~const bounds do not work when impling const Drop

[beepster4096]

I encountered this issue while trying to remove box_free.

I tried this code:

#![feature(const_trait_impl, const_fn_trait_bound, const_mut_refs)]

trait X {
    fn x();
}

struct Y<T: X>(T);

impl<T: ~const X> const Drop for Y<T> {
    fn drop(&mut self) {
        T::x();
    }
}

I expected the code to compile.

Instead, I got the following error:

error[E0367]: `Drop` impl requires `T: X` but the struct it is implemented for does not
 --> src/lib.rs:9:9
  |
9 | impl<T: ~const X> const Drop for Y<T> {
  |         ^^^^^^^^
  |
note: the implementor must specify the same requirement
 --> src/lib.rs:7:1
  |
7 | struct Y<T: X>(T);
  | ^^^^^^^^^^^^^^^^^^

Meta

rustc --version --verbose:

rustc 1.60.0-nightly (22e491ac7 2022-01-13)
binary: rustc
commit-hash: 22e491ac7ed454d34669151a8b6464cb643c9b41
commit-date: 2022-01-13
host: x86_64-pc-windows-msvc
release: 1.60.0-nightly
LLVM version: 13.0.0

[lilasta]

workaround: play

#![feature(const_trait_impl, const_fn_trait_bound, const_mut_refs)]

trait X {
    fn x();
}

struct Y<T: X>(T);

impl<T: X> const Drop for Y<T> {
    fn drop(&mut self)
    where
        T: ~const X
    {
        T::x();
    }
}

[compiler-errors]

Might be enough to just replace ConstIfConst with NotConst when comparing the list of predicates in dropck, if someone wants to try that.

[crlf0710]

cc @fee1-dead

[beepster4096]

workaround: play

This workaround causes the type to impl ~const Drop when it shouldn't causing errors when interpreting instead of during type checking. (playground) This needs to be fixed too.

Might be enough to just replace ConstIfConst with NotConst when comparing the list of predicates in dropck, if someone wants to try that.

This isn't enough. The constness of the bound seems to not get checked at all.

[beepster4096]

I'm not exactly sure how trait selection works, but I think the problem may be here, where only the constness of the impl is checked, and not any of the bounds.

[compiler-errors]

@drmeepster you wouldn't believe that the same exact line was on my clipboard, and I was going to say that it was somewhere around assemble_const_drop_candidates 😆

Yeah, it seems like we need to enforce that the ~const bounds on the drop impl are actually const during confirmation.

[compiler-errors]

Perhaps need to add some real confirmation logic here: https://github.com/rust-lang/rust/blob/master/compiler/rustc_trait_selection/src/traits/select/confirmation.rs#L162

[compiler-errors]

I'll take a stab at this, if nobody else is planning on it already!
@rustbot claim

cc @fee1-dead:
I'm planning on rewriting assemble_const_drop_candidates to prefer selecting a user-provided impl const Drop if it is provided (e.g. by an ADT), and fleshing out a real confirm_const_drop_candidate so we make sure to check those obligations that come from the user-provided impl const Drop.