Rust Terminologies

extern crate

In source code of some libraries, I see extern crate. I cannot use it in my code, how does it work?

https://doc.rust-lang.org/reference/items/extern-crates.html

An extern crate declaration specifies a dependency on an external crate.

Procedural Macros

https://doc.rust-lang.org/reference/procedural-macros.html

• Function-like macros - custom!(...)
• Derive macros - #[derive(CustomDerive)]
• Attribute macros - #[CustomAttribute]