Rollup merge of #127482 - compiler-errors:closure-two-par-sig-inference, r=oli-obk

Infer async closure signature from (old-style) two-part `Fn` + `Future` bounds

When an async closure is passed to a function that has a "two-part" `Fn` and `Future` trait bound, like:

```rust
use std::future::Future;

fn not_exactly_an_async_closure(_f: F)
where
    F: FnOnce(String) -> Fut,
    Fut: Future<Output = ()>,
{}
```

The we want to be able to extract the signature to guide inference in the async closure, like:

```rust
not_exactly_an_async_closure(async |string| {
    for x in string.split('\n') { ... }
    //~^ We need to know that the type of `string` is `String` to call methods on it.
})
```

Closure signature inference will see two bounds: `<?F as FnOnce<Args>>::Output = ?Fut`, `<?Fut as Future>::Output = String`. We need to extract the signature by looking through both projections.

### Why?

I expect the ecosystem's move onto `async Fn` trait bounds (which are not affected by this PR, and already do signature inference fine) to be slow. In the mean time, I don't see major overhead to supporting this "old–style" of trait bounds that were used to model async closures.

r? oli-obk
Fixes #127468
Fixes #127425

compiler/rustc_hir_typeck/src/closure.rs

@@ -424,9 +424,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             if let Some(trait_def_id) = trait_def_id {
                 let found_kind = match closure_kind {
                     hir::ClosureKind::Closure => self.tcx.fn_trait_kind_from_def_id(trait_def_id),
-                    hir::ClosureKind::CoroutineClosure(hir::CoroutineDesugaring::Async) => {
-                        self.tcx.async_fn_trait_kind_from_def_id(trait_def_id)
-                    }
+                    hir::ClosureKind::CoroutineClosure(hir::CoroutineDesugaring::Async) => self
+                        .tcx
+                        .async_fn_trait_kind_from_def_id(trait_def_id)
+                        .or_else(|| self.tcx.fn_trait_kind_from_def_id(trait_def_id)),
                     _ => None,
                 };
 
@@ -470,14 +471,37 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         // for closures and async closures, respectively.
         match closure_kind {
             hir::ClosureKind::Closure
-                if self.tcx.fn_trait_kind_from_def_id(trait_def_id).is_some() => {}
+                if self.tcx.fn_trait_kind_from_def_id(trait_def_id).is_some() =>
+            {
+                self.extract_sig_from_projection(cause_span, projection)
+            }
             hir::ClosureKind::CoroutineClosure(hir::CoroutineDesugaring::Async)
-                if self.tcx.async_fn_trait_kind_from_def_id(trait_def_id).is_some() => {}
-            _ => return None,
+                if self.tcx.async_fn_trait_kind_from_def_id(trait_def_id).is_some() =>
+            {
+                self.extract_sig_from_projection(cause_span, projection)
+            }
+            // It's possible we've passed the closure to a (somewhat out-of-fashion)
+            // `F: FnOnce() -> Fut, Fut: Future<Output = T>` style bound. Let's still
+            // guide inference here, since it's beneficial for the user.
+            hir::ClosureKind::CoroutineClosure(hir::CoroutineDesugaring::Async)
+                if self.tcx.fn_trait_kind_from_def_id(trait_def_id).is_some() =>
+            {
+                self.extract_sig_from_projection_and_future_bound(cause_span, projection)
+            }
+            _ => None,
         }
+    }
+
+    /// Given an `FnOnce::Output` or `AsyncFn::Output` projection, extract the args
+    /// and return type to infer a [`ty::PolyFnSig`] for the closure.
+    fn extract_sig_from_projection(
+        &self,
+        cause_span: Option<Span>,
+        projection: ty::PolyProjectionPredicate<'tcx>,
+    ) -> Option<ExpectedSig<'tcx>> {
+        let projection = self.resolve_vars_if_possible(projection);
 
         let arg_param_ty = projection.skip_binder().projection_term.args.type_at(1);
-        let arg_param_ty = self.resolve_vars_if_possible(arg_param_ty);
         debug!(?arg_param_ty);
 
         let ty::Tuple(input_tys) = *arg_param_ty.kind() else {
@@ -486,7 +510,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
         // Since this is a return parameter type it is safe to unwrap.
         let ret_param_ty = projection.skip_binder().term.expect_type();
-        let ret_param_ty = self.resolve_vars_if_possible(ret_param_ty);
         debug!(?ret_param_ty);
 
         let sig = projection.rebind(self.tcx.mk_fn_sig(
@@ -500,6 +523,69 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         Some(ExpectedSig { cause_span, sig })
     }
 
+    /// When an async closure is passed to a function that has a "two-part" `Fn`
+    /// and `Future` trait bound, like:
+    ///
+    /// ```rust
+    /// use std::future::Future;
+    ///
+    /// fn not_exactly_an_async_closure<F, Fut>(_f: F)
+    /// where
+    ///     F: FnOnce(String, u32) -> Fut,
+    ///     Fut: Future<Output = i32>,
+    /// {}
+    /// ```
+    ///
+    /// The we want to be able to extract the signature to guide inference in the async
+    /// closure. We will have two projection predicates registered in this case. First,
+    /// we identify the `FnOnce<Args, Output = ?Fut>` bound, and if the output type is
+    /// an inference variable `?Fut`, we check if that is bounded by a `Future<Output = Ty>`
+    /// projection.
+    fn extract_sig_from_projection_and_future_bound(
+        &self,
+        cause_span: Option<Span>,
+        projection: ty::PolyProjectionPredicate<'tcx>,
+    ) -> Option<ExpectedSig<'tcx>> {
+        let projection = self.resolve_vars_if_possible(projection);
+
+        let arg_param_ty = projection.skip_binder().projection_term.args.type_at(1);
+        debug!(?arg_param_ty);
+
+        let ty::Tuple(input_tys) = *arg_param_ty.kind() else {
+            return None;
+        };
+
+        // If the return type is a type variable, look for bounds on it.
+        // We could theoretically support other kinds of return types here,
+        // but none of them would be useful, since async closures return
+        // concrete anonymous future types, and their futures are not coerced
+        // into any other type within the body of the async closure.
+        let ty::Infer(ty::TyVar(return_vid)) = *projection.skip_binder().term.expect_type().kind()
+        else {
+            return None;
+        };
+
+        // FIXME: We may want to elaborate here, though I assume this will be exceedingly rare.
+        for bound in self.obligations_for_self_ty(return_vid) {
+            if let Some(ret_projection) = bound.predicate.as_projection_clause()
+                && let Some(ret_projection) = ret_projection.no_bound_vars()
+                && self.tcx.is_lang_item(ret_projection.def_id(), LangItem::FutureOutput)
+            {
+                let sig = projection.rebind(self.tcx.mk_fn_sig(
+                    input_tys,
+                    ret_projection.term.expect_type(),
+                    false,
+                    hir::Safety::Safe,
+                    Abi::Rust,
+                ));
+
+                return Some(ExpectedSig { cause_span, sig });
+            }
+        }
+
+        None
+    }
+
     fn sig_of_closure(
         &self,
         expr_def_id: LocalDefId,

tests/ui/async-await/async-closures/signature-inference-from-two-part-bound.rs

@@ -0,0 +1,27 @@
+//@ edition: 2021
+//@ check-pass
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+
+#![feature(async_closure)]
+
+use std::future::Future;
+use std::any::Any;
+
+struct Struct;
+impl Struct {
+    fn method(&self) {}
+}
+
+fn fake_async_closure<F, Fut>(_: F)
+where
+    F: Fn(Struct) -> Fut,
+    Fut: Future<Output = ()>,
+{}
+
+fn main() {
+    fake_async_closure(async |s| {
+        s.method();
+    })
+}