Rollup merge of #80614 - 1000teslas:issue-78938-fix, r=tmandry
Explain why borrows can't be held across yield point in async blocks For https://github.com/rust-lang/rust/issues/78938.
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@ -50,3 +50,24 @@ fn foo() -> Box<Fn(u32) -> u32> {
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Now that the closure has its own copy of the data, there's no need to worry
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about safety.
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This error may also be encountered while using `async` blocks:
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```compile_fail,E0373,edition2018
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use std::future::Future;
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async fn f() {
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let v = vec![1, 2, 3i32];
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spawn(async { //~ ERROR E0373
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println!("{:?}", v)
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});
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}
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fn spawn<F: Future + Send + 'static>(future: F) {
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unimplemented!()
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}
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```
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Similarly to closures, `async` blocks are not executed immediately and may
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capture closed-over data by reference. For more information, see
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https://rust-lang.github.io/async-book/03_async_await/01_chapter.html.
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@ -1318,21 +1318,30 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
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Applicability::MachineApplicable,
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);
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let msg = match category {
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match category {
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ConstraintCategory::Return(_) | ConstraintCategory::OpaqueType => {
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format!("{} is returned here", kind)
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let msg = format!("{} is returned here", kind);
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err.span_note(constraint_span, &msg);
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}
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ConstraintCategory::CallArgument => {
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fr_name.highlight_region_name(&mut err);
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format!("function requires argument type to outlive `{}`", fr_name)
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if matches!(use_span.generator_kind(), Some(GeneratorKind::Async(_))) {
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err.note(
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"async blocks are not executed immediately and must either take a \
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reference or ownership of outside variables they use",
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);
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} else {
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let msg = format!("function requires argument type to outlive `{}`", fr_name);
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err.span_note(constraint_span, &msg);
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}
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}
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_ => bug!(
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"report_escaping_closure_capture called with unexpected constraint \
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category: `{:?}`",
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category
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),
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};
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err.span_note(constraint_span, &msg);
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}
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err
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}
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33
src/test/ui/async-await/issues/issue-78938-async-block.rs
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33
src/test/ui/async-await/issues/issue-78938-async-block.rs
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@ -0,0 +1,33 @@
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// edition:2018
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use std::{sync::Arc, future::Future, pin::Pin, task::{Context, Poll}};
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async fn f() {
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let room_ref = Arc::new(Vec::new());
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let gameloop_handle = spawn(async { //~ ERROR E0373
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game_loop(Arc::clone(&room_ref))
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});
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gameloop_handle.await;
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}
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fn game_loop(v: Arc<Vec<usize>>) {}
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fn spawn<F>(future: F) -> JoinHandle
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where
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F: Future + Send + 'static,
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F::Output: Send + 'static,
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{
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loop {}
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}
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struct JoinHandle;
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impl Future for JoinHandle {
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type Output = ();
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fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
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loop {}
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}
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}
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fn main() {}
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@ -0,0 +1,21 @@
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error[E0373]: async block may outlive the current function, but it borrows `room_ref`, which is owned by the current function
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--> $DIR/issue-78938-async-block.rs:8:39
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LL | let gameloop_handle = spawn(async {
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| _______________________________________^
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LL | | game_loop(Arc::clone(&room_ref))
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| | -------- `room_ref` is borrowed here
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LL | | });
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| |_____^ may outlive borrowed value `room_ref`
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= note: async blocks are not executed immediately and must either take a reference or ownership of outside variables they use
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help: to force the async block to take ownership of `room_ref` (and any other referenced variables), use the `move` keyword
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LL | let gameloop_handle = spawn(async move {
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LL | game_loop(Arc::clone(&room_ref))
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LL | });
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error: aborting due to previous error
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For more information about this error, try `rustc --explain E0373`.
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