`super let` in `const` with interior mutability has inconsistent behavior

I tried this code:

#![feature(super_let)]

use std::sync::atomic::AtomicI32;

const WORKS: &Option<AtomicI32> = {
    super let a: Option<AtomicI32> = None;
    &a
};

const WORKS2: &Option<AtomicI32> = {
    super let a: Option<AtomicI32> = None;
    let x = &a;
    &*x
};

const FAILS: &Option<AtomicI32> = {
    super let a: Option<AtomicI32> = None;
    let _x = &a;
    &a
};

I expected to see this happen: Either all three constants compile, or all three fail.

Instead, this happened: Only the FAILS constant fails to compile

error[E0492]: constants cannot refer to interior mutable data
  --> src/lib.rs:19:5
   |
19 |     &a
   |     ^^ this borrow of an interior mutable value may end up in the final value

For more information about this error, try `rustc --explain E0492`.
error: could not compile `playground` (lib) due to 1 previous error

See also #142229, which also involves super let in const, and issues with mutability.

Meta

Reproducible on the playground with 1.89.0-nightly (2025-06-11 e703dff8fe220b78195c)

@rustbot labels +F-super_let +A-const-eval

Start by compiling the provided three-constant reproducer on the noted nightly and compare the handling of WORKS, WORKS2, and FAILS, including E0492. Trace the const-evaluation and interior-mutability checks involved in super let; done means the three cases have consistent behavior and a regression test covers it.