Using the format!() macro with an async function makes the whole Future non-Send

[Hocuri]

I tried this code: (minimal reproducible example; for the actual code see chatmail/core#3591)

use core::future::Future;
use core::pin::Pin;

fn build_string() -> Pin<Box<dyn Future<Output = String> + Send>> {
    Box::pin(async move {
        let mut string_builder = "<h>".to_string();
        string_builder += &format!("Hello {}", helper().await);
        string_builder += "</h>";
        string_builder
    })
}

async fn helper() -> String {
    "World".to_string()
}

Playground

Note that we are using,

format!("Hello {}", helper().await)

in a context where the Future needs to be Send; only this line without the string_builder seems not to be able to reproduce the issue though.

I expected to see this happen: That the code behaves the same as

let s = helper().await;
format!("Hello {}", s)

Instead, this happened:

There are two error messages (about line 5 and line 7); this issue is about the first one:

error: future cannot be sent between threads safely
 --> src/lib.rs:5:5
  |
5 |     Box::pin(async move {
  |     ^^^^^^^^^^^^^^^^^^^ future created by async block is not `Send`
  |
  = help: the trait `Sync` is not implemented for `core::fmt::Opaque`
note: future is not `Send` as this value is used across an await
 --> src/lib.rs:7:56
  |
7 |         string_builder += &format!("Hello {}", helper().await);
  |                            ----------------------------^^^^^^-
  |                            |                   |       |
  |                            |                   |       await occurs here, with `helper().await` maybe used later
  |                            |                   has type `ArgumentV1<'_>` which is not `Send`
  |                            `helper().await` is later dropped here
  = note: required for the cast to the object type `dyn Future<Output = String> + Send`

The second error message didn't appear in the original issue in chatmail/core#3591, possibly because there the call to Box::pin() and the format!() call are in two different crates:

error: generator cannot be sent between threads safely
  --> deltachat-jsonrpc/src/api/mod.rs:74:1
   |
74 | #[rpc(all_positional, ts_outdir = "typescript/generated")]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ future created by async block is not `Send`
   |
   = help: the trait `Sync` is not implemented for `core::fmt::Opaque`
   = note: required for the cast to the object type `dyn Future<Output = Result<Value, yerpc::Error>> + Send`
   = note: this error originates in the attribute macro `rpc` (in Nightly builds, run with -Z macro-backtrace for more info)

Meta

rustc --version --verbose

Probably doesn't matter, anyway:

rustc 1.61.0 (fe5b13d68 2022-05-18)
binary: rustc
commit-hash: fe5b13d681f25ee6474be29d748c65adcd91f69e
commit-date: 2022-05-18
host: x86_64-unknown-linux-gnu
release: 1.61.0
LLVM version: 14.0.0

@rustbot label +A-fmt

[Hocuri]

Notes:

• Maybe this can be fixed with Tracking issue for improving std::fmt::Arguments and format_args!() #99012 cc @m-ou-se
• See Better error messages for sending format! uses across thread boundaries. #82696, an issue about improving the error message.

I'll guess this is
@rustbot label +T-libs

[lolbinarycat]

triage: seems to have been fixed, now we just need someone to add this as a test.

[m-ou-se]

The issue of yield points in formatting arguments was actually already fixed by #93461, but that was already merged far before this issue was even opened.

Looks like this specific issue is not triggered just by the format!(..) invocation. It was only triggered when doing string += &format!(..);.

And that was fixed by #107421