lowering: extend temporary lifetimes around await

This commit changes the HIR lowering around `await` so that temporary
lifetimes are extended. Previously, await was lowered as:

```rust
{
    let mut pinned = future;
    loop {
        match ::std::future::poll_with_tls_context(unsafe {
            <::std::pin::Pin>::new_unchecked(&mut pinned)
        }) {
            ::std::task::Poll::Ready(result) => break result,
            ::std::task::Poll::Pending => {}
        }
        yield ();
    }
}
```

With this commit, await is lowered as:

```rust
match future {
    mut pinned => loop {
        match ::std::future::poll_with_tls_context(unsafe {
            <::std::pin::Pin>::new_unchecked(&mut pinned)
        }) {
            ::std::task::Poll::Ready(result) => break result,
            ::std::task::Poll::Pending => {}
        }
        yield ();
    }
}
```

However, this change has the following side-effects:

- All temporaries in future will be considered to live across a
  yield for the purpose of auto-traits.
- Borrowed temporaries in future are likely to be considered to be live
  across the yield for the purpose of the generator transform.

Signed-off-by: David Wood <david@davidtw.co>

diff --git a/src/librustc/hir/lowering/expr.rs b/src/librustc/hir/lowering/expr.rs
index 0d8986ddec3c7fe7e0a86d08aecf10dd145878cb..a46cdabbb518fd74c1dd8270e60fd872b97095ae 100644 (file)
--- a/src/librustc/hir/lowering/expr.rs
+++ b/src/librustc/hir/lowering/expr.rs
@@ -507,14 +507,13 @@ pub(super) fn make_async_expr(
 
     /// Desugar `<expr>.await` into:
     /// ```rust
-    /// {
-    ///     let mut pinned = <expr>;
-    ///     loop {
+    /// match <expr> {
+    ///     mut pinned => loop {
     ///         match ::std::future::poll_with_tls_context(unsafe {
-    ///             ::std::pin::Pin::new_unchecked(&mut pinned)
+    ///             <::std::pin::Pin>::new_unchecked(&mut pinned)
     ///         }) {
     ///             ::std::task::Poll::Ready(result) => break result,
-    ///             ::std::task::Poll::Pending => {},
+    ///             ::std::task::Poll::Pending => {}
     ///         }
     ///         yield ();
     ///     }
@@ -549,21 +548,12 @@ fn lower_expr_await(&mut self, await_span: Span, expr: &Expr) -> hir::ExprKind {
             self.allow_gen_future.clone(),
         );
 
-        // let mut pinned = <expr>;
-        let expr = P(self.lower_expr(expr));
         let pinned_ident = Ident::with_dummy_span(sym::pinned);
         let (pinned_pat, pinned_pat_hid) = self.pat_ident_binding_mode(
             span,
             pinned_ident,
             hir::BindingAnnotation::Mutable,
         );
-        let pinned_let = self.stmt_let_pat(
-            ThinVec::new(),
-            span,
-            Some(expr),
-            pinned_pat,
-            hir::LocalSource::AwaitDesugar,
-        );
 
         // ::std::future::poll_with_tls_context(unsafe {
         //     ::std::pin::Pin::new_unchecked(&mut pinned)
@@ -621,7 +611,7 @@ fn lower_expr_await(&mut self, await_span: Span, expr: &Expr) -> hir::ExprKind {
             self.arm(hir_vec![pending_pat], empty_block)
         };
 
-        let match_stmt = {
+        let inner_match_stmt = {
             let match_expr = self.expr_match(
                 span,
                 poll_expr,
@@ -643,10 +633,11 @@ fn lower_expr_await(&mut self, await_span: Span, expr: &Expr) -> hir::ExprKind {
 
         let loop_block = P(self.block_all(
             span,
-            hir_vec![match_stmt, yield_stmt],
+            hir_vec![inner_match_stmt, yield_stmt],
             None,
         ));
 
+        // loop { .. }
         let loop_expr = P(hir::Expr {
             hir_id: loop_hir_id,
             node: hir::ExprKind::Loop(
@@ -658,10 +649,14 @@ fn lower_expr_await(&mut self, await_span: Span, expr: &Expr) -> hir::ExprKind {
             attrs: ThinVec::new(),
         });
 
-        hir::ExprKind::Block(
-            P(self.block_all(span, hir_vec![pinned_let], Some(loop_expr))),
-            None,
-        )
+        // mut pinned => loop { ... }
+        let pinned_arm = self.arm(hir_vec![pinned_pat], loop_expr);
+
+        // match <expr> {
+        //     mut pinned => loop { .. }
+        // }
+        let expr = P(self.lower_expr(expr));
+        hir::ExprKind::Match(expr, hir_vec![pinned_arm], hir::MatchSource::AwaitDesugar)
     }
 
     fn lower_expr_closure(

diff --git a/src/test/ui/async-await/async-fn-nonsend.stderr b/src/test/ui/async-await/async-fn-nonsend.stderr
index fad90b29c0e6e62e89448f94a5a0439279da5945..d2f92f04f40a716c078e7ca4b72c757b02b635d6 100644 (file)
--- a/src/test/ui/async-await/async-fn-nonsend.stderr
+++ b/src/test/ui/async-await/async-fn-nonsend.stderr
@@ -9,9 +9,9 @@ LL |     assert_send(local_dropped_before_await());
    |
    = help: within `impl std::future::Future`, the trait `std::marker::Send` is not implemented for `std::rc::Rc<()>`
    = note: required because it appears within the type `impl std::fmt::Debug`
-   = note: required because it appears within the type `{impl std::fmt::Debug, impl std::future::Future, ()}`
-   = note: required because it appears within the type `[static generator@$DIR/async-fn-nonsend.rs:21:39: 26:2 {impl std::fmt::Debug, impl std::future::Future, ()}]`
-   = note: required because it appears within the type `std::future::GenFuture<[static generator@$DIR/async-fn-nonsend.rs:21:39: 26:2 {impl std::fmt::Debug, impl std::future::Future, ()}]>`
+   = note: required because it appears within the type `{impl std::fmt::Debug, fn() -> impl std::future::Future {fut}, impl std::future::Future, ()}`
+   = note: required because it appears within the type `[static generator@$DIR/async-fn-nonsend.rs:21:39: 26:2 {impl std::fmt::Debug, fn() -> impl std::future::Future {fut}, impl std::future::Future, ()}]`
+   = note: required because it appears within the type `std::future::GenFuture<[static generator@$DIR/async-fn-nonsend.rs:21:39: 26:2 {impl std::fmt::Debug, fn() -> impl std::future::Future {fut}, impl std::future::Future, ()}]>`
    = note: required because it appears within the type `impl std::future::Future`
    = note: required because it appears within the type `impl std::future::Future`
 
@@ -26,9 +26,9 @@ LL |     assert_send(non_send_temporary_in_match());
    |
    = help: within `impl std::future::Future`, the trait `std::marker::Send` is not implemented for `std::rc::Rc<()>`
    = note: required because it appears within the type `impl std::fmt::Debug`
-   = note: required because it appears within the type `{fn(impl std::fmt::Debug) -> std::option::Option<impl std::fmt::Debug> {std::option::Option::<impl std::fmt::Debug>::Some}, fn() -> impl std::fmt::Debug {non_send}, impl std::fmt::Debug, std::option::Option<impl std::fmt::Debug>, impl std::future::Future, ()}`
-   = note: required because it appears within the type `[static generator@$DIR/async-fn-nonsend.rs:28:40: 37:2 {fn(impl std::fmt::Debug) -> std::option::Option<impl std::fmt::Debug> {std::option::Option::<impl std::fmt::Debug>::Some}, fn() -> impl std::fmt::Debug {non_send}, impl std::fmt::Debug, std::option::Option<impl std::fmt::Debug>, impl std::future::Future, ()}]`
-   = note: required because it appears within the type `std::future::GenFuture<[static generator@$DIR/async-fn-nonsend.rs:28:40: 37:2 {fn(impl std::fmt::Debug) -> std::option::Option<impl std::fmt::Debug> {std::option::Option::<impl std::fmt::Debug>::Some}, fn() -> impl std::fmt::Debug {non_send}, impl std::fmt::Debug, std::option::Option<impl std::fmt::Debug>, impl std::future::Future, ()}]>`
+   = note: required because it appears within the type `{fn(impl std::fmt::Debug) -> std::option::Option<impl std::fmt::Debug> {std::option::Option::<impl std::fmt::Debug>::Some}, fn() -> impl std::fmt::Debug {non_send}, impl std::fmt::Debug, std::option::Option<impl std::fmt::Debug>, fn() -> impl std::future::Future {fut}, impl std::future::Future, ()}`
+   = note: required because it appears within the type `[static generator@$DIR/async-fn-nonsend.rs:28:40: 37:2 {fn(impl std::fmt::Debug) -> std::option::Option<impl std::fmt::Debug> {std::option::Option::<impl std::fmt::Debug>::Some}, fn() -> impl std::fmt::Debug {non_send}, impl std::fmt::Debug, std::option::Option<impl std::fmt::Debug>, fn() -> impl std::future::Future {fut}, impl std::future::Future, ()}]`
+   = note: required because it appears within the type `std::future::GenFuture<[static generator@$DIR/async-fn-nonsend.rs:28:40: 37:2 {fn(impl std::fmt::Debug) -> std::option::Option<impl std::fmt::Debug> {std::option::Option::<impl std::fmt::Debug>::Some}, fn() -> impl std::fmt::Debug {non_send}, impl std::fmt::Debug, std::option::Option<impl std::fmt::Debug>, fn() -> impl std::future::Future {fut}, impl std::future::Future, ()}]>`
    = note: required because it appears within the type `impl std::future::Future`
    = note: required because it appears within the type `impl std::future::Future`
 
@@ -45,9 +45,9 @@ LL |     assert_send(non_sync_with_method_call());
    = note: required because of the requirements on the impl of `std::marker::Send` for `&mut dyn std::fmt::Write`
    = note: required because it appears within the type `std::fmt::Formatter<'_>`
    = note: required because of the requirements on the impl of `std::marker::Send` for `&mut std::fmt::Formatter<'_>`
-   = note: required because it appears within the type `for<'r, 's> {&'r mut std::fmt::Formatter<'s>, bool, impl std::future::Future, ()}`
-   = note: required because it appears within the type `[static generator@$DIR/async-fn-nonsend.rs:39:38: 45:2 for<'r, 's> {&'r mut std::fmt::Formatter<'s>, bool, impl std::future::Future, ()}]`
-   = note: required because it appears within the type `std::future::GenFuture<[static generator@$DIR/async-fn-nonsend.rs:39:38: 45:2 for<'r, 's> {&'r mut std::fmt::Formatter<'s>, bool, impl std::future::Future, ()}]>`
+   = note: required because it appears within the type `for<'r, 's> {&'r mut std::fmt::Formatter<'s>, bool, fn() -> impl std::future::Future {fut}, impl std::future::Future, ()}`
+   = note: required because it appears within the type `[static generator@$DIR/async-fn-nonsend.rs:39:38: 45:2 for<'r, 's> {&'r mut std::fmt::Formatter<'s>, bool, fn() -> impl std::future::Future {fut}, impl std::future::Future, ()}]`
+   = note: required because it appears within the type `std::future::GenFuture<[static generator@$DIR/async-fn-nonsend.rs:39:38: 45:2 for<'r, 's> {&'r mut std::fmt::Formatter<'s>, bool, fn() -> impl std::future::Future {fut}, impl std::future::Future, ()}]>`
    = note: required because it appears within the type `impl std::future::Future`
    = note: required because it appears within the type `impl std::future::Future`
 
@@ -68,9 +68,9 @@ LL |     assert_send(non_sync_with_method_call());
    = note: required because of the requirements on the impl of `std::marker::Send` for `std::slice::Iter<'_, std::fmt::ArgumentV1<'_>>`
    = note: required because it appears within the type `std::fmt::Formatter<'_>`
    = note: required because of the requirements on the impl of `std::marker::Send` for `&mut std::fmt::Formatter<'_>`
-   = note: required because it appears within the type `for<'r, 's> {&'r mut std::fmt::Formatter<'s>, bool, impl std::future::Future, ()}`
-   = note: required because it appears within the type `[static generator@$DIR/async-fn-nonsend.rs:39:38: 45:2 for<'r, 's> {&'r mut std::fmt::Formatter<'s>, bool, impl std::future::Future, ()}]`
-   = note: required because it appears within the type `std::future::GenFuture<[static generator@$DIR/async-fn-nonsend.rs:39:38: 45:2 for<'r, 's> {&'r mut std::fmt::Formatter<'s>, bool, impl std::future::Future, ()}]>`
+   = note: required because it appears within the type `for<'r, 's> {&'r mut std::fmt::Formatter<'s>, bool, fn() -> impl std::future::Future {fut}, impl std::future::Future, ()}`
+   = note: required because it appears within the type `[static generator@$DIR/async-fn-nonsend.rs:39:38: 45:2 for<'r, 's> {&'r mut std::fmt::Formatter<'s>, bool, fn() -> impl std::future::Future {fut}, impl std::future::Future, ()}]`
+   = note: required because it appears within the type `std::future::GenFuture<[static generator@$DIR/async-fn-nonsend.rs:39:38: 45:2 for<'r, 's> {&'r mut std::fmt::Formatter<'s>, bool, fn() -> impl std::future::Future {fut}, impl std::future::Future, ()}]>`
    = note: required because it appears within the type `impl std::future::Future`
    = note: required because it appears within the type `impl std::future::Future`
 

diff --git a/src/test/ui/async-await/issue-63832-await-short-temporary-lifetime-1.rs b/src/test/ui/async-await/issue-63832-await-short-temporary-lifetime-1.rs
new file mode 100644 (file)
index 0000000..54059b2
--- /dev/null
+++ b/src/test/ui/async-await/issue-63832-await-short-temporary-lifetime-1.rs
@@ -0,0 +1,19 @@
+// check-pass
+// edition:2018
+
+struct Test(String);
+
+impl Test {
+    async fn borrow_async(&self) {}
+
+    fn with(&mut self, s: &str) -> &mut Self {
+        self.0 = s.into();
+        self
+    }
+}
+
+async fn test() {
+    Test("".to_string()).with("123").borrow_async().await;
+}
+
+fn main() { }

diff --git a/src/test/ui/async-await/issue-63832-await-short-temporary-lifetime.rs b/src/test/ui/async-await/issue-63832-await-short-temporary-lifetime.rs
new file mode 100644 (file)
index 0000000..c5ea2b8
--- /dev/null
+++ b/src/test/ui/async-await/issue-63832-await-short-temporary-lifetime.rs
@@ -0,0 +1,12 @@
+// check-pass
+// edition:2018
+
+async fn foo(x: &[Vec<u32>]) -> u32 {
+    0
+}
+
+async fn bar() {
+    foo(&[vec![123]]).await;
+}
+
+fn main() { }