Constant promotion with interior mutability exposes whether a function was used

use std::cell::Cell;

const fn make_none<T>() -> Option<T> {
    None
}

const A: Option<Cell<i32>> = None;
const B: Option<Cell<i32>> = make_none();

fn works() {
    let _: &'static _ = &A;
}

fn fails() {
    let _: &'static _ = &B;
}

In the above code, I expected works and fails to either both compile or both not compile. Instead, only works compiles, and fails gives the following error:

error[E0716]: temporary value dropped while borrowed
  --> src/lib.rs:15:26
   |
15 |     let _: &'static _ = &B;
   |            ----------    ^ creates a temporary value which is freed while still in use
   |            |
   |            type annotation requires that borrow lasts for `'static`
16 | }
   | - temporary value is freed at the end of this statement

For more information about this error, try `rustc --explain E0716`.

That is, for some reason, &A is constant-promoted, but &B isn't.

I'm not sure if this is a bug, but it's at least extremely surprising, and is a semver hazard.

See also https://github.com/rust-lang/unsafe-code-guidelines/issues/493

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Reproducible on the playground with version 1.92.0-nightly (2025-09-20 dd7fda570040e8a736f7)

Run the supplied playground reproduction on the reported nightly version and compare the behavior of direct constant promotion with promotion through make_none. Read the linked unsafe-code-guidelines issue to understand the interior-mutability constraints; the work is done when the promotion behavior and intended semver-compatible outcome are established and documented with a regression test or specification change.