Apple's C language extension of blocks in Rust.
This crate provides functionality for interacting with C blocks, which is the C-equivalent of Rust's closures.
They are technically not limited to only being used in Objective-C, though in practice it's likely the only place you'll ever encounter them.
See the docs for a more thorough overview.
This crate is part of the objc2 project,
see that for related crates.