`pin!()` macro no longer constant-promotes `&mut` references to empty arrays

[theemathas]

Code

I tried this code:

use std::pin::{pin, Pin};

fn require_static(_: Pin<&'static mut [i32; 0]>) {}

pub fn weird() {
    require_static(pin!([]));
}

I expected to see this happen: The code compiles

Instead, this happened (when compiled with beta rust):

error[E0716]: temporary value dropped while borrowed
 --> src/lib.rs:6:20
  |
6 |     require_static(pin!([]));
  |     ---------------^^^^^^^^-- temporary value is freed at the end of this statement
  |     |              |
  |     |              creates a temporary value which is freed while still in use
  |     argument requires that borrow lasts for `'static`
  |
  = note: this error originates in the macro `pin` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0716`.
error: could not compile `playground` (lib) due to 1 previous error

See also #140126, which is about the fact that &mut references to empty arrays are, for some reason, sometimes allowed in const contexts.

This was found while messing around. I don't know if any code relies on this.

Version it worked on

It most recently worked on: stable Rust 1.87.0

Version with regression

The error is reproducible on the playground with 1.88.0-beta.5 (2025-06-01 645b44edd3717f02838d)

The regression presumably happened due to #139114. cc @m-ou-se

@rustbot modify labels: +regression-from-stable-to-beta -regression-untriaged A-pin F-super_let

[m-ou-se]

Did you come across this in a real world use case? A pinned static mutable reference to a zero sized type seems like a bit of a wild concept to me. ^^'

[theemathas]

I was just messing around, because I felt like something with this combination of stuff had to be broken somehow 😅

[theemathas]

Another variant:

use std::pin::{pin, Pin};

const fn identity<T>(x: T) -> T { x }

pub const X: Pin<&'static mut [i32; 0]> = identity(pin!([]));

This compiles on stable, but gives the following error on beta:

error[E0716]: temporary value dropped while borrowed
 --> src/lib.rs:5:52
  |
5 | pub const X: Pin<&'static mut [i32; 0]> = identity(pin!([]));
  |                                           ---------^^^^^^^^-
  |                                           |        |       |
  |                                           |        |       temporary value is freed at the end of this statement
  |                                           |        creates a temporary value which is freed while still in use
  |                                           using this value as a constant requires that borrow lasts for `'static`
  |
  = note: this error originates in the macro `pin` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0716`.
error: could not compile `playground` (lib) due to 1 previous error