Correct handling of arguments in async fn

src/libsyntax/parse/parser.rs

@@ -1576,7 +1576,7 @@ impl<'a> Parser<'a> {
             let ident = self.parse_ident()?;
             let mut generics = self.parse_generics()?;
 
-            let d = self.parse_fn_decl_with_self(|p: &mut Parser<'a>| {
+            let mut decl = self.parse_fn_decl_with_self(|p: &mut Parser<'a>| {
                 // This is somewhat dubious; We don't want to allow
                 // argument names to be left off if there is a
                 // definition...
@@ -1585,7 +1585,7 @@ impl<'a> Parser<'a> {
                 p.parse_arg_general(p.span.rust_2018(), true, false)
             })?;
             generics.where_clause = self.parse_where_clause()?;
-            self.construct_async_arguments(&mut asyncness, &d);
+            self.construct_async_arguments(&mut asyncness, &mut decl);
 
             let sig = ast::MethodSig {
                 header: FnHeader {
@@ -1594,7 +1594,7 @@ impl<'a> Parser<'a> {
                     abi,
                     asyncness,
                 },
-                decl: d,
+                decl,
             };
 
             let body = match self.token {
@@ -6479,10 +6479,10 @@ impl<'a> Parser<'a> {
                      -> PResult<'a, ItemInfo> {
         let (ident, mut generics) = self.parse_fn_header()?;
         let allow_c_variadic = abi == Abi::C && unsafety == Unsafety::Unsafe;
-        let decl = self.parse_fn_decl(allow_c_variadic)?;
+        let mut decl = self.parse_fn_decl(allow_c_variadic)?;
         generics.where_clause = self.parse_where_clause()?;
         let (inner_attrs, body) = self.parse_inner_attrs_and_block()?;
-        self.construct_async_arguments(&mut asyncness, &decl);
+        self.construct_async_arguments(&mut asyncness, &mut decl);
         let header = FnHeader { unsafety, asyncness, constness, abi };
         Ok((ident, ItemKind::Fn(decl, header, generics, body), Some(inner_attrs)))
     }
@@ -6666,9 +6666,9 @@ impl<'a> Parser<'a> {
             let (constness, unsafety, mut asyncness, abi) = self.parse_fn_front_matter()?;
             let ident = self.parse_ident()?;
             let mut generics = self.parse_generics()?;
-            let decl = self.parse_fn_decl_with_self(|p| p.parse_arg())?;
+            let mut decl = self.parse_fn_decl_with_self(|p| p.parse_arg())?;
             generics.where_clause = self.parse_where_clause()?;
-            self.construct_async_arguments(&mut asyncness, &decl);
+            self.construct_async_arguments(&mut asyncness, &mut decl);
             *at_end = true;
             let (inner_attrs, body) = self.parse_inner_attrs_and_block()?;
             let header = ast::FnHeader { abi, unsafety, constness, asyncness };
@@ -8714,9 +8714,9 @@ impl<'a> Parser<'a> {
     ///
     /// The arguments of the function are replaced in HIR lowering with the arguments created by
     /// this function and the statements created here are inserted at the top of the closure body.
-    fn construct_async_arguments(&mut self, asyncness: &mut Spanned<IsAsync>, decl: &FnDecl) {
+    fn construct_async_arguments(&mut self, asyncness: &mut Spanned<IsAsync>, decl: &mut FnDecl) {
         if let IsAsync::Async { ref mut arguments, .. } = asyncness.node {
-            for (index, input) in decl.inputs.iter().enumerate() {
+            for (index, input) in decl.inputs.iter_mut().enumerate() {
                 let id = ast::DUMMY_NODE_ID;
                 let span = input.pat.span;
 
@@ -8728,8 +8728,10 @@ impl<'a> Parser<'a> {
                 // `let <pat> = __argN;` statement, instead just adding a `let <pat> = <pat>;`
                 // statement.
                 let (binding_mode, ident, is_simple_pattern) = match input.pat.node {
-                    PatKind::Ident(binding_mode, ident, _) => (binding_mode, ident, true),
-                    _ => (BindingMode::ByValue(Mutability::Immutable), ident, false),
+                    PatKind::Ident(binding_mode @ BindingMode::ByValue(_), ident, _) => {
+                        (binding_mode, ident, true)
+                    }
+                    _ => (BindingMode::ByValue(Mutability::Mutable), ident, false),
                 };
 
                 // Construct an argument representing `__argN: <ty>` to replace the argument of the
@@ -8796,6 +8798,15 @@ impl<'a> Parser<'a> {
                     })
                 };
 
+                // Remove mutability from arguments. If this is not a simple pattern,
+                // those arguments are replaced by `__argN`, so there is no need to do this.
+                if let PatKind::Ident(BindingMode::ByValue(mutability @ Mutability::Mutable), ..) =
+                    &mut input.pat.node
+                {
+                    assert!(is_simple_pattern);
+                    *mutability = Mutability::Immutable;
+                }
+
                 let move_stmt = Stmt { id, node: StmtKind::Local(P(move_local)), span };
                 arguments.push(AsyncArgument { ident, arg, pat_stmt, move_stmt });
             }

src/test/ui/async-await/argument-patterns.rs

@@ -0,0 +1,30 @@
+// edition:2018
+// run-pass
+
+#![allow(unused_variables)]
+#![deny(unused_mut)]
+#![feature(async_await)]
+
+type A = Vec<u32>;
+
+async fn a(n: u32, mut vec: A) {
+    vec.push(n);
+}
+
+async fn b(n: u32, ref mut vec: A) {
+    vec.push(n);
+}
+
+async fn c(ref vec: A) {
+    vec.contains(&0);
+}
+
+async fn d((a, mut b): (A, A)) {
+    b.push(1);
+}
+
+async fn f((ref mut a, ref b): (A, A)) {}
+
+async fn g(((ref a, ref mut b), (ref mut c, ref d)): ((A, A), (A, A))) {}
+
+fn main() {}

src/test/ui/async-await/drop-order-for-async-fn-parameters-by-ref-binding.rs

@@ -0,0 +1,271 @@
+// aux-build:arc_wake.rs
+// edition:2018
+// run-pass
+
+#![allow(unused_variables)]
+#![feature(async_await, await_macro)]
+
+// Test that the drop order for parameters in a fn and async fn matches up. Also test that
+// parameters (used or unused) are not dropped until the async fn completes execution.
+// See also #54716.
+
+extern crate arc_wake;
+
+use arc_wake::ArcWake;
+use std::cell::RefCell;
+use std::future::Future;
+use std::marker::PhantomData;
+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 that unused bindings are dropped after the function is polled.
+async fn foo_async(ref mut x: D, ref mut _y: D) {
+    x.1.borrow_mut().push(DropOrder::Function);
+}
+
+fn foo_sync(ref mut x: D, ref mut _y: D) {
+    x.1.borrow_mut().push(DropOrder::Function);
+}
+
+/// Check that underscore patterns are dropped after the function is polled.
+async fn bar_async(ref mut x: D, _: D) {
+    x.1.borrow_mut().push(DropOrder::Function);
+}
+
+fn bar_sync(ref mut x: D, _: D) {
+    x.1.borrow_mut().push(DropOrder::Function);
+}
+
+/// Check that underscore patterns within more complex patterns are dropped after the function
+/// is polled.
+async fn baz_async((ref mut x, _): (D, D)) {
+    x.1.borrow_mut().push(DropOrder::Function);
+}
+
+fn baz_sync((ref mut x, _): (D, D)) {
+    x.1.borrow_mut().push(DropOrder::Function);
+}
+
+/// Check that underscore and unused bindings within and outwith more complex patterns are dropped
+/// after the function is polled.
+async fn foobar_async(ref mut x: D, (ref mut a, _, ref mut _c): (D, D, D), _: D, ref mut _y: D) {
+    x.1.borrow_mut().push(DropOrder::Function);
+}
+
+fn foobar_sync(ref mut x: D, (ref mut a, _, ref mut _c): (D, D, D), _: D, ref mut _y: D) {
+    x.1.borrow_mut().push(DropOrder::Function);
+}
+
+struct Foo;
+
+impl Foo {
+    /// Check that unused bindings are dropped after the method is polled.
+    async fn foo_async(ref mut x: D, ref mut _y: D) {
+        x.1.borrow_mut().push(DropOrder::Function);
+    }
+
+    fn foo_sync(ref mut x: D, ref mut _y: D) {
+        x.1.borrow_mut().push(DropOrder::Function);
+    }
+
+    /// Check that underscore patterns are dropped after the method is polled.
+    async fn bar_async(ref mut x: D, _: D) {
+        x.1.borrow_mut().push(DropOrder::Function);
+    }
+
+    fn bar_sync(ref mut x: D, _: D) {
+        x.1.borrow_mut().push(DropOrder::Function);
+    }
+
+    /// Check that underscore patterns within more complex patterns are dropped after the method
+    /// is polled.
+    async fn baz_async((ref mut x, _): (D, D)) {
+        x.1.borrow_mut().push(DropOrder::Function);
+    }
+
+    fn baz_sync((ref mut x, _): (D, D)) {
+        x.1.borrow_mut().push(DropOrder::Function);
+    }
+
+    /// Check that underscore and unused bindings within and outwith more complex patterns are
+    /// dropped after the method is polled.
+    async fn foobar_async(
+        ref mut x: D, (ref mut a, _, ref mut _c): (D, D, D), _: D, ref mut _y: D,
+    ) {
+        x.1.borrow_mut().push(DropOrder::Function);
+    }
+
+    fn foobar_sync(
+        ref mut x: D, (ref mut a, _, ref mut _c): (D, D, D), _: D, ref mut _y: D,
+    ) {
+        x.1.borrow_mut().push(DropOrder::Function);
+    }
+}
+
+struct Bar<'a>(PhantomData<&'a ()>);
+
+impl<'a> Bar<'a> {
+    /// Check that unused bindings are dropped after the method with self is polled.
+    async fn foo_async(&'a self, ref mut x: D, ref mut _y: D) {
+        x.1.borrow_mut().push(DropOrder::Function);
+    }
+
+    fn foo_sync(&'a self, ref mut x: D, ref mut _y: D) {
+        x.1.borrow_mut().push(DropOrder::Function);
+    }
+
+    /// Check that underscore patterns are dropped after the method with self is polled.
+    async fn bar_async(&'a self, ref mut x: D, _: D) {
+        x.1.borrow_mut().push(DropOrder::Function);
+    }
+
+    fn bar_sync(&'a self, ref mut x: D, _: D) {
+        x.1.borrow_mut().push(DropOrder::Function);
+    }
+
+    /// Check that underscore patterns within more complex patterns are dropped after the method
+    /// with self is polled.
+    async fn baz_async(&'a self, (ref mut x, _): (D, D)) {
+        x.1.borrow_mut().push(DropOrder::Function);
+    }
+
+    fn baz_sync(&'a self, (ref mut x, _): (D, D)) {
+        x.1.borrow_mut().push(DropOrder::Function);
+    }
+
+    /// Check that underscore and unused bindings within and outwith more complex patterns are
+    /// dropped after the method with self is polled.
+    async fn foobar_async(
+        &'a self, ref mut x: D, (ref mut a, _, ref mut _c): (D, D, D), _: D, ref mut _y: D,
+    ) {
+        x.1.borrow_mut().push(DropOrder::Function);
+    }
+
+    fn foobar_sync(
+        &'a self, ref mut x: D, (ref mut a, _, ref mut _c): (D, D, D), _: D, ref mut _y: D,
+    ) {
+        x.1.borrow_mut().push(DropOrder::Function);
+    }
+}
+
+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 _ = fut.as_mut().poll(&mut cx);
+
+    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())));
+    assert_drop_order_after_poll(|l| bar_async(D("x", l.clone()), D("_", l.clone())),
+                                 |l| bar_sync(D("x", l.clone()), D("_", l.clone())));
+    assert_drop_order_after_poll(|l| baz_async((D("x", l.clone()), D("_", l.clone()))),
+                                 |l| baz_sync((D("x", l.clone()), D("_", l.clone()))));
+    assert_drop_order_after_poll(
+        |l| {
+            foobar_async(
+                D("x", l.clone()),
+                (D("a", l.clone()), D("_", l.clone()), D("_c", l.clone())),
+                D("_", l.clone()),
+                D("_y", l.clone()),
+            )
+        },
+        |l| {
+            foobar_sync(
+                D("x", l.clone()),
+                (D("a", l.clone()), D("_", l.clone()), D("_c", l.clone())),
+                D("_", l.clone()),
+                D("_y", l.clone()),
+            )
+        },
+    );
+
+    // Methods w/out self (see doc comment on function for what it tests).
+    assert_drop_order_after_poll(|l| Foo::foo_async(D("x", l.clone()), D("_y", l.clone())),
+                                 |l| Foo::foo_sync(D("x", l.clone()), D("_y", l.clone())));
+    assert_drop_order_after_poll(|l| Foo::bar_async(D("x", l.clone()), D("_", l.clone())),
+                                 |l| Foo::bar_sync(D("x", l.clone()), D("_", l.clone())));
+    assert_drop_order_after_poll(|l| Foo::baz_async((D("x", l.clone()), D("_", l.clone()))),
+                                 |l| Foo::baz_sync((D("x", l.clone()), D("_", l.clone()))));
+    assert_drop_order_after_poll(
+        |l| {
+            Foo::foobar_async(
+                D("x", l.clone()),
+                (D("a", l.clone()), D("_", l.clone()), D("_c", l.clone())),
+                D("_", l.clone()),
+                D("_y", l.clone()),
+            )
+        },
+        |l| {
+            Foo::foobar_sync(
+                D("x", l.clone()),
+                (D("a", l.clone()), D("_", l.clone()), D("_c", l.clone())),
+                D("_", l.clone()),
+                D("_y", l.clone()),
+            )
+        },
+    );
+
+    // Methods (see doc comment on function for what it tests).
+    let b = Bar(Default::default());
+    assert_drop_order_after_poll(|l| b.foo_async(D("x", l.clone()), D("_y", l.clone())),
+                                 |l| b.foo_sync(D("x", l.clone()), D("_y", l.clone())));
+    assert_drop_order_after_poll(|l| b.bar_async(D("x", l.clone()), D("_", l.clone())),
+                                 |l| b.bar_sync(D("x", l.clone()), D("_", l.clone())));
+    assert_drop_order_after_poll(|l| b.baz_async((D("x", l.clone()), D("_", l.clone()))),
+                                 |l| b.baz_sync((D("x", l.clone()), D("_", l.clone()))));
+    assert_drop_order_after_poll(
+        |l| {
+            b.foobar_async(
+                D("x", l.clone()),
+                (D("a", l.clone()), D("_", l.clone()), D("_c", l.clone())),
+                D("_", l.clone()),
+                D("_y", l.clone()),
+            )
+        },
+        |l| {
+            b.foobar_sync(
+                D("x", l.clone()),
+                (D("a", l.clone()), D("_", l.clone()), D("_c", l.clone())),
+                D("_", l.clone()),
+                D("_y", l.clone()),
+            )
+        },
+    );
+}