axum-errors-handler-trait

To fix the error E0277 when an Axum handler does not satisfy the Handler trait, apply the `#[debug_handler]` attribute to get precise error spans. This maps the opaque error to one of five root causes, such as body extractor ordering or non-async handlers, for targeted resolution.

An Axum route compilation fails with an unsatisfied blanket impl because the handler function violates trait requirements. Common causes include non-extractor arguments, body extractor placement, return types not implementing `IntoResponse`, non-async handlers, or `!Send` futures.

Use `axum::debug_handler` by adding the attribute macro to your rejected handler and recompiling. This converts opaque compiler output into precise spans, allowing you to read the exact error and apply the matching fix from the five-cause decision tree.

Yes, the `debug_handler` macro works with Axum 0.7 and 0.8 state type overrides. You can specify the state type using `#[debug_handler(state = ...)]` to accurately diagnose handler trait failures across both versions.

The `macros` feature flag is required to use `axum::debug_handler` for compile errors. You must enable this feature on the `axum` crate dependency before applying the attribute to identify handler trait issues.

A limitation of `axum::debug_handler` when resolving opaque compile failures is the `impl`-block-without-`self` limitation. Additionally, it requires the `macros` feature flag and cannot diagnose issues outside the five primary handler trait failure causes.