T is a superset of disjoint sets &T and &mut T

• T, &T, and &mut T are all infinite sets, since it’s possible to borrow a type ad-infinitum.
• T is a superset of both &T and &mut T.
• &T and &mut T are disjoint sets.

(from ¹)

trait Trait {}
impl<T> Trait for &T {}
impl<T> Trait for &mut T {}

Type Variable	T	&T	&mut T
Examples	i32, &i32, &mut i32, &&i32, &mut &mut i32, …	&i32, &&i32, &&mut i32, …	&mut i32, &mut &mut i32, &mut &i32, …

Footnotes

1. https://github.com/pretzelhammer/rust-blog/blob/master/posts/common-rust-lifetime-misconceptions.md#1-t-only-contains-owned-types ↩