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

[theemathas]

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

[theemathas]

An interesting error that I haven't seen before:

#![feature(super_let)]

use std::cell::UnsafeCell;

const WUT: &Option<UnsafeCell<i32>> = {
    super let a: Option<UnsafeCell<i32>> = None;
    let x = &a;
    let ptr = x as *const _ as *mut i32;
    unsafe { ptr.write(1); }
    x
};

error[E0080]: constructing invalid value at .<deref>.<enum-variant(Some)>.0: encountered `UnsafeCell` in read-only memory
 --> src/lib.rs:5:1
  |
5 | const WUT: &Option<UnsafeCell<i32>> = {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ it is undefined behavior to use this value
  |
  = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
  = note: the raw bytes of the constant (size: 8, align: 8) {
              ╾───────alloc2────────╼                         │ ╾──────╼
          }

For more information about this error, try `rustc --explain E0080`.
error: could not compile `playground` (lib) due to 1 previous error

I don't know if this is a problem or not though.