This library provides interior mutability that can be borrowed
as plain immutable references
&T in exchange for the write-once,
std::cell::RefCell, a plain
&T can be taken from
Once an immutable reference is taken, the value can never be mutated
(even after all references are dropped).
The use cases include caching getter and delayed evaluation.
A mutable memory location that is write-once and can be borrowed as
A wrapper type for a mutably borrowed value from a