diff --git a/compiler/rustc_mir_transform/src/coroutine/by_move_body.rs b/compiler/rustc_mir_transform/src/coroutine/by_move_body.rs index 99dbb34226834..0866205dfd0db 100644 --- a/compiler/rustc_mir_transform/src/coroutine/by_move_body.rs +++ b/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` +//! 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 +//! { +//! type CallOnceFuture: Future; +//! 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); diff --git a/src/tools/miri/tests/pass/async-closure-captures.rs b/src/tools/miri/tests/pass/async-closure-captures.rs new file mode 100644 index 0000000000000..3e33de32efb04 --- /dev/null +++ b/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(fut: impl Future) -> 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(f: &impl async Fn() -> T) -> T { + f().await +} + +async fn call_once(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; + } +} diff --git a/src/tools/miri/tests/pass/async-closure-captures.stdout b/src/tools/miri/tests/pass/async-closure-captures.stdout new file mode 100644 index 0000000000000..a0db6d236fef1 --- /dev/null +++ b/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) diff --git a/tests/ui/async-await/async-closures/captures.rs b/tests/ui/async-await/async-closures/captures.rs new file mode 100644 index 0000000000000..e3ab8713709d4 --- /dev/null +++ b/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(f: &impl async Fn() -> T) -> T { + f().await +} + +async fn call_once(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; + } +} diff --git a/tests/ui/async-await/async-closures/captures.run.stdout b/tests/ui/async-await/async-closures/captures.run.stdout new file mode 100644 index 0000000000000..a0db6d236fef1 --- /dev/null +++ b/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)