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
```

3 files changed, 3 insertions, 5 deletions

diff --git a/clippy_lints/src/doc.rs b/clippy_lints/src/doc.rs
index 4557e432885..ae5f9424b23 100644
--- a/clippy_lints/src/doc.rs
+++ b/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/clippy_lints/src/manual_async_fn.rs b/clippy_lints/src/manual_async_fn.rs
index 5c6a342b3d0..6a98df49912 100644
--- a/clippy_lints/src/manual_async_fn.rs
+++ b/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/tests/ui/author/blocks.stdout b/tests/ui/author/blocks.stdout
index 9de0550d81d..c6acf24c21e 100644
--- a/tests/ui/author/blocks.stdout
+++ b/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