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

[zachs18]

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

Fixes #119749

Example code with error and Miri output

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

Code (playground link):

#![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:

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

[rustbot]

r? @joshtriplett

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[steffahn]

This PR fixes #119749 which was recently independently discovered; feel free to put that into the OP in order to auto-close the issue on merge.

[Noratrieb]

@bors r=steffahn,Nilstrieb

[bors]

📌 Commit bfe04e0 has been approved by steffahn,Nilstrieb

It is now in the queue for this repository.