Rust Axum/Actix handler type errors — extractor ordering

Rust web framework (Axum/Actix) gives cryptic type errors when defining route handlers. The error mentions traits not implemented or type mismatches for Handler, FromRequest, or similar traits.

(1) Axum: extractors must be in the correct order. Body-consuming extractors (Json, String, Bytes) must be LAST — only one can consume the body. (2) State extractor: State(state) can be anywhere. Query, Path, Headers can be in any order before body. (3) Common error: returning the wrong type. Handlers must return impl IntoResponse. (4) If using custom types as extractors: implement FromRequestParts (for non-body) or FromRequest (for body). (5) The error messages are long — read the 'note' at the bottom for the actual missing trait. (6) Actix: similar rules but uses web::Json, web::Path, web::Query. (7) Use #[debug_handler] (axum) macro for better error messages.

Rust web frameworks use the type system to extract request data. Each extractor implements a trait (FromRequest), and the framework generates code to call them in order. Type errors mean the compiler can't find the right trait implementation.