Recursive diagnostics when using consts in arrays

[SeeSpring]

I tried this code:

trait A {
    const N: usize;
    const ARRAY: [u8; Self::N];
}

trait B<T: A> {
    fn f() -> [u8; <T as A>::N];
}

Playground

I expected to see this happen: the compiler gives useful diagnostics

Instead, this happened: the compiler gives diagnostics which don't work

   Compiling playground v0.0.1 (/playground)
error[E0599]: no associated item named `N` found for type parameter `Self` in the current scope
 --> src/lib.rs:3:29
  |
3 |     const ARRAY: [u8; Self::N];
  |                             ^ associated item not found in `Self`
  |
  = help: items from traits can only be used if the type parameter is bounded by the trait
help: the following trait defines an item `N`, perhaps you need to add a supertrait for it:
  |
1 | trait A: A {
  |        ^^^

error[E0277]: the trait bound `T: A` is not satisfied
 --> src/lib.rs:7:20
  |
2 |     const N: usize;
  |     --------------- required by `A::N`
...
7 |     fn f() -> [u8; <T as A>::N];
  |                    ^^^^^^^^^^^ the trait `A` is not implemented for `T`
  |
help: consider further restricting this bound
  |
6 | trait B<T: A + A> {
  |              ^^^

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0277, E0599.
For more information about an error, try `rustc --explain E0277`.
error: could not compile `playground`.

To learn more, run the command again with --verbose.

Meta

rustc --version --verbose:

rustc 1.45.0-nightly (74e804683 2020-05-30)
binary: rustc
commit-hash: 74e80468347471779be6060d8d7d6d04e98e467f
commit-date: 2020-05-30
host: x86_64-pc-windows-msvc
release: 1.45.0-nightly
LLVM version: 10.0

[estebank]

cc @eddyb

[Dylan-DPC]

Current output:

error: generic parameters may not be used in const operations
 --> src/lib.rs:3:23
  |
3 |     const ARRAY: [u8; Self::N];
  |                       ^^^^^^^ cannot perform const operation using `Self`
  |
  = note: type parameters may not be used in const expressions
  = help: use `#![feature(generic_const_exprs)]` to allow generic const expressions

error: generic parameters may not be used in const operations
 --> src/lib.rs:7:21
  |
7 |     fn f() -> [u8; <T as A>::N];
  |                     ^ cannot perform const operation using `T`
  |
  = note: type parameters may not be used in const expressions
  = help: use `#![feature(generic_const_exprs)]` to allow generic const expressions

error: could not compile `playground` (lib) due to 2 previous errors

[estebank]

The only thing I'd like to change is to make it clear that the array length is a const expression, besides that I believe we can close this ticket.