Rollup merge of #95011 - michaelwoerister:awaitee_field, r=tmandry

async: Give predictable 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.

This will be useful for async stack traces, as discussed in https://internals.rust-lang.org/t/async-debugging-logical-stack-traces-setting-goals-collecting-examples/15547.

I don't know if this needs some discussion by ````@rust-lang/compiler,```` e.g. about the name of the binding (`__awaitee`) or about the fact that this PR introduces a (soft) guarantee about a compiler generated name. Although, regarding the later, I think the same reasoning applies here as it does for debuginfo in general.

r? ````@tmandry````

2 files changed, 17 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 ee29a1e96eb..0bca0a99e15 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,
@@ -1019,7 +1020,6 @@ symbols! {
         pattern_parentheses,
         phantom_data,
         pin,
-        pinned,
         platform_intrinsics,
         plugin,
         plugin_registrar,