`&pin!()` stopped being deref-coerced in rust 1.88

[theemathas]

I expected the following code to compile, since Pin implements Deref, so it should be possible to do a deref coercion from the type &Pin<i32> to the type &i32.

fn main() {
    let _x: &i32 = &std::pin::pin!(1i32);
}

The above code compiles without errors in rust 1.87.0. However, it produces the following compile error in rust 1.88.0

error[E0308]: mismatched types
 --> <source>:2:21
  |
2 |     let _x: &i32 = &std::pin::pin!(1i32);
  |                     ^^^^^^^^^^^^^^^^^^^^ expected `i32`, found `Pin<&mut i32>`
  |
  = note: expected type `i32`
           found struct `Pin<&mut i32>`
  = note: this error originates in the macro `std::pin::pin` (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0308`.

This presumably regressed in #139114, which changed pin!() to use super let. This uses a block expression in the syntax, which then presumably runs into the bug in #23014.

I don't know if there are any realistic use cases which run into this problem. (Hopefully not.)

cc @dianne @m-ou-se (who worked on super let).

cc @jackh726 (who seemed to have recently been working on coercions).

Meta

Tested on Godbolt with rustc versions 1.87.0 and 1.88.0.

[theemathas]

Additionally, the following code compiles in 1.88.0, but compile errors in 1.87.0:

use std::pin::{Pin, pin};
fn main() {
    let x = Box::new(1i32);
    let _y: Pin<&mut i32> = pin!(x);
}

I think that in versions 1.88.0+, it's coercing a &mut Box<i32> into a &mut i32 inside the pin!() macro.

Error in 1.87.0:

error[E0308]: mismatched types
 --> <source>:4:34
  |
4 |     let _y: Pin<&mut i32> = pin!(x);
  |                                  ^ expected `i32`, found `Box<i32>`
  |
  = note: expected type `i32`
           found struct `Box<i32>`
help: consider unboxing the value
  |
4 |     let _y: Pin<&mut i32> = pin!(*x);
  |                                  +

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0308`.

That is, in version 1.88.0, we both have code that's newly rejected (original code in the issue), and code that's newly accepted (code in this comment).

[theemathas]

Using a github search, I found a piece of code in the wild that I think is newly allowed: https://github.com/restatedev/restate/blob/a472bad4e12427012f6215947edd35561d62d0b3/crates/vqueues/src/scheduler/drr.rs#L563