I just came across a line of code in the first example on this page: State in axum::extract - Rust
async fn handler(
// access the state via the `State` extractor
// extracting a state of the wrong type results in a compile error
State(state): State<AppState>,
) {
// use `state`...
}
Notice the left hand side of the parameter. I can't find this type of syntax mentioned anywhere in the book. Why and how is this used and where can I read more about it?