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Auto merge of #64525 - nikomatsakis:issue-64512-drop-order-tail-temp, r=davidtwco

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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
This commit is contained in:
bors 2019-09-17 18:23:30 +00:00
commit 7efe1c6e67
5 changed files with 161 additions and 26 deletions
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15
src/librustc/hir/lowering.rs
View File

@ -2682,12 +2682,8 @@ fn lower_param_bounds(&mut self, bounds: &[GenericBound], mut itctx: ImplTraitCo
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_with_stmts(
})
}
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> {

15
src/librustc/hir/lowering/expr.rs
View File

@ -90,10 +90,7 @@ pub(super) fn lower_expr(&mut self, e: &Expr) -> hir::Expr {
),
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 @@ fn lower_expr_if(
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 @@ fn lower_expr_while_in_loop_scope(
};
// 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 @@ fn lower_expr_while_in_loop_scope(
//
// ```
// 'label: loop {
// match DropTemps($cond) {
// match drop-temps { $cond } {
// true => $body,
// _ => break,
// }
@ -1310,7 +1305,7 @@ pub(super) fn expr_bool(&mut self, span: Span, val: bool) -> hir::Expr {
/// `{ 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>,

48
src/librustc/hir/lowering/item.rs
View File

@ -1071,10 +1071,7 @@ pub(super) fn lower_fn_body(
}
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 @@ fn lower_maybe_async_body(
// from:
//
// async fn foo(<pattern>: <ty>, <pattern>: <ty>, <pattern>: <ty>) {
// async move {
// }
// <body>
// }
//
// into:
@ -1116,11 +1112,19 @@ fn lower_maybe_async_body(
// 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 @@ fn lower_maybe_async_body(
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()))
})

95
src/test/ui/async-await/drop-order/drop-order-for-temporary-in-tail-return-expr.rs Normal file
View File

@ -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())));
}

14
src/test/ui/async-await/issue-64391.rs Normal file
View File

@ -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() { }