Fix capture analysis for by-move closure bodies

compiler/rustc_mir_transform/src/coroutine/by_move_body.rs

@@ -1,7 +1,66 @@
-//! A MIR pass which duplicates a coroutine's body and removes any derefs which
-//! would be present for upvars that are taken by-ref. The result of which will
-//! be a coroutine body that takes all of its upvars by-move, and which we stash
-//! into the `CoroutineInfo` for all coroutines returned by coroutine-closures.
+//! This pass constructs a second coroutine body sufficient for return from
+//! `FnOnce`/`AsyncFnOnce` implementations for coroutine-closures (e.g. async closures).
+//!
+//! Consider an async closure like:
+//! ```rust
+//! #![feature(async_closure)]
+//!
+//! let x = vec![1, 2, 3];
+//!
+//! let closure = async move || {
+//! println!("{x:#?}");
+//! };
+//! ```
+//!
+//! This desugars to something like:
+//! ```rust,ignore (invalid-borrowck)
+//! let x = vec![1, 2, 3];
+//!
+//! let closure = move || {
+//! async {
+//! println!("{x:#?}");
+//! }
+//! };
+//! ```
+//!
+//! Important to note here is that while the outer closure *moves* `x: Vec<i32>`
+//! into its upvars, the inner `async` coroutine simply captures a ref of `x`.
+//! This is the "magic" of async closures -- the futures that they return are
+//! allowed to borrow from their parent closure's upvars.
+//!
+//! However, what happens when we call `closure` with `AsyncFnOnce` (or `FnOnce`,
+//! since all async closures implement that too)? Well, recall the signature:
+//! ```
+//! use std::future::Future;
+//! pub trait AsyncFnOnce<Args>
+//! {
+//! type CallOnceFuture: Future<Output = Self::Output>;
+//! type Output;
+//! fn async_call_once(
+//! self,
+//! args: Args
+//! ) -> Self::CallOnceFuture;
+//! }
+//! ```
+//!
+//! This signature *consumes* the async closure (`self`) and returns a `CallOnceFuture`.
+//! How do we deal with the fact that the coroutine is supposed to take a reference
+//! to the captured `x` from the parent closure, when that parent closure has been
+//! destroyed?
+//!
+//! This is the second piece of magic of async closures. We can simply create a
+//! *second* `async` coroutine body where that `x` that was previously captured
+//! by reference is now captured by value. This means that we consume the outer
+//! closure and return a new coroutine that will hold onto all of these captures,
+//! and drop them when it is finished (i.e. after it has been `.await`ed).
+//!
+//! We do this with the analysis below, which detects the captures that come from
+//! borrowing from the outer closure, and we simply peel off a `deref` projection
+//! from them. This second body is stored alongside the first body, and optimized
+//! with it in lockstep. When we need to resolve a body for `FnOnce` or `AsyncFnOnce`,
+//! we use this "by move" body instead.
+
+use itertools::Itertools;
 
 use rustc_data_structures::unord::UnordSet;
 use rustc_hir as hir;
@@ -14,6 +73,8 @@ pub struct ByMoveBody;
 
 impl<'tcx> MirPass<'tcx> for ByMoveBody {
  fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut mir::Body<'tcx>) {
+ // We only need to generate by-move coroutine bodies for coroutines that come
+ // from coroutine-closures.
  let Some(coroutine_def_id) = body.source.def_id().as_local() else {
  return;
  };
@@ -22,44 +83,70 @@ impl<'tcx> MirPass<'tcx> for ByMoveBody {
  else {
  return;
  };
+
+ // Also, let's skip processing any bodies with errors, since there's no guarantee
+ // the MIR body will be constructed well.
  let coroutine_ty = body.local_decls[ty::CAPTURE_STRUCT_LOCAL].ty;
  if coroutine_ty.references_error() {
  return;
  }
- let ty::Coroutine(_, args) = *coroutine_ty.kind() else { bug!("{body:#?}") };
 
- let coroutine_kind = args.as_coroutine().kind_ty().to_opt_closure_kind().unwrap();
+ let ty::Coroutine(_, coroutine_args) = *coroutine_ty.kind() else { bug!("{body:#?}") };
+ // We don't need to generate a by-move coroutine if the kind of the coroutine is
+ // already `FnOnce` -- that means that any upvars that the closure consumes have
+ // already been taken by-value.
+ let coroutine_kind = coroutine_args.as_coroutine().kind_ty().to_opt_closure_kind().unwrap();
  if coroutine_kind == ty::ClosureKind::FnOnce {
  return;
  }
 
+ let parent_def_id = tcx.local_parent(coroutine_def_id);
+ let ty::CoroutineClosure(_, parent_args) =
+ *tcx.type_of(parent_def_id).instantiate_identity().kind()
+ else {
+ bug!();
+ };
+ let parent_closure_args = parent_args.as_coroutine_closure();
+ let num_args = parent_closure_args
+ .coroutine_closure_sig()
+ .skip_binder()
+ .tupled_inputs_ty
+ .tuple_fields()
+ .len();
+
  let mut by_ref_fields = UnordSet::default();
- let by_move_upvars = Ty::new_tup_from_iter(
- tcx,
- tcx.closure_captures(coroutine_def_id).iter().enumerate().map(|(idx, capture)| {
- if capture.is_by_ref() {
- by_ref_fields.insert(FieldIdx::from_usize(idx));
- }
- capture.place.ty()
- }),
- );
- let by_move_coroutine_ty = Ty::new_coroutine(
- tcx,
- coroutine_def_id.to_def_id(),
- ty::CoroutineArgs::new(
+ for (idx, (coroutine_capture, parent_capture)) in tcx
+ .closure_captures(coroutine_def_id)
+ .iter()
+ // By construction we capture all the args first.
+ .skip(num_args)
+ .zip_eq(tcx.closure_captures(parent_def_id))
+ .enumerate()
+ {
+ // This upvar is captured by-move from the parent closure, but by-ref
+ // from the inner async block. That means that it's being borrowed from
+ // the outer closure body -- we need to change the coroutine to take the
+ // upvar by value.
+ if coroutine_capture.is_by_ref() && !parent_capture.is_by_ref() {
+ by_ref_fields.insert(FieldIdx::from_usize(num_args + idx));
+ }
+
+ // Make sure we're actually talking about the same capture.
+ // FIXME(async_closures): We could look at the `hir::Upvar` instead?
+ assert_eq!(coroutine_capture.place.ty(), parent_capture.place.ty());
+ }
+
+ let by_move_coroutine_ty = tcx
+ .instantiate_bound_regions_with_erased(parent_closure_args.coroutine_closure_sig())
+ .to_coroutine_given_kind_and_upvars(
  tcx,
- ty::CoroutineArgsParts {
- parent_args: args.as_coroutine().parent_args(),
- kind_ty: Ty::from_closure_kind(tcx, ty::ClosureKind::FnOnce),
- resume_ty: args.as_coroutine().resume_ty(),
- yield_ty: args.as_coroutine().yield_ty(),
- return_ty: args.as_coroutine().return_ty(),
- witness: args.as_coroutine().witness(),
- tupled_upvars_ty: by_move_upvars,
- },
- )
- .args,
- );
+ parent_closure_args.parent_args(),
+ coroutine_def_id.to_def_id(),
+ ty::ClosureKind::FnOnce,
+ tcx.lifetimes.re_erased,
+ parent_closure_args.tupled_upvars_ty(),
+ parent_closure_args.coroutine_captures_by_ref_ty(),
+ );
 
  let mut by_move_body = body.clone();
  MakeByMoveBody { tcx, by_ref_fields, by_move_coroutine_ty }.visit_body(&mut by_move_body);

src/tools/miri/tests/pass/async-closure-captures.rs

@@ -0,0 +1,91 @@
+// Same as rustc's `tests/ui/async-await/async-closures/captures.rs`, keep in sync
+
+#![feature(async_closure, noop_waker)]
+
+use std::future::Future;
+use std::pin::pin;
+use std::task::*;
+
+pub fn block_on<T>(fut: impl Future<Output = T>) -> T {
+ let mut fut = pin!(fut);
+ let ctx = &mut Context::from_waker(Waker::noop());
+
+ loop {
+ match fut.as_mut().poll(ctx) {
+ Poll::Pending => {}
+ Poll::Ready(t) => break t,
+ }
+ }
+}
+
+fn main() {
+ block_on(async_main());
+}
+
+async fn call<T>(f: &impl async Fn() -> T) -> T {
+ f().await
+}
+
+async fn call_once<T>(f: impl async FnOnce() -> T) -> T {
+ f().await
+}
+
+#[derive(Debug)]
+#[allow(unused)]
+struct Hello(i32);
+
+async fn async_main() {
+ // Capture something by-ref
+ {
+ let x = Hello(0);
+ let c = async || {
+ println!("{x:?}");
+ };
+ call(&c).await;
+ call_once(c).await;
+
+ let x = &Hello(1);
+ let c = async || {
+ println!("{x:?}");
+ };
+ call(&c).await;
+ call_once(c).await;
+ }
+
+ // Capture something and consume it (force to `AsyncFnOnce`)
+ {
+ let x = Hello(2);
+ let c = async || {
+ println!("{x:?}");
+ drop(x);
+ };
+ call_once(c).await;
+ }
+
+ // Capture something with `move`, don't consume it
+ {
+ let x = Hello(3);
+ let c = async move || {
+ println!("{x:?}");
+ };
+ call(&c).await;
+ call_once(c).await;
+
+ let x = &Hello(4);
+ let c = async move || {
+ println!("{x:?}");
+ };
+ call(&c).await;
+ call_once(c).await;
+ }
+
+ // Capture something with `move`, also consume it (so `AsyncFnOnce`)
+ {
+ let x = Hello(5);
+ let c = async move || {
+ println!("{x:?}");
+ drop(x);
+ };
+ call_once(c).await;
+ }
+}

src/tools/miri/tests/pass/async-closure-captures.stdout

@@ -0,0 +1,10 @@
+Hello(0)
+Hello(0)
+Hello(1)
+Hello(1)
+Hello(2)
+Hello(3)
+Hello(3)
+Hello(4)
+Hello(4)
+Hello(5)

tests/ui/async-await/async-closures/captures.rs

@@ -0,0 +1,82 @@
+//@ aux-build:block-on.rs
+//@ edition:2021
+//@ run-pass
+//@ check-run-results
+
+// Same as miri's `tests/pass/async-closure-captures.rs`, keep in sync
+
+#![feature(async_closure)]
+
+extern crate block_on;
+
+fn main() {
+ block_on::block_on(async_main());
+}
+
+async fn call<T>(f: &impl async Fn() -> T) -> T {
+ f().await
+}
+
+async fn call_once<T>(f: impl async FnOnce() -> T) -> T {
+ f().await
+}
+
+#[derive(Debug)]
+#[allow(unused)]
+struct Hello(i32);
+
+async fn async_main() {
+ // Capture something by-ref
+ {
+ let x = Hello(0);
+ let c = async || {
+ println!("{x:?}");
+ };
+ call(&c).await;
+ call_once(c).await;
+
+ let x = &Hello(1);
+ let c = async || {
+ println!("{x:?}");
+ };
+ call(&c).await;
+ call_once(c).await;
+ }
+
+ // Capture something and consume it (force to `AsyncFnOnce`)
+ {
+ let x = Hello(2);
+ let c = async || {
+ println!("{x:?}");
+ drop(x);
+ };
+ call_once(c).await;
+ }
+
+ // Capture something with `move`, don't consume it
+ {
+ let x = Hello(3);
+ let c = async move || {
+ println!("{x:?}");
+ };
+ call(&c).await;
+ call_once(c).await;
+
+ let x = &Hello(4);
+ let c = async move || {
+ println!("{x:?}");
+ };
+ call(&c).await;
+ call_once(c).await;
+ }
+
+ // Capture something with `move`, also consume it (so `AsyncFnOnce`)
+ {
+ let x = Hello(5);
+ let c = async move || {
+ println!("{x:?}");
+ drop(x);
+ };
+ call_once(c).await;
+ }
+}

tests/ui/async-await/async-closures/captures.run.stdout

@@ -0,0 +1,10 @@
+Hello(0)
+Hello(0)
+Hello(1)
+Hello(1)
+Hello(2)
+Hello(3)
+Hello(3)
+Hello(4)
+Hello(4)
+Hello(5)