Why does the index method require ownership?

Question

From the docs, the Index trait is defined:

pub trait Index<Idx> where Idx: ?Sized {
    type Output: ?Sized;
    fn index(&self, index: Idx) -> &Self::Output;
}

Since the type of the index parameter is Idx and not &Idx, the index method needs to take ownership of the value it is passed.

Is there a reason for this restriction? I know 9 times out of 10 one will be using something like an integer type that derives Copy, but I'm just curious why a borrowed value would be any less capable of acting as an index.

Answer 1

A borrowed value can be a perfectly good index, and the definition of the Index trait allows for that. Just use a reference as the index type. Nonsense example:

impl <'a> Index<&'a IndexType> for Foo {
    type Output = u8;
    fn index(&self, index: &IndexType) -> &u8 {
        unimplemented!()
    }
}

So the "restriction" of passing the index by value isn't really a restriction at all, because it allows the person implementing Index to choose if the index should be passed by value or by reference.