For watching and loading a Rust library in Rust.

You are likely looking for the watch function docs.

hotlib

A library for watching, dynamically compiling, and hot-loading Rust libraries.

Try running:

cargo run --example demo

And while the demo is running, try editing the test_crate/src/lib.rs file. Each time you write your changes to disk, the demo should automatically detect the change, return the package, build the package and then load the corresponding dynamic library.