Fix Ptr inconsistency in {Rc,Arc}

[DiuDiu777]

PR Description

This pr aims to address the problem discussed on zulip.

Problem Clarification

As this post presents, the {Rc, Arc}::{in/de-crement_strong_count_/in} do not limit the layout of the memory that ptr points to, while internally Rc::from_raw_in is called directly.

UB doesn't just appear when the strong count is decremented to zero. Miri also detects the UB of out-of-bounds pointer use when increment strong count is called on a pointer with an incorrect layout(shown as below).

use std::rc::Rc;
#[repr(align(8))]
struct Aligned8(u64);

#[repr(align(16))]
struct Aligned16(u64);

fn main() {
    let rc: Rc<Aligned8> = Rc::new(Aligned8(42));
    let raw_ptr = Rc::into_raw(rc);

    unsafe {
        Rc::increment_strong_count(raw_ptr as *const Aligned16);
    }
}

Miri output:

error: Undefined Behavior: out-of-bounds pointer use: expected a pointer to 32 bytes of memory, but got alloc954 which is only 24 bytes from the end of the allocation

[rustbot]

r? @Mark-Simulacrum

rustbot has assigned @Mark-Simulacrum.
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

[Mark-Simulacrum]

@bors r+

[bors]

📌 Commit fa183ad has been approved by Mark-Simulacrum

It is now in the queue for this repository.