refcell_ref.md

Example

The Ref and RefMut types taken from core::cell::RefCell:

pub struct Ref<'b, T: ?Sized + 'b> {
    value: &'b T,
    borrow: BorrowRef<'b>,
}

pub struct RefMut<'b, T: ?Sized + 'b> {
    value: &'b mut T,
    borrow: BorrowRefMut<'b>,
}

Explanation

What is interesting about these types are the value fields. They use a normal Rust type, but in fact the guarantees for this are slightly weaker than normal: the reference is valid until either (a) the end of 'b OR (b) the field borrow is dropped, whichever happens first. This means that the type appears entirely "safe", but in fact there are extra requirements, which speaks to the idea of an "unsafety frontier" that is also explored in other litmus tests such as usize_transfer.

Source

An email from Ralf Jung.