Associated things not found even though they do exist when used as array length

[djc]

struct Foo;

impl Bar for Foo {
    const BAR: usize = 1;
}

trait Bar {
    const BAR: usize;
}

struct Array<S> where S: Bar {
    data: [u8; S::BAR],
    s: S,
}

This errors out with:

error[E0599]: no associated item named `BAR` found for type `S` in the current scope
  --> src/lib.rs:12:19
   |
12 |     data: [u8; S::BAR],
   |                   ^^^ associated item not found in `S`
   |
   = help: items from traits can only be used if the trait is implemented and in scope
   = note: the following trait defines an item `BAR`, perhaps you need to implement it:
           candidate #1: `Bar`

Even though S::BAR clearly does exist. The same thing happens for constants within associated types (so T::Bar::BAR), with E0220, and this confused me a whole bunch. Instead apparently the problem is that array sizes currently require a literal or some other more consty consts (is that part of #133)?

cc @estebank.

[hellow554]

Probably dup of #43408

[estebank]

It seems to be the same underlying problem, although it is hit in a separate part of the evaluation.

From #43408 (comment):

This is a limitation of array length - it can't use any parameters in scope - which also came up during the stabilization of associated consts (...) and the decision taken was to stabilize associated consts without it.

The exact error here comes from the type parameter being resolved, but the ty::Generics and ty::GenericPredicates (bounds, including the default Sized on type parameters) of the array length are actually both empty. They have to be (for now), otherwise we get cycle errors.

[estebank]

CC @rust-lang/wg-diagnostics if anyone wants to improve the error message while we await for this code to be accepted, but I would much rather we actually made the changes to make this code valid.

[estebank]

If you try to outsmart the compiler and write this in a more explicit way, you get the a similar error as #43408, but even more galling because it recommends the same code that already exists:

error[E0277]: the trait bound `S: Bar` is not satisfied
  --> src/lib.rs:12:16
   |
12 |     data: [u8; <S as Bar>::BAR],
   |                ^^^^^^^^^^^^^^^ the trait `Bar` is not implemented for `S`
   |
   = help: consider adding a `where S: Bar` bound
note: required by `Bar::BAR`
  --> src/lib.rs:8:5
   |
8  |     const BAR: usize;
   |     ^^^^^^^^^^^^^^^^^

rustc should say something along the lines of "you can't do this just yet".

[estebank]

Current output, still problematic:

error[E0599]: no associated item named `BAR` found for type `S` in the current scope
  --> file12.rs:12:19
   |
12 |     data: [u8; S::BAR],
   |                   ^^^ associated item not found in `S`
   |
   = help: items from traits can only be used if the type parameter is bounded by the trait
help: the following trait defines an item `BAR`, perhaps you need to restrict type parameter `S` with it:
   |
11 | struct Array<S: Bar> where S: Bar {
   |              ^^^^^^

[brunoczim]

Are there any ideas about when this will be possible (using generic associated constants on array lenght)?

[eddyb]

Closing as duplicate of #43408.