Auto merge of #104321 - Swatinem:async-gen, r=oli-obk

Avoid `GenFuture` shim when compiling async constructs

Previously, async constructs would be lowered to "normal" generators, with an additional `from_generator` / `GenFuture` shim in between to convert from `Generator` to `Future`.

The compiler will now special-case these generators internally so that async constructs will *directly* implement `Future` without the need to go through the `from_generator` / `GenFuture` shim.

The primary motivation for this change was hiding this implementation detail in stack traces and debuginfo, but it can in theory also help the optimizer as there is less abstractions to see through.

---

Given this demo code:

```rust
pub async fn a(arg: u32) -> Backtrace {
    let bt = b().await;
    let _arg = arg;
    bt
}

pub async fn b() -> Backtrace {
    Backtrace::force_capture()
}
```

I would get the following with the latest stable compiler (on Windows):

```
   4: async_codegen::b::async_fn$0
             at .\src\lib.rs:10
   5: core::future::from_generator::impl$1::poll<enum2$<async_codegen::b::async_fn_env$0> >
             at /rustc/897e37553bba8b42751c67658967889d11ecd120\library\core\src\future\mod.rs:91
   6: async_codegen::a::async_fn$0
             at .\src\lib.rs:4
   7: core::future::from_generator::impl$1::poll<enum2$<async_codegen::a::async_fn_env$0> >
             at /rustc/897e37553bba8b42751c67658967889d11ecd120\library\core\src\future\mod.rs:91
```

whereas now I get a much cleaner stack trace:

```
   3: async_codegen::b::async_fn$0
             at .\src\lib.rs:10
   4: async_codegen::a::async_fn$0
             at .\src\lib.rs:4
```

16 files changed, 83 insertions, 79 deletions

diff --git a/src/test/codegen/async-fn-debug-awaitee-field.rs b/src/test/codegen/async-fn-debug-awaitee-field.rs
index 909cd0062a6..bc268615814 100644
--- a/src/test/codegen/async-fn-debug-awaitee-field.rs
+++ b/src/test/codegen/async-fn-debug-awaitee-field.rs
@@ -11,12 +11,14 @@ async fn async_fn_test() {
     foo().await;
 }
 
-// NONMSVC: [[GEN:!.*]] = !DICompositeType(tag: DW_TAG_structure_type, name: "{async_fn_env#0}",
+// NONMSVC: [[GEN:!.*]] = !DICompositeType(tag: DW_TAG_structure_type, name: "{async_fn_env#0}", scope: [[GEN_SCOPE:![0-9]*]],
 // MSVC: [[GEN:!.*]] = !DICompositeType(tag: DW_TAG_union_type, name: "enum2$<async_fn_debug_awaitee_field::async_fn_test::async_fn_env$0>",
+// NONMSVC: [[GEN_SCOPE:!.*]] = !DINamespace(name: "async_fn_test",
 // CHECK: [[SUSPEND_STRUCT:!.*]] = !DICompositeType(tag: DW_TAG_structure_type, name: "Suspend0", scope: [[GEN]],
 // CHECK: !DIDerivedType(tag: DW_TAG_member, name: "__awaitee", scope: [[SUSPEND_STRUCT]], {{.*}}, baseType: [[AWAITEE_TYPE:![0-9]*]],
-// NONMSVC: [[AWAITEE_TYPE]] = !DICompositeType(tag: DW_TAG_structure_type, name: "GenFuture<async_fn_debug_awaitee_field::foo::{async_fn_env#0}>",
-// MSVC: [[AWAITEE_TYPE]] = !DICompositeType(tag: DW_TAG_structure_type, name: "GenFuture<enum2$<async_fn_debug_awaitee_field::foo::async_fn_env$0> >",
+// NONMSVC: [[AWAITEE_TYPE]] = !DICompositeType(tag: DW_TAG_structure_type, name: "{async_fn_env#0}", scope: [[AWAITEE_SCOPE:![0-9]*]],
+// MSVC: [[AWAITEE_TYPE]] = !DICompositeType(tag: DW_TAG_union_type, name: "enum2$<async_fn_debug_awaitee_field::foo::async_fn_env$0>",
+// NONMSVC: [[AWAITEE_SCOPE]] = !DINamespace(name: "foo",
 
 fn main() {
     let _fn = async_fn_test();
diff --git a/src/test/run-make/coverage-reports/expected_show_coverage.async2.txt b/src/test/run-make/coverage-reports/expected_show_coverage.async2.txt
index dc06a485a8f..500dde1f269 100644
--- a/src/test/run-make/coverage-reports/expected_show_coverage.async2.txt
+++ b/src/test/run-make/coverage-reports/expected_show_coverage.async2.txt
@@ -72,7 +72,7 @@
    67|       |        }
    68|      2|    }
   ------------------
-  | async2::executor::block_on::<core::future::from_generator::GenFuture<async2::async_func::{closure#0}>>:
+  | async2::executor::block_on::<async2::async_func::{closure#0}>:
   |   51|      1|    pub fn block_on<F: Future>(mut future: F) -> F::Output {
   |   52|      1|        let mut future = unsafe { Pin::new_unchecked(&mut future) };
   |   53|      1|        use std::hint::unreachable_unchecked;
@@ -92,7 +92,7 @@
   |   67|       |        }
   |   68|      1|    }
   ------------------
-  | async2::executor::block_on::<core::future::from_generator::GenFuture<async2::async_func_just_println::{closure#0}>>:
+  | async2::executor::block_on::<async2::async_func_just_println::{closure#0}>:
   |   51|      1|    pub fn block_on<F: Future>(mut future: F) -> F::Output {
   |   52|      1|        let mut future = unsafe { Pin::new_unchecked(&mut future) };
   |   53|      1|        use std::hint::unreachable_unchecked;
diff --git a/src/test/ui/async-await/async-block-control-flow-static-semantics.stderr b/src/test/ui/async-await/async-block-control-flow-static-semantics.stderr
index 2a08d5d6ce5..b8ca64fae83 100644
--- a/src/test/ui/async-await/async-block-control-flow-static-semantics.stderr
+++ b/src/test/ui/async-await/async-block-control-flow-static-semantics.stderr
@@ -1,8 +1,7 @@
 error[E0267]: `break` inside of an `async` block
   --> $DIR/async-block-control-flow-static-semantics.rs:32:9
    |
-LL |       async {
-   |  ___________-
+LL | /     async {
 LL | |         break 0u8;
    | |         ^^^^^^^^^ cannot `break` inside of an `async` block
 LL | |     };
@@ -11,8 +10,7 @@ LL | |     };
 error[E0267]: `break` inside of an `async` block
   --> $DIR/async-block-control-flow-static-semantics.rs:39:13
    |
-LL |           async {
-   |  _______________-
+LL | /         async {
 LL | |             break 0u8;
    | |             ^^^^^^^^^ cannot `break` inside of an `async` block
 LL | |         };
diff --git a/src/test/ui/async-await/async-borrowck-escaping-block-error.stderr b/src/test/ui/async-await/async-borrowck-escaping-block-error.stderr
index f21c8115124..190c59e32eb 100644
--- a/src/test/ui/async-await/async-borrowck-escaping-block-error.stderr
+++ b/src/test/ui/async-await/async-borrowck-escaping-block-error.stderr
@@ -1,11 +1,11 @@
 error[E0373]: async block may outlive the current function, but it borrows `x`, which is owned by the current function
-  --> $DIR/async-borrowck-escaping-block-error.rs:6:20
+  --> $DIR/async-borrowck-escaping-block-error.rs:6:14
    |
 LL |     Box::new(async { x } )
-   |                    ^^-^^
-   |                    | |
-   |                    | `x` is borrowed here
-   |                    may outlive borrowed value `x`
+   |              ^^^^^^^^-^^
+   |              |       |
+   |              |       `x` is borrowed here
+   |              may outlive borrowed value `x`
    |
 note: async block is returned here
   --> $DIR/async-borrowck-escaping-block-error.rs:6:5
@@ -18,13 +18,13 @@ LL |     Box::new(async move { x } )
    |                    ++++
 
 error[E0373]: async block may outlive the current function, but it borrows `x`, which is owned by the current function
-  --> $DIR/async-borrowck-escaping-block-error.rs:11:11
+  --> $DIR/async-borrowck-escaping-block-error.rs:11:5
    |
 LL |     async { *x }
-   |           ^^--^^
-   |           | |
-   |           | `x` is borrowed here
-   |           may outlive borrowed value `x`
+   |     ^^^^^^^^--^^
+   |     |       |
+   |     |       `x` is borrowed here
+   |     may outlive borrowed value `x`
    |
 note: async block is returned here
   --> $DIR/async-borrowck-escaping-block-error.rs:11:5
diff --git a/src/test/ui/async-await/generator-desc.stderr b/src/test/ui/async-await/generator-desc.stderr
index 2494c3feb2a..774c97966b1 100644
--- a/src/test/ui/async-await/generator-desc.stderr
+++ b/src/test/ui/async-await/generator-desc.stderr
@@ -1,20 +1,20 @@
 error[E0308]: mismatched types
-  --> $DIR/generator-desc.rs:10:25
+  --> $DIR/generator-desc.rs:10:19
    |
 LL |     fun(async {}, async {});
-   |               --        ^^
-   |               |         |
-   |               |         expected `async` block, found a different `async` block
-   |               |         arguments to this function are incorrect
-   |               the expected `async` block
+   |         --------  ^^^^^^^^
+   |         |         |
+   |         |         expected `async` block, found a different `async` block
+   |         |         arguments to this function are incorrect
+   |         the expected `async` block
    |
-   = note: expected `async` block `[static generator@$DIR/generator-desc.rs:10:15: 10:17]`
-              found `async` block `[static generator@$DIR/generator-desc.rs:10:25: 10:27]`
+   = note: expected `async` block `impl Future<Output = ()>` (`async` block)
+              found `async` block `impl Future<Output = ()>` (`async` block)
 note: function defined here
   --> $SRC_DIR/core/src/future/mod.rs:LL:COL
    |
-LL | pub const fn from_generator<T>(gen: T) -> impl Future<Output = T::Return>
-   |              ^^^^^^^^^^^^^^
+LL | pub const fn identity_future<O, Fut: Future<Output = O>>(f: Fut) -> Fut {
+   |              ^^^^^^^^^^^^^^^
 
 error[E0308]: mismatched types
   --> $DIR/generator-desc.rs:12:16
@@ -53,16 +53,8 @@ LL |     fun((async || {})(), (async || {})());
    |     |             the expected `async` closure body
    |     arguments to this function are incorrect
    |
-  ::: $SRC_DIR/core/src/future/mod.rs:LL:COL
-   |
-LL | pub const fn from_generator<T>(gen: T) -> impl Future<Output = T::Return>
-   |                                           -------------------------------
-   |                                           |
-   |                                           the expected opaque type
-   |                                           the found opaque type
-   |
-   = note: expected opaque type `impl Future<Output = ()>` (`async` closure body)
-              found opaque type `impl Future<Output = ()>` (`async` closure body)
+   = note: expected `async` closure body `impl Future<Output = ()>` (`async` closure body)
+              found `async` closure body `impl Future<Output = ()>` (`async` closure body)
 note: function defined here
   --> $DIR/generator-desc.rs:8:4
    |
diff --git a/src/test/ui/async-await/issue-68112.drop_tracking.stderr b/src/test/ui/async-await/issue-68112.drop_tracking.stderr
index c915164cfce..f2802698fd5 100644
--- a/src/test/ui/async-await/issue-68112.drop_tracking.stderr
+++ b/src/test/ui/async-await/issue-68112.drop_tracking.stderr
@@ -59,10 +59,10 @@ LL | fn make_non_send_future2() -> impl Future<Output = Arc<RefCell<i32>>> {
    |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: required because it captures the following types: `ResumeTy`, `impl Future<Output = Arc<RefCell<i32>>>`, `()`, `Ready<i32>`
 note: required because it's used within this `async` block
-  --> $DIR/issue-68112.rs:60:26
+  --> $DIR/issue-68112.rs:60:20
    |
 LL |       let send_fut = async {
-   |  __________________________^
+   |  ____________________^
 LL | |         let non_send_fut = make_non_send_future2();
 LL | |         let _ = non_send_fut.await;
 LL | |         ready(0).await;
diff --git a/src/test/ui/async-await/issue-68112.no_drop_tracking.stderr b/src/test/ui/async-await/issue-68112.no_drop_tracking.stderr
index 11b7d1aaaa6..38eb85b302f 100644
--- a/src/test/ui/async-await/issue-68112.no_drop_tracking.stderr
+++ b/src/test/ui/async-await/issue-68112.no_drop_tracking.stderr
@@ -59,10 +59,10 @@ LL | fn make_non_send_future2() -> impl Future<Output = Arc<RefCell<i32>>> {
    |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: required because it captures the following types: `ResumeTy`, `impl Future<Output = Arc<RefCell<i32>>>`, `()`, `i32`, `Ready<i32>`
 note: required because it's used within this `async` block
-  --> $DIR/issue-68112.rs:60:26
+  --> $DIR/issue-68112.rs:60:20
    |
 LL |       let send_fut = async {
-   |  __________________________^
+   |  ____________________^
 LL | |         let non_send_fut = make_non_send_future2();
 LL | |         let _ = non_send_fut.await;
 LL | |         ready(0).await;
diff --git a/src/test/ui/async-await/issue-70935-complex-spans.drop_tracking.stderr b/src/test/ui/async-await/issue-70935-complex-spans.drop_tracking.stderr
index 7fb88116665..721234aa4a7 100644
--- a/src/test/ui/async-await/issue-70935-complex-spans.drop_tracking.stderr
+++ b/src/test/ui/async-await/issue-70935-complex-spans.drop_tracking.stderr
@@ -20,10 +20,9 @@ LL | | }
    | |_^
    = note: required because it captures the following types: `ResumeTy`, `impl Future<Output = ()>`, `()`
 note: required because it's used within this `async` block
-  --> $DIR/issue-70935-complex-spans.rs:16:16
+  --> $DIR/issue-70935-complex-spans.rs:16:5
    |
-LL |       async move {
-   |  ________________^
+LL | /     async move {
 LL | |         baz(|| async{
 LL | |             foo(tx.clone());
 LL | |         }).await;
diff --git a/src/test/ui/async-await/issues/issue-78938-async-block.stderr b/src/test/ui/async-await/issues/issue-78938-async-block.stderr
index 29aa8372f87..c1a4b467f10 100644
--- a/src/test/ui/async-await/issues/issue-78938-async-block.stderr
+++ b/src/test/ui/async-await/issues/issue-78938-async-block.stderr
@@ -1,8 +1,8 @@
 error[E0373]: async block may outlive the current function, but it borrows `room_ref`, which is owned by the current function
-  --> $DIR/issue-78938-async-block.rs:8:39
+  --> $DIR/issue-78938-async-block.rs:8:33
    |
 LL |       let gameloop_handle = spawn(async {
-   |  _______________________________________^
+   |  _________________________________^
 LL | |         game_loop(Arc::clone(&room_ref))
    | |                               -------- `room_ref` is borrowed here
 LL | |     });
diff --git a/src/test/ui/async-await/try-on-option-in-async.stderr b/src/test/ui/async-await/try-on-option-in-async.stderr
index a55850d76c3..4c7b4fa41fa 100644
--- a/src/test/ui/async-await/try-on-option-in-async.stderr
+++ b/src/test/ui/async-await/try-on-option-in-async.stderr
@@ -1,8 +1,7 @@
 error[E0277]: the `?` operator can only be used in an async block that returns `Result` or `Option` (or another type that implements `FromResidual`)
   --> $DIR/try-on-option-in-async.rs:8:10
    |
-LL |       async {
-   |  ___________-
+LL | /     async {
 LL | |         let x: Option<u32> = None;
 LL | |         x?;
    | |          ^ cannot use the `?` operator in an async block that returns `{integer}`
diff --git a/src/test/ui/chalkify/bugs/async.stderr b/src/test/ui/chalkify/bugs/async.stderr
index f53ed53f73c..6f22d2c593a 100644
--- a/src/test/ui/chalkify/bugs/async.stderr
+++ b/src/test/ui/chalkify/bugs/async.stderr
@@ -1,32 +1,47 @@
-error[E0277]: the trait bound `[static generator@$DIR/async.rs:7:29: 9:2]: Generator<ResumeTy>` is not satisfied
+error[E0277]: `impl Future<Output = u32>` is not a future
   --> $DIR/async.rs:7:29
    |
 LL |   async fn foo(x: u32) -> u32 {
-   |  _____________________________^
+   |  _____________________________-
 LL | |     x
 LL | | }
-   | |_^ the trait `Generator<ResumeTy>` is not implemented for `[static generator@$DIR/async.rs:7:29: 9:2]`
+   | | ^
+   | | |
+   | |_`impl Future<Output = u32>` is not a future
+   |   required by a bound introduced by this call
    |
-note: required by a bound in `std::future::from_generator`
+   = help: the trait `Future` is not implemented for `impl Future<Output = u32>`
+   = note: impl Future<Output = u32> must be a future or must implement `IntoFuture` to be awaited
+note: required by a bound in `identity_future`
   --> $SRC_DIR/core/src/future/mod.rs:LL:COL
    |
-LL |     T: Generator<ResumeTy, Yield = ()>,
-   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `std::future::from_generator`
+LL | pub const fn identity_future<O, Fut: Future<Output = O>>(f: Fut) -> Fut {
+   |                                      ^^^^^^^^^^^^^^^^^^ required by this bound in `identity_future`
 
-error[E0280]: the requirement `<[static generator@$DIR/async.rs:7:29: 9:2] as Generator<ResumeTy>>::Yield == ()` is not satisfied
+error[E0277]: the size for values of type `<impl Future<Output = u32> as Future>::Output` cannot be known at compilation time
   --> $DIR/async.rs:7:29
    |
 LL |   async fn foo(x: u32) -> u32 {
    |  _____________________________^
 LL | |     x
 LL | | }
-   | |_^
+   | |_^ doesn't have a size known at compile-time
    |
-note: required by a bound in `std::future::from_generator`
+   = help: the trait `Sized` is not implemented for `<impl Future<Output = u32> as Future>::Output`
+note: required by a bound in `identity_future`
   --> $SRC_DIR/core/src/future/mod.rs:LL:COL
    |
-LL |     T: Generator<ResumeTy, Yield = ()>,
-   |                            ^^^^^^^^^^ required by this bound in `std::future::from_generator`
+LL | pub const fn identity_future<O, Fut: Future<Output = O>>(f: Fut) -> Fut {
+   |                              ^ required by this bound in `identity_future`
+
+error[E0277]: `impl Future<Output = u32>` is not a future
+  --> $DIR/async.rs:7:25
+   |
+LL | async fn foo(x: u32) -> u32 {
+   |                         ^^^ `impl Future<Output = u32>` is not a future
+   |
+   = help: the trait `Future` is not implemented for `impl Future<Output = u32>`
+   = note: impl Future<Output = u32> must be a future or must implement `IntoFuture` to be awaited
 
 error[E0280]: the requirement `<impl Future<Output = u32> as Future>::Output == u32` is not satisfied
   --> $DIR/async.rs:7:25
@@ -34,6 +49,6 @@ error[E0280]: the requirement `<impl Future<Output = u32> as Future>::Output ==
 LL | async fn foo(x: u32) -> u32 {
    |                         ^^^
 
-error: aborting due to 3 previous errors
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0277`.
diff --git a/src/test/ui/lazy-type-alias-impl-trait/freeze_cycle.rs b/src/test/ui/lazy-type-alias-impl-trait/freeze_cycle.rs
index 10f6bd74031..f02a93ed41b 100644
--- a/src/test/ui/lazy-type-alias-impl-trait/freeze_cycle.rs
+++ b/src/test/ui/lazy-type-alias-impl-trait/freeze_cycle.rs
@@ -1,6 +1,6 @@
 // check-pass
 
-#![feature(gen_future, generator_trait, negative_impls)]
+#![feature(generator_trait, negative_impls)]
 
 use std::ops::{Generator, GeneratorState};
 use std::task::{Poll, Context};
diff --git a/src/test/ui/suggestions/expected-boxed-future-isnt-pinned.stderr b/src/test/ui/suggestions/expected-boxed-future-isnt-pinned.stderr
index 77cef485f30..918d37e6559 100644
--- a/src/test/ui/suggestions/expected-boxed-future-isnt-pinned.stderr
+++ b/src/test/ui/suggestions/expected-boxed-future-isnt-pinned.stderr
@@ -78,20 +78,21 @@ LL | impl<P: Deref<Target: Unpin>> Pin<P> {
 error[E0308]: mismatched types
   --> $DIR/expected-boxed-future-isnt-pinned.rs:28:5
    |
-LL |   fn zap() -> BoxFuture<'static, i32> {
-   |               ----------------------- expected `Pin<Box<(dyn Future<Output = i32> + Send + 'static)>>` because of return type
 LL | /     async {
 LL | |         42
 LL | |     }
-   | |_____^ expected struct `Pin`, found opaque type
-   |
-  ::: $SRC_DIR/core/src/future/mod.rs:LL:COL
-   |
-LL |   pub const fn from_generator<T>(gen: T) -> impl Future<Output = T::Return>
-   |                                             ------------------------------- the found opaque type
-   |
-   = note:   expected struct `Pin<Box<(dyn Future<Output = i32> + Send + 'static)>>`
-           found opaque type `impl Future<Output = {integer}>`
+   | |     ^
+   | |     |
+   | |_____expected struct `Pin`, found `async` block
+   |       arguments to this function are incorrect
+   |
+   = note:     expected struct `Pin<Box<dyn Future<Output = i32> + Send>>`
+           found `async` block `impl Future<Output = {integer}>`
+note: function defined here
+  --> $SRC_DIR/core/src/future/mod.rs:LL:COL
+   |
+LL | pub const fn identity_future<O, Fut: Future<Output = O>>(f: Fut) -> Fut {
+   |              ^^^^^^^^^^^^^^^
 help: you need to pin and box this expression
    |
 LL ~     Box::pin(async {
diff --git a/src/tools/clippy/clippy_lints/src/doc.rs b/src/tools/clippy/clippy_lints/src/doc.rs
index 4557e432885..ae5f9424b23 100644
--- a/src/tools/clippy/clippy_lints/src/doc.rs
+++ b/src/tools/clippy/clippy_lints/src/doc.rs
@@ -427,9 +427,7 @@ fn lint_for_missing_headers(
                 let body = cx.tcx.hir().body(body_id);
                 let ret_ty = typeck.expr_ty(body.value);
                 if implements_trait(cx, ret_ty, future, &[]);
-                if let ty::Opaque(_, subs) = ret_ty.kind();
-                if let Some(gen) = subs.types().next();
-                if let ty::Generator(_, subs, _) = gen.kind();
+                if let ty::Generator(_, subs, _) = ret_ty.kind();
                 if is_type_diagnostic_item(cx, subs.as_generator().return_ty(), sym::Result);
                 then {
                     span_lint(
diff --git a/src/tools/clippy/clippy_lints/src/manual_async_fn.rs b/src/tools/clippy/clippy_lints/src/manual_async_fn.rs
index 5c6a342b3d0..6a98df49912 100644
--- a/src/tools/clippy/clippy_lints/src/manual_async_fn.rs
+++ b/src/tools/clippy/clippy_lints/src/manual_async_fn.rs
@@ -177,7 +177,7 @@ fn desugared_async_block<'tcx>(cx: &LateContext<'tcx>, block: &'tcx Block<'tcx>)
         if let Some(args) = cx
             .tcx
             .lang_items()
-            .from_generator_fn()
+            .identity_future_fn()
             .and_then(|def_id| match_function_call_with_def_id(cx, block_expr, def_id));
         if args.len() == 1;
         if let Expr {
diff --git a/src/tools/clippy/tests/ui/author/blocks.stdout b/src/tools/clippy/tests/ui/author/blocks.stdout
index 9de0550d81d..c6acf24c21e 100644
--- a/src/tools/clippy/tests/ui/author/blocks.stdout
+++ b/src/tools/clippy/tests/ui/author/blocks.stdout
@@ -45,7 +45,7 @@ if let ExprKind::Closure(CaptureBy::Value, fn_decl, body_id, _, None) = expr.kin
     && expr1 = &cx.tcx.hir().body(body_id).value
     && let ExprKind::Call(func, args) = expr1.kind
     && let ExprKind::Path(ref qpath) = func.kind
-    && matches!(qpath, QPath::LangItem(LangItem::FromGenerator, _))
+    && matches!(qpath, QPath::LangItem(LangItem::IdentityFuture, _))
     && args.len() == 1
     && let ExprKind::Closure(CaptureBy::Value, fn_decl1, body_id1, _, Some(Movability::Static)) = args[0].kind
     && let FnRetTy::DefaultReturn(_) = fn_decl1.output