panic does not compile intoResponse in Rust 1.85 and edition2024

[Havunen]

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

Bug Report

Version

cargo tree | grep axum

├── axum v0.8.1
│   ├── axum-core v0.5.0

Platform

Linux linuxdesktop 6.12.8-arch1-1 #1 SMP PREEMPT_DYNAMIC Thu, 02 Jan 2025 22:52:26 +0000 x86_64 GNU/Linux

Steps to reproduce code:

Cargo.toml

edition = "2024"
rust-version = "1.85.0"

main.rs

use axum::Router;
use axum::routing::get;

#[tokio::main]
async fn main() {
    let app = Router::new()
        .route("/fail2", get(|| async { panic!("panic") }));

    let listener = tokio::net::TcpListener::bind("0.0.0.0").await.unwrap();
    axum::serve(listener, app).await.unwrap();
}

compiler errors:

error[E0277]: the trait bound `!: IntoResponse` is not satisfied
   --> crates/ae_api/src/main.rs:7:30
    |
7   |         .route("/fail2", get(|| async { panic!("panic") }));
    |                          --- ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `IntoResponse` is not implemented for `!`
    |                          |
    |                          required by a bound introduced by this call
    |
    = help: the following other types implement trait `IntoResponse`:
              &'static [u8; N]
              &'static [u8]
              &'static str
              ()
              (R,)
              (Response<()>, R)
              (Response<()>, T1, R)
              (Response<()>, T1, T2, R)
            and 120 others
    = note: this error might have been caused by changes to Rust's type-inference algorithm (see issue #48950 <https://github.com/rust-lang/rust/issues/48950> for more information)
    = help: did you intend to use the type `()` here instead?
    = note: required for `{closure@crates/ae_api/src/main.rs:7:30: 7:32}` to implement `Handler<((),), ()>`
note: required by a bound in `axum::routing::get`
   --> /.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/axum-0.8.1/src/routing/method_routing.rs:440:1
    |
440 | top_level_handler_fn!(get, GET);
    | ^^^^^^^^^^^^^^^^^^^^^^---^^^^^^
    | |                     |
    | |                     required by a bound in this function
    | required by this bound in `get`
    = note: this error originates in the macro `top_level_handler_fn` (in Nightly builds, run with -Z macro-backtrace for more info)

edition2021 compiles without errors.

Maybe its related to this: rust-lang/rust#123748

[Turbo87]

Since we can't impl IntoResponse for ! due to rust-lang/rust#35121, I don't think there is much we can do about this 🤔

[jplatte]

Yeah we can impl this once ! is stable.

[Havunen]

I quickly skimmed through that issue.
Can Infallible be used to impl IntoResponse?

https://doc.rust-lang.org/std/convert/enum.Infallible.html

[jplatte]

That's already implemented: https://docs.rs/axum-core/0.5.0/axum_core/response/trait.IntoResponse.html#impl-IntoResponse-for-Infallible