Add `Box<[T; N]>: TryFrom<Vec<T>>` We have `[T; N]: TryFrom<Vec<T>>` (#76310) and `Box<[T; N]>: TryFrom<Box<[T]>>`, but not this combination. `vec.into_boxed_slice().try_into()` isn't quite a replacement for this, as that'll reallocate unnecessarily in the error case. **Insta-stable, so needs an FCP** (I tried to make this work with `, A`, but that's disallowed because of `#[fundamental]` https://github.com/rust-lang/rust/issues/29635#issuecomment-1247598385)