Why is the bound `T: 'a` required in order to store a reference `&'a T`?

Question

Given this code:

struct RefWrapper<\'a, T> {
    r: &\'a T,
}

... the compiler complains:

error: the param

Answer 1

This is part of the well-formedness rules. The type &'a T is only well-formed if T: 'a (“T outlives 'a”; it is required because we have a reference which we can access during the scope 'a; the pointed-to value in T needs to be valid for at least that scope, too).

struct RefWrapper<'a, T> is a generic type and it says you can input a lifetime 'x and a type U and get a RefWrapper<'x, U> type back. However, this type is not necessarily well-formed or even implemented unless the requirement T: 'a is respected.

This requirement comes from an implementation detail; it's not necessarily so that T and 'a are used together like &'a T in the struct's internals. The well formedness requirement needs to be promoted to the public interface of the RefWrapper struct, so that the requirements of forming a RefWrapper<'_, _> type are public, even if the internal implementation is not.

(There are other places where the same requirement T: 'a comes back but is implict:

pub fn foo<'a, T>(x: &'a T) { }

we spot a difference: here the type &'a T is part of the public api, too.)