How to get Deref coercion when using impl Trait

Question

This function returns the first element of a list-like collection. It works for a variety of different list-like types:

fn first<T: Copy>(x: impl Deref<Target=[T]>) -> T {
    x[0]
}

For example, this compiles and runs:

let data: Vec<usize> = vec![3, 4];
assert_eq!(first(data), 3);

let data: &[usize] = &[3, 4];
assert_eq!(first(data), 3);

let data: Rc<[usize]> = Rc::new([3, 4]);
assert_eq!(first(data), 3);

This also compiles and runs:

fn stub(x: &[usize]) -> usize {
    first(x)
}

let data: &[usize; 2] = &[3, 4];
assert_eq!(stub(data), 3);

assert_eq!(stub(&[3, 4]), 3);

But this fails to compile:

let data: &[usize; 2] = &[3, 4];
assert_eq!(first(data), 3); // Fails.

assert_eq!(first(&[3, 4]), 3); // Fails.

The error message is:

type mismatch resolving `<&[usize; 2] as std::ops::Deref>::Target == [_]`

I think I understand what is going on. For each type T there is a unique type <T as Deref>::Target. When T is &[usize; 2] the target is [usize; 2], not [usize]. The compiler is able to coerce &[T; 2] to &[T] if I explicitly ask it to, e.g. by using let or stub(), but if I don't then it's not able to work out that the coercion is required.

But it's frustrating. It's perfectly obvious to a human what the failing calls are intended to do, and the compiler understands what's required for Vec<usize>, Box<[usize]>, Rc<[usize]>, &[usize] and so on, so it doesn't seem unreasonable to try to make it work for [usize; 2] as well.

Question: Is there a convenient way to write first() so that the last two calls work too? If not, is there a syntax to ask the compiler to coerce a &[usize; 2] to a &[usize] inline, i.e. without using let or stub()?

Playground.

Answer 1

You want to use AsRef, not Deref:

use std::rc::Rc;

fn first<T: Copy>(x: impl AsRef<[T]>) -> T {
    x.as_ref()[0]
}

fn main() {
    let data: Vec<usize> = vec![3, 4];
    assert_eq!(first(data), 3);

    let data: &[usize] = &[3, 4];
    assert_eq!(first(data), 3);

    let data: Rc<[usize]> = Rc::new([3, 4]);
    assert_eq!(first(data), 3);

    let data: &[usize; 2] = &[3, 4];
    assert_eq!(first(data), 3);
}