1.59 accidentally stabilized dropping Box<T>s in const fns

[steffahn]

const fn f<T>(_: Box<T>) {}

(playground)

Note that this works even if T implements Drop in a non-const way. This seems hightly problematic, especially provided we eventually want to support constructing Box at compile time.

Relevant PR: #91884

Of course that impl claims to be unstable, but as we all know trait implementations are never unstable, unless the trait is unstable. (For ordinary traits, implementations of const Trait are thus unstable merely due to the fact that no user can use any such implementations in stable code yet.)

@rustbot label regression-from-stable-to-stable

I don’t know what team to tag.

The fix should be straightforward: Remove the const Drop for Box<T, A> implementation (i.e. revert it to be non-const), and add a test.

[oli-obk]

cc @rust-lang/wg-const-eval

[lcnr]

#[rustc_const_unstable(feature = "const_box", issue = "92521")]
unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> const Drop for Box<T, A> {
    fn drop(&mut self) {
        // FIXME: Do nothing, drop is currently performed by compiler.
    }
}

that impl should be illegal, shouldn't it? I would expect us to need an explicit T: ~const Drop here. But also, reverting and maybe even backporting that impl for now seems like the right choice to me

[RalfJung]

Good catch!

Actually calling this leads to odd behavior:

const fn f(_: Box<i32>) {}

pub const C: () = {
    unsafe { f(std::mem::transmute(&0)) };
};

error: any use of this value will cause an error
    |
   ::: src/lib.rs:3:1
    |
3   | / pub const C: () = {
4   | |     unsafe { f(std::mem::transmute(&0)) };
5   | | };
    | |__-
    |
    = note: `#[deny(const_err)]` on by default
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, [see issue #71800 <https://github.com/rust-lang/rust/issues/71800>](https://github.com/rust-lang/rust/issues/71800)

It doesn't even say what went wrong. I expected an error saying a non-const function was called as part of the Box drop glue.