Using the format!() macro with an async function makes the whole Future non-Send #101650
Description
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() }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