Reintroduce `into_future` in `.await` desugaring

This is a reintroduction of the remaining parts from
https://github.com/rust-lang/rust/pull/65244 that have not been relanded
yet.

Issues GH-67644, GH-67982

2 files changed, 23 insertions, 5 deletions

diff --git a/compiler/rustc_ast_lowering/src/expr.rs b/compiler/rustc_ast_lowering/src/expr.rs
index a0a63620c08..8f3bf5d6341 100644
--- a/compiler/rustc_ast_lowering/src/expr.rs
+++ b/compiler/rustc_ast_lowering/src/expr.rs
@@ -593,7 +593,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
 
     /// Desugar `<expr>.await` into:
     /// ```rust
-    /// match <expr> {
+    /// match ::std::future::IntoFuture::into_future(<expr>) {
     ///     mut pinned => loop {
     ///         match unsafe { ::std::future::Future::poll(
     ///             <::std::pin::Pin>::new_unchecked(&mut pinned),
@@ -629,7 +629,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
             await_span,
             self.allow_gen_future.clone(),
         );
-        let expr = self.lower_expr(expr);
+        let expr = self.lower_expr_mut(expr);
 
         let pinned_ident = Ident::with_dummy_span(sym::pinned);
         let (pinned_pat, pinned_pat_hid) =
@@ -746,10 +746,26 @@ impl<'hir> LoweringContext<'_, 'hir> {
         // mut pinned => loop { ... }
         let pinned_arm = self.arm(pinned_pat, loop_expr);
 
-        // match <expr> {
-        //     mut pinned => loop { .. }
+        // `match ::std::future::IntoFuture::into_future(<expr>) { ... }`
+        let into_future_span = self.mark_span_with_reason(
+            DesugaringKind::Await,
+            await_span,
+            self.allow_into_future.clone(),
+        );
+        //let expr = self.lower_expr_mut(expr);
+        let into_future_expr = self.expr_call_lang_item_fn(
+            into_future_span,
+            hir::LangItem::IntoFutureIntoFuture,
+            arena_vec![self; expr],
+        );
+
+        // match <into_future_expr> {        //     mut pinned => loop { .. }
         // }
-        hir::ExprKind::Match(expr, arena_vec![self; pinned_arm], hir::MatchSource::AwaitDesugar)
+        hir::ExprKind::Match(
+            into_future_expr,
+            arena_vec![self; pinned_arm],
+            hir::MatchSource::AwaitDesugar,
+        )
     }
 
     fn lower_expr_closure(
diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index 2b3a538772e..f5aafda9297 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -162,6 +162,7 @@ struct LoweringContext<'a, 'hir: 'a> {
 
     allow_try_trait: Option<Lrc<[Symbol]>>,
     allow_gen_future: Option<Lrc<[Symbol]>>,
+    allow_into_future: Option<Lrc<[Symbol]>>,
 }
 
 pub trait ResolverAstLowering {
@@ -320,6 +321,7 @@ pub fn lower_crate<'a, 'hir>(
         in_scope_lifetimes: Vec::new(),
         allow_try_trait: Some([sym::try_trait_v2][..].into()),
         allow_gen_future: Some([sym::gen_future][..].into()),
+        allow_into_future: Some([sym::into_future][..].into()),
     }
     .lower_crate(krate)
 }