Nested fallback() routes regression with merge()

[birkmose]

• I have looked for existing issues (including closed) about this

Bug Report

Version

The error is observed in all versions between 0.6.13 - 0.6.18.

├── axum v0.6.13
│   ├── axum-core v0.3.4

Platform

Linux  5.19.0-45-generic #46~22.04.1-Ubuntu SMP PREEMPT_DYNAMIC Wed Jun 7 15:06:04 UTC 20 x86_64 x86_64 x86_64 GNU/Linux

Description

In versions 0.6.13 and beyond a regression with nested routers using fallback() and merge() is observed.
I suspect it relates to the flattening of routes introduced in #1711 .

The snippet below behaves differently between 0.6.12 and 0.6.13 - the nested fallback() is stripped from the flattened router.
If the merge(other_router) is removed the fallback will work as expected.

// 0.6.12 routes:
// /other -> "other"
// /nested/some-route -> "nested"
// /nested/foo -> "fallback"

// 0.6.13+ routes:
// /other -> "other"
// /nested/some-route -> "nested"
// /nested/foo -> 404
        
let nested_routes = Router::new()
    .route("/some-route", get(|| async { "nested" }))
    .fallback(|| async { "fallback" });

let other_router = Router::new()
    .route("/other", get(|| async { "other" }));

let app = Router::new()
    .nest("/nested", nested_routes)
    .merge(other_router); // <-- If removed the fallback() works as expected.

Interestingly if you add an outer fallback it will behave as expected:

// 0.6.13+ routes:
// /other -> "other"
// /nested/some-route -> "nested"
// /nested/foo -> "fallback"
// /foo/foo -> "outer fallback"
    
let nested_routes = Router::new()
    .route("/some-route", get(|| async { "nested" }))
    .fallback(|| async { "fallback" });

let other_router = Router::new()
    .route("/other", get(|| async { "other" }));

let app = Router::new()
    .nest("/nested", nested_routes)
    .fallback(|| async { "outer fallback" }) // <-- This seems to make the inner fallback work?
    .merge(other_router);

[davidpdrsn]

The fix is released in axum 0.6.19