What can be bound to a slice?

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1

What mechanism makes &Vec<T> bindable to &[T] (as in the following code)?

let v = vec![1,2,3];
let x: &[i32] = &v;

Put another way, is it possible to make the last line in the following sample, compile?

struct W(Vec<i32>);
let w = W(vec![1,2,3]);
let y: &[i32] = &w;

I'm expecting there to be some trait that is responsible for this, but I'm failing to find it.

2

You are quite right regarding this. There are the Deref and DerefMut traits which allow a type to be borrowed as another type, ie, by default borrowing T will always create &T or &mut T. However, as you have noted for Vec, it can be borrowed as &Vec<T> or as &[T]. This is because Vec has impls for Deref and for DerefMut.
This automatic "conversion", ie, deciding when to apply the Deref trait is known as deref coercion.

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There is more information on coercions (including a list of places where the compiler allows them) here: Type coercions - The Rust Reference

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