Suggest `.try_into().unwrap()` or `as _` for const parameters of the wrong type (e.g., in array types) if applicable and possible on stable

[scvalex]

I tried this code:

pub struct MaxLengthString<const N: u8> {
    len: u8,
    buf: [u8; N],
}

And I got this compilation error:

error[E0308]: mismatched types
 --> src/lib.rs:3:15
  |
3 |     buf: [u8; N],
  |               ^ expected `usize`, found `u8`

For more information about this error, try `rustc --explain E0308`.

This works fine if the const parameter is of type usize, but I think it should work for the other unsigned types that are smaller than usize. Or at the very least, maybe there should be a different error message that says this is definitely not supported.

Meta

rustc --version --verbose:

rustc 1.67.0-nightly (8681d4cff 2022-11-25)
binary: rustc
commit-hash: 8681d4cffcd23bbe619984ab62772a91827a40dc
commit-date: 2022-11-25
host: x86_64-unknown-linux-gnu
release: 1.67.0-nightly
LLVM version: 15.0.4

[Noratrieb]

Only allowing usize is intended, but improvements to error messages are of course always welcome :)

[HintringerFabian]

@rustbot claim

[jruderman]

This alternative works, but I don't know if it's appropriate for diagnostics to suggest it:

#![feature(generic_const_exprs)]
#![allow(incomplete_features)]

pub struct MaxLengthString<const N: u8> 
where [(); N as usize]:
{
    len: u8,
    buf: [u8; N as usize],
}

[estebank]

The logic to produce that suggestion exists in rustc, but it is dormant under the assumption that we want to move people as much as possible towards into and try_into, but we already know they are not available in const context, so we might want to start giving them in that case.

[jruderman]

Why aren't into and try_into available in const?

[Noratrieb]

Const traits are far from stable.

[gyscos]

So to confirm, currently, there is no way on stable to turn a generic <const N: u8> into an array size?