Type which uses const generics leads to ICE when using Derive

[Matthias247]

This code

#![feature(const_generics)]

#[derive(Debug)]
struct X<const N: usize> {
    a: [u32; N],
}

leads to

 --> src/lib.rs:1:12
  |
1 | #![feature(const_generics)]
  |            ^^^^^^^^^^^^^^

error[E0107]: wrong number of const arguments: expected 1, found 0

thread 'rustc' panicked at 'assertion failed: `(left == right)`
  left: `[type error]`,
 right: `usize`', src/librustc/ty/sty.rs:2279:9
note: Run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
error: aborting due to previous error

For more information about this error, try `rustc --explain E0107`.

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

It doesn't seem Debug related, since the same happens for deriving Copy.

[hellow554]

@rustbot modify labels: I-ICE C-Bug A-const-generics

[HadrienG2]

I strongly suspect this might be because the derive(Debug) macro implementation will copy-paste N everywhere, whereas the current const generics implementation requires using {N} when instantiating a const generic type.

[varkor]

This now has only the error[E0107]: wrong number of const arguments: expected 1, found 0 error, which is the same as #61346, so maybe a fix for one will also address the other.

[andreytkachenko]

Can confirm, in case:

pub struct Test<const X: usize>(pub [usize; X]);

compiler complain that [T, _] not derives Debug, what is expected because it does not exists in core yet:

4 | pub struct Test<const X: usize>(pub [usize; X]);
  |                                 ^^^^^^^^^^^^^^ `[usize; _]` cannot be formatted using `{:?}` because it doesn't implement `std::fmt::Debug`
  |
  = help: the trait `std::fmt::Debug` is not implemented for `[usize; _]`
  = note: required because of the requirements on the impl of `std::fmt::Debug` for `&[usize; _]`
  = note: required for the cast to the object type `dyn std::fmt::Debug`