async function being !Send due to match retaining type of a consumed value.

[Stormshield-robinc]

Hi,

I tried this code:

// is Send
async fn good() {
    let life = NonSend::new().life();

    match life {
        _ => f().await,
    }
}

// is not Send
async fn bad() {
    match NonSend::new().life() {
        _ => f().await,
    }
}

playground will the full code : https://play.rust-lang.org/?version=nightly&mode=release&edition=2021&gist=e0c1095262f83a6f62cea40d6059fe3d

I expected to see this happen:

In this case I would expect both good() and bad() async function to be Send and the whole code to compile.

This code basically create a !Send value that is consumed by some function that return something Send.
After this point, a .await is called. Since the non sendable value have been consumed, I would expect the async function to be Send

Instead, this happened:

The function good() is Send but not the function bad().
It seem that the match somehow retain the non-sendable type even though it should have been consumed.
What is even more surprising is to see the difference of behavior between the let and the match, since I would have expected those to be identical.

I dug a bit through the existing issues and found #93883 and #94067 that kinda look like what I have here, but I cannot for sure know if this it is exactly the same issue.

Meta

rustc --version --verbose:

rustc 1.71.0 (8ede3aae2 2023-07-12)
binary: rustc
commit-hash: 8ede3aae28fe6e4d52b38157d7bfe0d3bceef225
commit-date: 2023-07-12
host: x86_64-unknown-linux-gnu
release: 1.71.0
LLVM version: 16.0.5

Note : also reproduced with the available versions in the playground

[fmease]

Fixed on nightly, not sure which PR fixed this though.

[stormshield-kg]

Fixed by 13e6f24, according to cargo-bisect-rustc.

[fmease]

@cjgillot (#107421), do you think it's worth adding the code from the issue description as a regression test or can I close this issue directly?

[cjgillot]

There are already tests on this kind of code.