NARR limbo due to const_evaluatable_unchecked

[peter-lyons-kehl]

Of course, everything here is on Nightly Rust:

warning: cannot use constants which depend on generic parameters in types
   --> src/slices/mod.rs:48:20
    |
48  |         type NARR: $trait_name<'a, T, $narr_size>;
    |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
...
    = note: `#[warn(const_evaluatable_unchecked)]` on by default
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!

https://github.com/ranging-rs/slicing-rs/blob/main/src/slices/mod.rs#L48=

[peter-lyons-kehl]

Can't workaround by having an if/else expression in generic bounds:

trait Foo<const N: usize>
where
    [(); if N>0 { N } else {panic!() }]: Sized,
{
    type Narr: Foo<0>;
}

-> overly complex generic constant

Can't workaround with helper const value-generating macros:

/// Used for `NARR` associative types only. Purpose: When `disable_empty_arrays` feature is enabled, this makes `NARR` not be an array with 0 items (which would fail), but with one item.
#[cfg(not(feature = "disable_empty_arrays"))]
macro_rules! non_array_size {
    () => { 0usize };
}
#[cfg(feature = "disable_empty_arrays")]
macro_rules! non_array_size {
    () => { 1usize };
}

//...
// inside the owner trait:
type NARR: $trait_name<'a, T, non_array_size!()>;

Neither helps const:

#[cfg(not(feature = "disable_empty_arrays"))]
const NARR_SIZE: usize = 0;
#[cfg(feature = "disable_empty_arrays")]
const NARR_SIZE: usize = 1;

//...
// inside the owner trait:
type NARR: $trait_name<'a, T, NARR_SIZE>;