> Rc<T, A> {
 
             // Free the allocation without dropping its contents
             let (bptr, alloc) = Box::into_raw_with_allocator(src);
-            let src = Box::from_raw(bptr as *mut mem::ManuallyDrop<T>);
+            let src = Box::from_raw_in(bptr as *mut mem::ManuallyDrop<T>, alloc.by_ref());
             drop(src);
 
             Self::from_ptr_in(ptr, alloc)

library/alloc/src/sync.rs

@@ -1869,7 +1869,7 @@ impl<T: ?Sized, A: Allocator> Arc<T, A> {
 
             // Free the allocation without dropping its contents
             let (bptr, alloc) = Box::into_raw_with_allocator(src);
-            let src = Box::from_raw(bptr as *mut mem::ManuallyDrop<T>);
+            let src = Box::from_raw_in(bptr as *mut mem::ManuallyDrop<T>, alloc.by_ref());
             drop(src);
 
             Self::from_ptr_in(ptr, alloc)