Auto merge of #90737 - eholk:intofuture, r=tmandry

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.

This isn't quite ready to merge yet. The last attempt was reverting due to performance regressions, so we need to make sure this does not introduce those issues again.

Issues #67644, #67982

/cc `@yoshuawuyts`

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> {
+        // `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 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(