Future is not Send anymore when it uses format! because core::fmt::Opaque is not Sync

[kangalio]

Code

I tried this code:

async fn say(_: String) {}
async fn mention() -> u32 {
    123
}

fn assert_send<T: Send>(_: T) {}

fn main() {
    assert_send(async {
        say(format!("{}{}", mention().await, mention().await)).await;
    });
}

I expected to see this happen: successful compilation

Instead, this happened:

error: future cannot be sent between threads safely
 --> src/main.rs:9:5
  |
9 |     assert_send(async {
  |     ^^^^^^^^^^^ future created by async block is not `Send`
  |
  = help: the trait `Sync` is not implemented for `core::fmt::Opaque`
note: required by a bound in `assert_send`
 --> src/main.rs:6:19
  |
6 | fn assert_send<T: Send>(_: T) {}
  |                   ^^^^ required by this bound in `assert_send`

Version it worked on

It most recently worked on: Rust 1.59.0-beta

Version with regression

rustc --version --verbose:

rustc 1.56.1 (59eed8a2a 2021-11-01)
binary: rustc
commit-hash: 59eed8a2aac0230a8b53e89d4e99d55912ba6b35
commit-date: 2021-11-01
host: x86_64-unknown-linux-gnu
release: 1.56.1
LLVM version: 13.0.0

Backtrace

Backtrace

<backtrace>

[Urgau]

This is probably related to #91359

cc @dtolnay

[danielhenrymantilla]

Duplicate of #93274 (but it's nice to have better keywords to find it 🙂)

[kangalio]

Duplicate of #93274

Ah, you're right