Support for `const` variable in array init

[JoshMcguigan]

Currently, arr_macro does not support const variables in array initialization as shown below. This is because the macro needs to know the exact value of SIZE, but the compiler expands macros before doing name resolution.

const SIZE: usize = 3;
arr![None; SIZE];

See additional details at dtolnay/syn#101

[kaimast]

Is there a rust bug associated with this? All the issues linked seem to be closed or archived.

[daxpedda]

Is there a rust bug associated with this?

https://github.com/rust-lang/rust/issues/52393

[yegor256]

@JoshMcguigan what is a possible workaround?

[DrRuhe]

I ran into this while using const generics to specify an array size and trying to fill that array with None without T being Copy or Default. I solved it by using this snippet:

let array: [Option<T>; N] = (0..N).map(|_| None).collect::<Vec<_>>().try_into().ok().unwrap();

The .ok() is needed to relax the Debug trait bound on T.

It is obviously not ideal but for my purposes a good enough workaround.

[PENGUINLIONG]

Is there a plan to support constant generic?