async: Give predictable, reserved name to binding generated from .await expressions.

This name makes it to debuginfo and allows debuggers to identify such bindings and
their captured versions in suspended async fns.

3 files changed, 40 insertions, 14 deletions

diff --git a/compiler/rustc_ast_lowering/src/expr.rs b/compiler/rustc_ast_lowering/src/expr.rs
index d64f1a05712..9442e0f1a1f 100644
--- a/compiler/rustc_ast_lowering/src/expr.rs
+++ b/compiler/rustc_ast_lowering/src/expr.rs
@@ -618,9 +618,9 @@ impl<'hir> LoweringContext<'_, 'hir> {
     /// Desugar `<expr>.await` into:
     /// ```rust
     /// match ::std::future::IntoFuture::into_future(<expr>) {
-    ///     mut pinned => loop {
+    ///     mut __awaitee => loop {
     ///         match unsafe { ::std::future::Future::poll(
-    ///             <::std::pin::Pin>::new_unchecked(&mut pinned),
+    ///             <::std::pin::Pin>::new_unchecked(&mut __awaitee),
     ///             ::std::future::get_context(task_context),
     ///         ) } {
     ///             ::std::task::Poll::Ready(result) => break result,
@@ -657,21 +657,24 @@ impl<'hir> LoweringContext<'_, 'hir> {
         let expr = self.lower_expr_mut(expr);
         let expr_hir_id = expr.hir_id;
 
-        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);
+        // Note that the name of this binding must not be changed to something else because
+        // debuggers and debugger extensions expect it to be called `__awaitee`. They use
+        // this name to identify what is being awaited by a suspended async functions.
+        let awaitee_ident = Ident::with_dummy_span(sym::__awaitee);
+        let (awaitee_pat, awaitee_pat_hid) =
+            self.pat_ident_binding_mode(span, awaitee_ident, hir::BindingAnnotation::Mutable);
 
         let task_context_ident = Ident::with_dummy_span(sym::_task_context);
 
         // unsafe {
         //     ::std::future::Future::poll(
-        //         ::std::pin::Pin::new_unchecked(&mut pinned),
+        //         ::std::pin::Pin::new_unchecked(&mut __awaitee),
         //         ::std::future::get_context(task_context),
         //     )
         // }
         let poll_expr = {
-            let pinned = self.expr_ident(span, pinned_ident, pinned_pat_hid);
-            let ref_mut_pinned = self.expr_mut_addr_of(span, pinned);
+            let awaitee = self.expr_ident(span, awaitee_ident, awaitee_pat_hid);
+            let ref_mut_awaitee = self.expr_mut_addr_of(span, awaitee);
             let task_context = if let Some(task_context_hid) = self.task_context {
                 self.expr_ident_mut(span, task_context_ident, task_context_hid)
             } else {
@@ -681,7 +684,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
             let new_unchecked = self.expr_call_lang_item_fn_mut(
                 span,
                 hir::LangItem::PinNewUnchecked,
-                arena_vec![self; ref_mut_pinned],
+                arena_vec![self; ref_mut_awaitee],
                 Some(expr_hir_id),
             );
             let get_context = self.expr_call_lang_item_fn_mut(
@@ -782,8 +785,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
             span: self.lower_span(span),
         });
 
-        // mut pinned => loop { ... }
-        let pinned_arm = self.arm(pinned_pat, loop_expr);
+        // mut __awaitee => loop { ... }
+        let awaitee_arm = self.arm(awaitee_pat, loop_expr);
 
         // `match ::std::future::IntoFuture::into_future(<expr>) { ... }`
         let into_future_span = self.mark_span_with_reason(
@@ -799,11 +802,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
         );
 
         // match <into_future_expr> {
-        //     mut pinned => loop { .. }
+        //     mut __awaitee => loop { .. }
         // }
         hir::ExprKind::Match(
             into_future_expr,
-            arena_vec![self; pinned_arm],
+            arena_vec![self; awaitee_arm],
             hir::MatchSource::AwaitDesugar,
         )
     }
diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index 4dd1c3fed6b..774cf6a19c7 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -272,6 +272,7 @@ symbols! {
         __D,
         __H,
         __S,
+        __awaitee,
         __try_var,
         _d,
         _e,
@@ -1022,7 +1023,6 @@ symbols! {
         pattern_parentheses,
         phantom_data,
         pin,
-        pinned,
         platform_intrinsics,
         plugin,
         plugin_registrar,
diff --git a/src/test/codegen/async-fn-debug-awaitee-field.rs b/src/test/codegen/async-fn-debug-awaitee-field.rs
new file mode 100644
index 00000000000..efb345fa9f3
--- /dev/null
+++ b/src/test/codegen/async-fn-debug-awaitee-field.rs
@@ -0,0 +1,23 @@
+// This test makes sure that the generator field capturing the awaitee in a `.await` expression
+// is called "__awaitee" in debuginfo. This name must not be changed since debuggers and debugger
+// extensions rely on the field having this name.
+
+// ignore-tidy-linelength
+// compile-flags: -C debuginfo=2 --edition=2018
+
+async fn foo() {}
+
+async fn async_fn_test() {
+    foo().await;
+}
+
+// NONMSVC: [[GEN:!.*]] = !DICompositeType(tag: DW_TAG_structure_type, name: "{async_fn_env#0}",
+// MSVC: [[GEN:!.*]] = !DICompositeType(tag: DW_TAG_union_type, name: "enum$<async_fn_debug_awaitee_field::async_fn_test::async_fn_env$0>",
+// CHECK: [[SUSPEND_STRUCT:!.*]] = !DICompositeType(tag: DW_TAG_structure_type, name: "Suspend0", scope: [[GEN]],
+// CHECK: !DIDerivedType(tag: DW_TAG_member, name: "__awaitee", scope: [[SUSPEND_STRUCT]], {{.*}}, baseType: [[AWAITEE_TYPE:![0-9]*]],
+// NONMSVC: [[AWAITEE_TYPE]] = !DICompositeType(tag: DW_TAG_structure_type, name: "GenFuture<async_fn_debug_awaitee_field::foo::{async_fn_env#0}>",
+// MSVC: [[AWAITEE_TYPE]] = !DICompositeType(tag: DW_TAG_structure_type, name: "GenFuture<enum$<async_fn_debug_awaitee_field::foo::async_fn_env$0> >",
+
+fn main() {
+    let _fn = async_fn_test();
+}