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

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>) { ... }`
-        //     mut pinned => loop { .. }
+        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(

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)
 }

compiler/rustc_hir/src/lang_items.rs

@@ -347,6 +347,7 @@ language_item_table! {
     ControlFlowContinue,     sym::Continue,            cf_continue_variant,        Target::Variant,        GenericRequirement::None;
     ControlFlowBreak,        sym::Break,               cf_break_variant,           Target::Variant,        GenericRequirement::None;
 
+    IntoFutureIntoFuture,    sym::into_future,         into_future_fn,             Target::Method(MethodKind::Trait { body: false }), GenericRequirement::None;
     IntoIterIntoIter,        sym::into_iter,           into_iter_fn,               Target::Method(MethodKind::Trait { body: false }), GenericRequirement::None;
     IteratorNext,            sym::next,                next_fn,                    Target::Method(MethodKind::Trait { body: false}), GenericRequirement::None;
 

compiler/rustc_span/src/symbol.rs

@@ -196,6 +196,7 @@ symbols! {
         Implied,
         Input,
         Into,
+        IntoFuture,
         IntoIterator,
         IoRead,
         IoWrite,
@@ -734,6 +735,7 @@ symbols! {
         inout,
         instruction_set,
         intel,
+        into_future,
         into_iter,
         intra_doc_pointers,
         intrinsics,

library/core/src/future/into_future.rs

@@ -13,6 +13,7 @@ pub trait IntoFuture {
 
     /// Creates a future from a value.
     #[unstable(feature = "into_future", issue = "67644")]
+    #[cfg_attr(not(bootstrap), lang = "into_future")]
     fn into_future(self) -> Self::Future;
 }
 

src/test/ui/async-await/async-fn-size-moved-locals.rs

@@ -112,7 +112,7 @@ async fn mixed_sizes() {
 fn main() {
     assert_eq!(1025, std::mem::size_of_val(&single()));
     assert_eq!(1026, std::mem::size_of_val(&single_with_noop()));
-    assert_eq!(3078, std::mem::size_of_val(&joined()));
+    assert_eq!(3076, std::mem::size_of_val(&joined()));
-    assert_eq!(3079, std::mem::size_of_val(&joined_with_noop()));
+    assert_eq!(3076, std::mem::size_of_val(&joined_with_noop()));
-    assert_eq!(7181, std::mem::size_of_val(&mixed_sizes()));
+    assert_eq!(6157, std::mem::size_of_val(&mixed_sizes()));
 }

src/test/ui/async-await/await-into-future.rs

@@ -0,0 +1,28 @@
+// check-pass
+
+// edition:2021
+
+#![feature(into_future)]
+
+use std::{future::{Future, IntoFuture}, pin::Pin};
+
+struct AwaitMe;
+
+impl IntoFuture for AwaitMe {
+    type Output = i32;
+    type Future = Pin<Box<dyn Future<Output = i32>>>;
+
+    fn into_future(self) -> Self::Future {
+        Box::pin(me())
+    }
+}
+
+async fn me() -> i32 {
+    41
+}
+
+async fn run() {
+    assert_eq!(AwaitMe.await, 41);
+}
+
+fn main() {}

src/test/ui/async-await/unresolved_type_param.rs

@@ -10,12 +10,20 @@ async fn foo() {
     //~^ ERROR type inside `async fn` body must be known in this context
     //~| ERROR type inside `async fn` body must be known in this context
     //~| ERROR type inside `async fn` body must be known in this context
+    //~| ERROR type inside `async fn` body must be known in this context
+    //~| ERROR type inside `async fn` body must be known in this context
+    //~| NOTE cannot infer type for type parameter `T`
+    //~| NOTE cannot infer type for type parameter `T`
     //~| NOTE cannot infer type for type parameter `T`
     //~| NOTE cannot infer type for type parameter `T`
     //~| NOTE cannot infer type for type parameter `T`
     //~| NOTE the type is part of the `async fn` body because of this `await`
     //~| NOTE the type is part of the `async fn` body because of this `await`
     //~| NOTE the type is part of the `async fn` body because of this `await`
+    //~| NOTE the type is part of the `async fn` body because of this `await`
+    //~| NOTE the type is part of the `async fn` body because of this `await`
+    //~| NOTE in this expansion of desugaring of `await`
+    //~| NOTE in this expansion of desugaring of `await`
     //~| NOTE in this expansion of desugaring of `await`
     //~| NOTE in this expansion of desugaring of `await`
     //~| NOTE in this expansion of desugaring of `await`

src/test/ui/async-await/unresolved_type_param.stderr

@@ -34,6 +34,30 @@ note: the type is part of the `async fn` body because of this `await`
 LL |     bar().await;
    |     ^^^^^^^^^^^
 
-error: aborting due to 3 previous errors
+error[E0698]: type inside `async fn` body must be known in this context
+  --> $DIR/unresolved_type_param.rs:9:5
+   |
+LL |     bar().await;
+   |     ^^^ cannot infer type for type parameter `T` declared on the function `bar`
+   |
+note: the type is part of the `async fn` body because of this `await`
+  --> $DIR/unresolved_type_param.rs:9:5
+   |
+LL |     bar().await;
+   |     ^^^^^^^^^^^
+
+error[E0698]: type inside `async fn` body must be known in this context
+  --> $DIR/unresolved_type_param.rs:9:5
+   |
+LL |     bar().await;
+   |     ^^^ cannot infer type for type parameter `T` declared on the function `bar`
+   |
+note: the type is part of the `async fn` body because of this `await`
+  --> $DIR/unresolved_type_param.rs:9:5
+   |
+LL |     bar().await;
+   |     ^^^^^^^^^^^
+
+error: aborting due to 5 previous errors
 
 For more information about this error, try `rustc --explain E0698`.