Prevent abuse of private module

[taiki-e]

This was originally reported by @danielhenrymantilla in https://discord.com/channels/273534239310479360/512792629516173323/870075511009857617.

Currently, you can break the soundness of the pin-project by overriding the private module as follows.
Considering that it uses a hidden private API that is not guaranteed to be stable, I don't think this will actually happen except in cases where users intentionally abuse it or use a malicious crate. Also, in such a case, fixing this would not help much because the attacker can do anything.
That said, it seems relatively easy to fix this, so I'm going to fix this.

extern crate pin_project as pin_project_orig;
extern crate self as pin_project;

pub use ::pin_project_orig::*;
mod __private {
    pub use ::pin_project_orig::__private::*;
    pub trait Drop {}
}

use std::{marker::PhantomPinned, mem};

#[pin_project]
struct S {
    #[pin]
    f: (u8, PhantomPinned),
}

impl Drop for S {
    fn drop(&mut self) {
        let prev = &self.f.0 as *const _ as usize;
        let moved = mem::take(&mut self.f); // move pinned field
        let moved = &moved.0 as *const _ as usize;
        assert_eq!(prev, moved); // panic
    }
}

fn main() {
    let mut x = Box::pin(S { f: (1, PhantomPinned) });
    let _f = x.as_mut().project().f; // first mutable access
}

playground

---

This patch forces the macro to refer to the pin-project crate by referring to the _pin_project imported into the const _ scope instead of ::pin_project.

pin-project/tests/expand/pub/struct.expanded.rs

Lines 21 to 23 in 7444436

	const _: () = {
	#[allow(unused_extern_crates)]
	extern crate pin_project as _pin_project;

pin-project/tests/expand/pub/struct.expanded.rs

Line 49 in 7444436

pinned: _pin_project::__private::Pin::new_unchecked(pinned),

In the above example, a user-implemented Drop is detected, resulting in an error.

pin-project/tests/ui/pin_project/override-priv-mod.stderr

Lines 1 to 10 in 7444436

	error[E0119]: conflicting implementations of trait `_::SMustNotImplDrop` for type `S`
	--> tests/ui/pin_project/override-priv-mod.rs:14:1
	|
	14 | #[pin_project] //~ ERROR conflicting implementations of trait `_::FooMustNotImplDrop`
	| ^^^^^^^^^^^^^^
	| |
	| first implementation here
	| conflicting implementation for `S`
	|
	= note: this error originates in the derive macro `::pin_project::__private::__PinProjectInternalDerive` (in Nightly builds, run with -Z macro-backtrace for more info)

If other items (e.g., PinnedDrop) are replaced in the same way, either nothing happens (nothing is affected because macro doesn't use them), or #[pin_project] and #[pinned_drop] refer to different items, resulting in an error.

[taiki-e]

bors r+

[bors]

Build succeeded:

• ci