What's your favorite Day 2 Rust language feature?

Let's say someone is transitioning from another language (e.g., Java or Python) to Rust. They've read The Rust Programming Language, completed Rustlings, and can now use Axum/Tokio to implement REST APIs using "Day 1" Rust features (e.g., enums, match, iterators, and all that jazz).

I’m curious, what non-basic (Day 2) Rust language features have enabled you the most? Something you discovered later on, but wish you had learned at the very start of your Rust journey?