dealing with cyclic (self-referential) values with const_heap

See the following snippet: (currently ICEs)

#![feature(core_intrinsics)]
#![feature(const_heap)]

use std::mem::{align_of, size_of};
use std::intrinsics;

struct SelfReferential {
    me: *const SelfReferential,
}

const Y: &'static SelfReferential = unsafe {
    let size = size_of::<SelfReferential>();
    let align = align_of::<SelfReferential>();
    let ptr_raw = intrinsics::const_allocate(size, align);
    let ptr = ptr_raw as *mut SelfReferential;
    let me_addr = &raw mut (*ptr).me;
    *me_addr = ptr;
    intrinsics::const_make_global(ptr_raw, size, align);
    &*ptr
};

fn main() {}

• me's provenance says it wasn't derived from an immutable reference. We can't really change that because it can't be mutated after we call const_make_global.
• The fact above hits "accepted a mutable pointer that should not have accepted" at https://github.com/rust-lang/rust/blob/040e2f8b9ff2d76fbe2146d6003e297ed4532088/compiler/rustc_const_eval/src/interpret/intern.rs#L247-L265
• We can check if alloc_id has kind Heap { was_made_immut: true } and skip the error that way.
• intern_shallow will keep returning alloc2 adding to our todo list being weird left and right. I added a .filter in https://github.com/rust-lang/rust/blob/040e2f8b9ff2d76fbe2146d6003e297ed4532088/compiler/rustc_const_eval/src/interpret/intern.rs#L311-L312 to skip provs that are in just_interned.
• Apparently that wasn't enough and rustc hits an infinite loop at some point. The stack trace isn't helpful enough and I didn't have enough time to find a way to debug the stack overflow.

Originally posted by @fee1-dead in https://github.com/rust-lang/rust/pull/143595#discussion_r2192969346

Start with the const_heap reproducer in the issue and inspect rustc_const_eval/src/interpret/intern.rs at the mutable-pointer check and the intern_shallow filtering around the linked lines. Trace the alloc_id kind, just_interned handling, and todo-list behavior to identify the infinite loop; done means the reproducer no longer ICEs or overflows the stack.