Auto merge of #91680 - saethlin:spare_capacity_mut-in-join, r=dtolnay

Auto merge of #91680 - saethlin:spare_capacity_mut-in-join, r=dtolnay

Use spare_capacity_mut instead of invalid unchecked indexing when joining str

This is a fix for https://github.com/rust-lang/rust/issues/91574

I think in general I'd prefer to see this code implemented with raw pointers or `MaybeUninit::write_slice`, but there's existing code in here based on copying from slice to slice, so converting everything from `&[T]` to `&[MaybeUninit<T>]` is less disruptive.