Cannot borrow from generic Copy types in a const context

[kupiakos]

I tried this code:

trait C: Copy + 'static {
    type Val: Copy + 'static;
    const VAL: Self::Val;
    const S: Self;
}

#[derive(Clone, Copy)]
struct FooGen<T: C>(T);
impl<T: C> FooGen<T> {
    const VAL: &'static Self = &FooGen(T::S);
}

struct FooAssoc<T: C>(T::Val);
impl<T: C> Clone for FooAssoc<T> { fn clone(&self) -> Self { *self } }
impl<T: C> Copy for FooAssoc<T> {}
impl<T: C> FooAssoc<T> {
    const VAL: &'static Self = &FooAssoc(T::VAL);
}

I expected the Copy bounds to be sufficient to allow this to compile. Copy is an imperfect analog for a lack of interior mutability, much like it is an imperfect analog for !Drop.

Instead, this happened:

error[E0492]: constants cannot refer to interior mutable data
  --> src/lib.rs:10:32
   |
10 |     const VAL: &'static Self = &FooGen(T::S);
   |                                ^^^^^^^^^^^^^ this borrow of an interior mutable value may end up in the final value

error[E0492]: constants cannot refer to interior mutable data
  --> src/lib.rs:21:32
   |
21 |     const VAL: &'static Self = &FooAssoc(T::VAL);
   |                                ^^^^^^^^^^^^^^^^^ this borrow of an interior mutable value may end up in the final value

Ideally, stable code would be able to bound on a lack of interior mutability (like a NoCell/Freeze trait) to allow this sort of generic code to work without needing const_refs_to_cell stabilized, as it's a useful property to be able to bound on for unsafe code.

Meta

1.7.0-stable (2023-12-21 82e1608)

[fmease]

cc #121675, #121840

[oli-obk]

Duplicate of #60715

[RalfJung]

Copy is an imperfect analog for a lack of interior mutability

It is not, and anyone relying on this is risking breakage in future versions of Rust. It is quite probable that we will eventually allow people to do something like unsafe impl Copy for, at which point one can easily have Freeze + Copy types.