Typecheck error with SmallVec crate

[y21]

The following produces a R-A error, but compiles just fine:

use smallvec::SmallVec;

const SIZE: usize = 1;

fn bug(ve: &SmallVec<[Option<()>; SIZE]>) -> Option<Option<&()>> {
    ve.iter().next().map(|x| x.as_ref())
}

fn main() {}

I haven't been able to come up with a minimal example that does not use SmallVec. ve.iter() is inferred as Iter<{unknown}> by RA in that function. Note that this does not error if the array size is written in place: [Option<()>; 1].

This does not happen if I simply replace SmallVec with some type that implements Deref<Target=[Option<()>]> or &[Option<()>; SIZE] itself.

rust-analyzer version: rust-analyzer version: 24cf957 2022-04-11 stable

rustc version: rustc 1.62.0-nightly (878c7833f 2022-04-16)

[bjorn3]

(@matklad looks like I lost triage and write permission with the migration to the rust-lang org)

[HKalbasi]

We don't evaluate const generics yet (#7434) so SmallVec<[Option<()>; SIZE]> becomes SmallVec<[Option<()>; _]>. And (unless you enable const_generic feature of small vec, with that it should work) there are some random impls for its Array trait (1, 2, 5, 1024, ...) which _ is not necessarily in them.