Lineage

A small rust crate that provides a type of cell that can replace its contained value while the previous value is still immutably borrowed:

impl Lineage {
    pub fn new(value: T) -> Self;

    pub fn get(&self) -> &T;

    pub fn set(&self, value: T);

    pub fn clear(&mut self);
}

Notice how a new value can be inserted into the cell with Lineage::set using only &self. This means the Lineage may still be borrowed by previous calls to Lineage::get but you can replace the contained value anyways. Internally the replaced value is added to a linked list which is not cleared until you call Lineage::clear or drop the Lineage.

let lineage: Lineage<String> = Lineage::new(String::from("ONE"));
let s1 = lineage.get();

lineage.set(String::from("TWO"));
let s2 = lineage.get();

assert_eq!(s1, "ONE");
assert_eq!(s2, "TWO");

Safety

As is expected for this kind of utility crate, the implementation makes use of unsafe. We have a number of tests to look for undefined behavior which can all be run natively or with miri. miri is a fantastic tool to execute rust applications in a virtual runtime that is sensitive to various kinds of undefined behavior.

# run tests normally
cargo test

# install miri
rustup toolchain install nightly
rustup +nightly component add miri

# run tests in miri
cargo clean
cargo +nightly miri test