# `unsized_tuple_coercion`

The tracking issue for this feature is: #42877

This is a part of RFC0401. According to the RFC, there should be an implementation like this:

    impl<..., T, U: ?Sized> Unsized<(..., U)> for (..., T) where T: Unsized<U> {}

This implementation is currently gated behind `#[feature(unsized_tuple_coercion)]` to avoid insta-stability. Therefore you can use it like this:

    #![feature(unsized_tuple_coercion)]

    fn main() {
        let x : ([i32; 3], [i32; 3]) = ([1, 2, 3], [4, 5, 6]);
        let y : &([i32; 3], [i32]) = &x;
        assert_eq!(y.1[0], 4);
    }