generic_const_exprs: unification does not look into const items

[lcnr]

#![feature(generic_const_exprs)]
use std::marker::PhantomData;

struct Nil;
struct Cons<N>(PhantomData<N>);

trait Len {
    const N: usize;
}

impl<N: Len> Len for Cons<N> {
    const N: usize = 1 + N::N;
}

impl Len for Nil {
    const N: usize = 0;
}

trait Piece<T> {
    type Size: Len;

    fn construct(self) -> [T; Self::Size::N];
}

impl<T: Copy + From<i32>> Piece<T> for i32 {
    type Size = Cons<Nil>;

    fn construct(self) -> [T; Self::Size::N] {
        [T::from(self)]
    }
}

impl<T: Copy, U: Piece<T>> Piece<T> for (U,) {
    type Size = U::Size;

    fn construct(self) -> [T; Self::Size::N] {
        todo!()
    }
}

this results in the following which definitely seems wrong

error: unconstrained generic constant
  --> src/lib.rs:36:31
   |
36 |     fn construct(self) -> [T; Self::Size::N] {
   |                               ^^^^^^^^^^^^^
   |
   = help: try adding a `where` bound using this expression: `where [(); Self::Size::N]:`
note: the requirement `the constant `<(U,) as Piece<T>>::construct::{constant#0}` can be evaluated` appears on the impl method `construct` but not on the corresponding trait method
  --> src/lib.rs:22:8
   |
19 | trait Piece<T> {
   |       ----- in this trait
...
22 |     fn construct(self) -> [T; Self::Size::N];
   |        ^^^^^^^^^ this trait method doesn't have the requirement `the constant `<(U,) as Piece<T>>::construct::{constant#0}` can be evaluated`

error[E0308]: method not compatible with trait
  --> src/lib.rs:36:5
   |
36 |     fn construct(self) -> [T; Self::Size::N] {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `Self::Size::N`, found `Self::Size::N`
   |
   = note: expected type `Self::Size::N`
              found type `Self::Size::N`

[lcnr]

minimized

#![feature(generic_const_exprs)]

trait Foo {
    const ASSOC: usize;
    fn construct(self) -> [u8; Self::ASSOC];
}

impl<const N: usize> Foo for [u8; N] {
    const ASSOC: usize = N;
    fn construct(self) -> [u8; N] {
        self
    }
}

when looking at the signature of construct, we expect Unevaluated(Self::ASSOC, [Self]) but get Param(N). As we don't look into associated consts rn this causes an error

[lcnr]

tracked in rust-lang/project-const-generics#31

[xbjfk]

I ran into a similar issue with this feature + adt_const_params. Playground. I'm not sure how related this is - let me know if this should be a new issue.

[BoxyUwU]

not related, you need to add that where clause. generic const exprs wants to avoid having generic constants that might not succesfully evaluate in the type system so we require those where clauses which look funny ✨