> When immutable values (whether it's the number 5 or a record, or whatever) are used with a pure function then the returned value can be essentially a direct replacement for the function invocation, making the operation essentially take zero time (i.e. there aren't intermediate mutation states, locks, or any other coordination).

The same can be said of any variable in Rust that is passed to a function via immutable reference. Borrow checking guarantees (at compile time, with no runtime overhead) that even if a user passes something that supports mutation (such as a BTree or an array) to a function, that that function may not mutate unless it has permission to.

This means that if you have an immutable reference to something, it can be treated as a value. For example: a Vec or BTreeMap (structures that supports mutation) can safely and easily be used as Map keys in Rust, because borrow checking guarantees that once the Map has taken ownership of the structure, it "is a value": ie, is immutable.