Motivation

The issue #2252 ia about a breaking change on fallback action of a nested Router. Previously, if the nested Router with a fallback is matched, it should call it's fallback when the route is not defined. Example:

https://github.com/dalton-oliveira/axum/blob/1313119b73663ea1f6e461197195ad89248b6e83/axum/src/routing/tests/nest.rs#L387-L397

This breaking change was introduced once it started to use matchit for routing to the fallback handler. It uses a special catch-all-parameter /*__private__axum_fallback which does't match empty path which would be the root of the nested router. There's no option for matching zero-or-more pattern. There's an open issue Support optional last param that, once solved, I think would be sufficient for this scenario and thus a better solution too.

Solution

This changes Router::nest to also nest the root for the fallback PathRouter

Fixes #2252