Auto merge of #64525 - nikomatsakis:issue-64512-drop-order-tail-temp, r=davidtwco
adjust desugaring for async fn to correct drop order
Old desugaring, given a user function body `{ $stmts; $expr }`
```
{
let $param_pattern0 = $raw_param0;
...
let $param_patternN = $raw_paramN;
$stmts;
$expr
}
```
New desugaring:
```
{
let $param_pattern0 = $raw_param0;
...
let $param_patternN = $raw_paramN;
drop-temps {
$stmts;
$expr
}
}
```
The drop-temps is an internal bit of HIR that drops temporaries from the resulting expression, but it should be equivalent to `return { $stmts; $expr }`.
Fixes #64512
Fixes #64391
diff --git a/src/librustc/hir/lowering.rs b/src/librustc/hir/lowering.rs
index 58789a1..48f7fc4 100644
--- a/src/librustc/hir/lowering.rs
+++ b/src/librustc/hir/lowering.rs
@@ -2682,12 +2682,8 @@
bounds.iter().map(|bound| self.lower_param_bound(bound, itctx.reborrow())).collect()
}
- fn lower_block_with_stmts(
- &mut self,
- b: &Block,
- targeted_by_break: bool,
- mut stmts: Vec<hir::Stmt>,
- ) -> P<hir::Block> {
+ fn lower_block(&mut self, b: &Block, targeted_by_break: bool) -> P<hir::Block> {
+ let mut stmts = vec![];
let mut expr = None;
for (index, stmt) in b.stmts.iter().enumerate() {
@@ -2712,8 +2708,11 @@
})
}
- fn lower_block(&mut self, b: &Block, targeted_by_break: bool) -> P<hir::Block> {
- self.lower_block_with_stmts(b, targeted_by_break, vec![])
+ /// Lowers a block directly to an expression, presuming that it
+ /// has no attributes and is not targeted by a `break`.
+ fn lower_block_expr(&mut self, b: &Block) -> hir::Expr {
+ let block = self.lower_block(b, false);
+ self.expr_block(block, ThinVec::new())
}
fn lower_pat(&mut self, p: &Pat) -> P<hir::Pat> {
diff --git a/src/librustc/hir/lowering/expr.rs b/src/librustc/hir/lowering/expr.rs
index a46cdab..990728f 100644
--- a/src/librustc/hir/lowering/expr.rs
+++ b/src/librustc/hir/lowering/expr.rs
@@ -90,10 +90,7 @@
),
ExprKind::Async(capture_clause, closure_node_id, ref block) => {
self.make_async_expr(capture_clause, closure_node_id, None, block.span, |this| {
- this.with_new_scopes(|this| {
- let block = this.lower_block(block, false);
- this.expr_block(block, ThinVec::new())
- })
+ this.with_new_scopes(|this| this.lower_block_expr(block))
})
}
ExprKind::Await(ref expr) => self.lower_expr_await(e.span, expr),
@@ -284,8 +281,7 @@
let else_arm = self.arm(hir_vec![else_pat], P(else_expr));
// Handle then + scrutinee:
- let then_blk = self.lower_block(then, false);
- let then_expr = self.expr_block(then_blk, ThinVec::new());
+ let then_expr = self.lower_block_expr(then);
let (then_pat, scrutinee, desugar) = match cond.node {
// `<pat> => <then>`:
ExprKind::Let(ref pat, ref scrutinee) => {
@@ -335,8 +331,7 @@
};
// Handle then + scrutinee:
- let then_blk = self.lower_block(body, false);
- let then_expr = self.expr_block(then_blk, ThinVec::new());
+ let then_expr = self.lower_block_expr(body);
let (then_pat, scrutinee, desugar, source) = match cond.node {
ExprKind::Let(ref pat, ref scrutinee) => {
// to:
@@ -356,7 +351,7 @@
//
// ```
// 'label: loop {
- // match DropTemps($cond) {
+ // match drop-temps { $cond } {
// true => $body,
// _ => break,
// }
@@ -1310,7 +1305,7 @@
/// `{ let _t = $expr; _t }` but should provide better compile-time performance.
///
/// The drop order can be important in e.g. `if expr { .. }`.
- fn expr_drop_temps(
+ pub(super) fn expr_drop_temps(
&mut self,
span: Span,
expr: P<hir::Expr>,
diff --git a/src/librustc/hir/lowering/item.rs b/src/librustc/hir/lowering/item.rs
index 5f82e42..61be40a 100644
--- a/src/librustc/hir/lowering/item.rs
+++ b/src/librustc/hir/lowering/item.rs
@@ -1071,10 +1071,7 @@
}
fn lower_fn_body_block(&mut self, decl: &FnDecl, body: &Block) -> hir::BodyId {
- self.lower_fn_body(decl, |this| {
- let body = this.lower_block(body, false);
- this.expr_block(body, ThinVec::new())
- })
+ self.lower_fn_body(decl, |this| this.lower_block_expr(body))
}
pub(super) fn lower_const_body(&mut self, expr: &Expr) -> hir::BodyId {
@@ -1102,8 +1099,7 @@
// from:
//
// async fn foo(<pattern>: <ty>, <pattern>: <ty>, <pattern>: <ty>) {
- // async move {
- // }
+ // <body>
// }
//
// into:
@@ -1116,11 +1112,19 @@
// let <pattern> = __arg1;
// let __arg0 = __arg0;
// let <pattern> = __arg0;
+ // drop-temps { <body> } // see comments later in fn for details
// }
// }
//
// If `<pattern>` is a simple ident, then it is lowered to a single
// `let <pattern> = <pattern>;` statement as an optimization.
+ //
+ // Note that the body is embedded in `drop-temps`; an
+ // equivalent desugaring would be `return { <body>
+ // };`. The key point is that we wish to drop all the
+ // let-bound variables and temporaries created in the body
+ // (and its tail expression!) before we drop the
+ // parameters (c.f. rust-lang/rust#64512).
for (index, parameter) in decl.inputs.iter().enumerate() {
let parameter = this.lower_param(parameter);
let span = parameter.pat.span;
@@ -1219,8 +1223,36 @@
let async_expr = this.make_async_expr(
CaptureBy::Value, closure_id, None, body.span,
|this| {
- let body = this.lower_block_with_stmts(body, false, statements);
- this.expr_block(body, ThinVec::new())
+ // Create a block from the user's function body:
+ let user_body = this.lower_block_expr(body);
+
+ // Transform into `drop-temps { <user-body> }`, an expression:
+ let desugared_span = this.mark_span_with_reason(
+ DesugaringKind::Async,
+ user_body.span,
+ None,
+ );
+ let user_body = this.expr_drop_temps(
+ desugared_span,
+ P(user_body),
+ ThinVec::new(),
+ );
+
+ // As noted above, create the final block like
+ //
+ // ```
+ // {
+ // let $param_pattern = $raw_param;
+ // ...
+ // drop-temps { <user-body> }
+ // }
+ // ```
+ let body = this.block_all(
+ desugared_span,
+ statements.into(),
+ Some(P(user_body)),
+ );
+ this.expr_block(P(body), ThinVec::new())
});
(HirVec::from(parameters), this.expr(body.span, async_expr, ThinVec::new()))
})
diff --git a/src/test/ui/async-await/drop-order/drop-order-for-temporary-in-tail-return-expr.rs b/src/test/ui/async-await/drop-order/drop-order-for-temporary-in-tail-return-expr.rs
new file mode 100644
index 0000000..e40acff
--- /dev/null
+++ b/src/test/ui/async-await/drop-order/drop-order-for-temporary-in-tail-return-expr.rs
@@ -0,0 +1,95 @@
+// aux-build:arc_wake.rs
+// edition:2018
+// run-pass
+
+#![allow(unused_variables)]
+
+// Test the drop order for parameters relative to local variables and
+// temporaries created in the tail return expression of the function
+// body. In particular, check that this drop order is the same between
+// a `async fn` and an ordinary `fn`. See #64512.
+
+extern crate arc_wake;
+
+use arc_wake::ArcWake;
+use std::cell::RefCell;
+use std::future::Future;
+use std::sync::Arc;
+use std::rc::Rc;
+use std::task::Context;
+
+struct EmptyWaker;
+
+impl ArcWake for EmptyWaker {
+ fn wake(self: Arc<Self>) {}
+}
+
+#[derive(Debug, Eq, PartialEq)]
+enum DropOrder {
+ Function,
+ Val(&'static str),
+}
+
+type DropOrderListPtr = Rc<RefCell<Vec<DropOrder>>>;
+
+struct D(&'static str, DropOrderListPtr);
+
+impl Drop for D {
+ fn drop(&mut self) {
+ self.1.borrow_mut().push(DropOrder::Val(self.0));
+ }
+}
+
+/// Check drop order of temporary "temp" as compared to `x`, `y`, and `z`.
+///
+/// Expected order:
+/// - `z`
+/// - temp
+/// - `y`
+/// - `x`
+async fn foo_async(x: D, _y: D) {
+ let l = x.1.clone();
+ let z = D("z", l.clone());
+ l.borrow_mut().push(DropOrder::Function);
+ helper_async(&D("temp", l)).await
+}
+
+async fn helper_async(v: &D) { }
+
+fn foo_sync(x: D, _y: D) {
+ let l = x.1.clone();
+ let z = D("z", l.clone());
+ l.borrow_mut().push(DropOrder::Function);
+ helper_sync(&D("temp", l))
+}
+
+fn helper_sync(v: &D) { }
+
+fn assert_drop_order_after_poll<Fut: Future<Output = ()>>(
+ f: impl FnOnce(DropOrderListPtr) -> Fut,
+ g: impl FnOnce(DropOrderListPtr),
+) {
+ let empty = Arc::new(EmptyWaker);
+ let waker = ArcWake::into_waker(empty);
+ let mut cx = Context::from_waker(&waker);
+
+ let actual_order = Rc::new(RefCell::new(Vec::new()));
+ let mut fut = Box::pin(f(actual_order.clone()));
+ let r = fut.as_mut().poll(&mut cx);
+
+ assert!(match r {
+ std::task::Poll::Ready(()) => true,
+ _ => false,
+ });
+
+ let expected_order = Rc::new(RefCell::new(Vec::new()));
+ g(expected_order.clone());
+
+ assert_eq!(*actual_order.borrow(), *expected_order.borrow());
+}
+
+fn main() {
+ // Free functions (see doc comment on function for what it tests).
+ assert_drop_order_after_poll(|l| foo_async(D("x", l.clone()), D("_y", l.clone())),
+ |l| foo_sync(D("x", l.clone()), D("_y", l.clone())));
+}
diff --git a/src/test/ui/async-await/issue-64391.rs b/src/test/ui/async-await/issue-64391.rs
new file mode 100644
index 0000000..c6faad3
--- /dev/null
+++ b/src/test/ui/async-await/issue-64391.rs
@@ -0,0 +1,14 @@
+// Regression test for Issue #64391. The goal here is that this
+// function compiles. In the past, due to incorrect drop order for
+// temporaries in the tail expression, we failed to compile this
+// example. The drop order itself is directly tested in
+// `drop-order/drop-order-for-temporary-in-tail-return-expr.rs`.
+//
+// check-pass
+// edition:2018
+
+async fn add(x: u32, y: u32) -> u32 {
+ async { x + y }.await
+}
+
+fn main() { }
