error: future cannot be sent between threads safely --> $DIR/issue-64130-non-send-future-diags.rs:23:13 | LL | is_send(foo()); | ^^^^^ future returned by `foo` is not `Send` | = help: within `impl Future`, the trait `Send` is not implemented for `MutexGuard<'_, u32>` note: future is not `Send` as this value is used across an await --> $DIR/issue-64130-non-send-future-diags.rs:17:11 | LL | let g = x.lock().unwrap(); | - has type `MutexGuard<'_, u32>` which is not `Send` LL | baz().await; | ^^^^^ await occurs here, with `g` maybe used later note: required by a bound in `is_send` --> $DIR/issue-64130-non-send-future-diags.rs:9:15 | LL | fn is_send(t: T) { } | ^^^^ required by this bound in `is_send` error: aborting due to 1 previous error