Disjoint capture in async blocks requires mut for Copy field modifications #146547
Description
I tried this code:
use std::future::Future; use std::pin::Pin; use std::thread::sleep; use std::time::Duration; async fn run_multiple_times<Fut>( mut f: impl FnMut() -> Pin<Box<dyn Future<Output = Fut> + Send>>, ) { for _ in 0..5 { f().await; sleep(Duration::from_secs(1)); } } struct Test { pub num: usize, pub name: String, } impl Test { fn inc(&mut self) { self.num += 1 } } #[tokio::main] async fn main() { let mut state = Test {num: 0, name: "test".to_string()}; run_multiple_times(move || { Box::pin(async move { println!("State: {}", state.num); state.num = state.num + 1; // This line doesn't capture state, yet requires it to be mut ??? // These two lines, as expected, do not compile: // state.inc() // error: cannot move out of `state` // state.name = "test".to_string(); // error: String doesn't implement Copy trait }) }).await; println!("State: {}", state.num); }I expected to see a compilation error due to state being moved twice. Instead, this code compiled without any warnings and printed "State: 0" 6 times. This issue also exists on the nightly version.
The way I understand this is that the compiler decides to move state.num instead of the whole state (which is already confusing) and, since usize implements Copy trait, it simply copies its value. The problem is that the compiler also requires state to be declared as mut which is definitely not how it should be.
Am I missing something or is it an actual bug?