implementation of `Reply` is not general enough

[dhedberg]

Version
warp v0.4.1

Platform
Linux fedora 6.15.8-200.fc42.x86_64 #1 SMP PREEMPT_DYNAMIC Thu Jul 24 13:26:52 UTC 2025 x86_64 GNU/Linux

Tested with rust 1.88.0, 1.89.0

Description

Code like this no longer compiles with v0.4.1, while it does compile with v0.3.7:

use warp::Filter;

#[tokio::main]
async fn main() {
    let task = tokio::spawn(async move {
        let hello = warp::path("test").map(|| "hello");
        let server = warp::serve(hello);
        server
            .run(([0, 0, 0, 0], 9000))
            .await;
    });

    let _ = tokio::join!(task);
}

.. giving the errors:

error: implementation of `Reply` is not general enough
  --> src/main.rs:5:16
   |
5  |       let task = tokio::spawn(async move {
   |  ________________^
6  | |         let hello = warp::path("test").map(|| "hello");
7  | |         let server = warp::serve(hello);
8  | |         server
9  | |             .run(([0, 0, 0, 0], 9000))
10 | |             .await;
11 | |     });
   | |______^ implementation of `Reply` is not general enough
   |
   = note: `&'0 str` must implement `Reply`, for any lifetime `'0`...
   = note: ...but `Reply` is actually implemented for the type `&'static str`

error: implementation of `Send` is not general enough
  --> src/main.rs:5:16
   |
5  |       let task = tokio::spawn(async move {
   |  ________________^
6  | |         let hello = warp::path("test").map(|| "hello");
7  | |         let server = warp::serve(hello);
8  | |         server
9  | |             .run(([0, 0, 0, 0], 9000))
10 | |             .await;
11 | |     });
   | |______^ implementation of `Send` is not general enough
   |
   = note: `Send` would have to be implemented for the type `&'0 str`, for any lifetime `'0`...
   = note: ...but `Send` is actually implemented for the type `&'1 str`, for some specific lifetime `'1`

[seanmonstar]

Hm, that looks pretty odd to me, there's this example that we compile as part of CI and it does nearly the same thing...

[dhedberg]

It does compile OK without running it through tokio::spawn. I did attempt to cut it down to a small example, so it's just the above code in a new project, with

[package]
name = "warp-bug"
version = "0.1.0"
edition = "2024"

[dependencies]
warp = { version = "0.4", features = ["server"] }
tokio = { version = "1", features = ["full"] }

Tell me if I'm doing something obviously wrong 🙂

We have a handful of small internal tools that fail on this and/or the (apparent?) lack of TLS support in 0.4, but we'll just stick to 0.3 for now. Thank you for creating warp!

[seanmonstar]

OK, I tried it out locally, and yea that change makes it break. I have no idea why... I tried changing back to 2018, not it. I wondered if it was a newer compiler, but warp v0.3.7 on the exact same code works. If I comment out the warp::serve() part, then it compiles, even if I move the routes into various sub async blocks.

So it seems like it's something that's different in Server. But I can't figure out what it is. I even tried changing the internal Run::run from an async fn in trait to one that returns an impl Future, but that doesn't fix it.

Really quite confused.

[dhedberg]

I did find that replacing ::path("test") with ::any() got rid of one of the errors, not sure if that narrows it down.

[yeoleobun]

Replace with all the str literal with String will work

[seanmonstar]

Yes but string literals should also work. I don't know why they don't, if anyone wants to dig in.

[d5ng4i]

I'm yet to figure out what's wrong, but replacing concrete types with corresponding impls, seems to eliminate the issue:

fn filter() -> impl Filter<Extract = impl Reply> + Clone + Send + Sync + 'static {
    warp::path("test").map(|| "hello")
}

[dhedberg]

This seems to work as a possible workaround:

use warp::Filter;

#[tokio::main]
async fn main() {
    let hello = warp::path("test").map(|| "hello");
    let server = warp::serve(hello).bind(([0, 0, 0, 0], 9000)).await;
    let task = tokio::spawn(server.run());
    let _ = tokio::join!(task);
}