1d76dd9ee7
`impl const Default for Box<[T]>` and `Box<str>` The unstable `const_default_impls` (#87864) already include empty `Vec<T>` and `String`. Now we extend that concept to `Box<[T]>` and `Box<str>` as well. This obviates a hack in `rustc_ast`'s `P::<[T]>::new`.