Axum Handler Trait Errors

Overview

Axum has no handler macro. A function becomes a handler purely by satisfying the Handler<T, S> trait, which has a single blanket implementation guarded by a long conjunction of bounds. When a function fails ANY one bound, the compiler reports the WHOLE impl as unsatisfied. It cannot point at the failing conjunct, so it emits the opaque error[E0277]: the trait bound ... Handler<_, _> is not satisfied, names the trait with inference placeholders, and lists unrelated Handler impls as "help".

Every such error reduces to exactly one of five root causes. The diagnostic tool is the #[debug_handler] attribute macro from axum-macros: it rewrites the opaque error into a precise, plain-language one that points at the exact argument or return type at fault.