Another form of the same issue AFAICT: #86510

On latest nightly, black_box(&x) prevents codegenning a constant return value, but it shouldn't.

This is possibly because of over pessimism about interior mutability, but we should be able to tell whether a type is interior mutable or not.

Godbolt

#![feature(bench_black_box)]

use std::hint::black_box;

pub fn foo() -> i32 {
    let x = 1;
    black_box(&x);
    x
}

The return instruction moves from a location on the stack:

        mov     eax, dword ptr [rsp + 4]

Whereas for this:

#![feature(bench_black_box)]

use std::hint::black_box;

pub fn foo() -> i32 {
    let x = 1;
    let y = x;
    black_box(&y);
    x
}

It optimizes correctly.

        mov     eax, 1