fix: Fix inference of AsyncFnX return type

crates/hir-ty/src/chalk_db.rs

@@ -259,7 +259,7 @@ impl chalk_solve::RustIrDatabase<Interner> for ChalkContext<'_> {
     }
     fn well_known_trait_id(
         &self,
-        well_known_trait: rust_ir::WellKnownTrait,
+        well_known_trait: WellKnownTrait,
     ) -> Option<chalk_ir::TraitId<Interner>> {
         let lang_attr = lang_item_from_well_known_trait(well_known_trait);
         let trait_ = lang_attr.resolve_trait(self.db, self.krate)?;

crates/hir-ty/src/display.rs

@@ -1463,6 +1463,8 @@ impl HirDisplay for Ty {
                     }
                     if f.closure_style == ClosureStyle::RANotation || !sig.ret().is_unit() {
                         write!(f, " -> ")?;
+                        // FIXME: We display `AsyncFn` as `-> impl Future`, but this is hard to fix because
+                        // we don't have a trait environment here, required to normalize `<Ret as Future>::Output`.
                         sig.ret().hir_fmt(f)?;
                     }
                 } else {

crates/hir-ty/src/infer/closure.rs

@@ -38,7 +38,7 @@ use crate::{
     infer::{BreakableKind, CoerceMany, Diverges, coerce::CoerceNever},
     make_binders,
     mir::{BorrowKind, MirSpan, MutBorrowKind, ProjectionElem},
-    to_chalk_trait_id,
+    to_assoc_type_id, to_chalk_trait_id,
     traits::FnTrait,
     utils::{self, elaborate_clause_supertraits},
 };
@@ -245,7 +245,7 @@ impl InferenceContext<'_> {
     }
 
     fn deduce_closure_kind_from_predicate_clauses(
-        &self,
+        &mut self,
         expected_ty: &Ty,
         clauses: impl DoubleEndedIterator<Item = WhereClause>,
         closure_kind: ClosureKind,
@@ -378,7 +378,7 @@ impl InferenceContext<'_> {
     }
 
     fn deduce_sig_from_projection(
-        &self,
+        &mut self,
         closure_kind: ClosureKind,
         projection_ty: &ProjectionTy,
         projected_ty: &Ty,
@@ -392,13 +392,16 @@ impl InferenceContext<'_> {
 
         // For now, we only do signature deduction based off of the `Fn` and `AsyncFn` traits,
         // for closures and async closures, respectively.
-        match closure_kind {
-            ClosureKind::Closure | ClosureKind::Async
-                if self.fn_trait_kind_from_trait_id(trait_).is_some() =>
-            {
-                self.extract_sig_from_projection(projection_ty, projected_ty)
-            }
-            _ => None,
+        let fn_trait_kind = self.fn_trait_kind_from_trait_id(trait_)?;
+        if !matches!(closure_kind, ClosureKind::Closure | ClosureKind::Async) {
+            return None;
+        }
+        if fn_trait_kind.is_async() {
+            // If the expected trait is `AsyncFn(...) -> X`, we don't know what the return type is,
+            // but we do know it must implement `Future<Output = X>`.
+            self.extract_async_fn_sig_from_projection(projection_ty, projected_ty)
+        } else {
+            self.extract_sig_from_projection(projection_ty, projected_ty)
         }
     }
 
@@ -424,6 +427,39 @@ impl InferenceContext<'_> {
         )))
     }
 
+    fn extract_async_fn_sig_from_projection(
+        &mut self,
+        projection_ty: &ProjectionTy,
+        projected_ty: &Ty,
+    ) -> Option<FnSubst<Interner>> {
+        let arg_param_ty = projection_ty.substitution.as_slice(Interner)[1].assert_ty_ref(Interner);
+
+        let TyKind::Tuple(_, input_tys) = arg_param_ty.kind(Interner) else {
+            return None;
+        };
+
+        let ret_param_future_output = projected_ty;
+        let ret_param_future = self.table.new_type_var();
+        let future_output =
+            LangItem::FutureOutput.resolve_type_alias(self.db, self.resolver.krate())?;
+        let future_projection = crate::AliasTy::Projection(crate::ProjectionTy {
+            associated_ty_id: to_assoc_type_id(future_output),
+            substitution: Substitution::from1(Interner, ret_param_future.clone()),
+        });
+        self.table.register_obligation(
+            crate::AliasEq { alias: future_projection, ty: ret_param_future_output.clone() }
+                .cast(Interner),
+        );
+
+        Some(FnSubst(Substitution::from_iter(
+            Interner,
+            input_tys.iter(Interner).map(|t| t.cast(Interner)).chain(Some(GenericArg::new(
+                Interner,
+                chalk_ir::GenericArgData::Ty(ret_param_future),
+            ))),
+        )))
+    }
+
     fn fn_trait_kind_from_trait_id(&self, trait_id: hir_def::TraitId) -> Option<FnTrait> {
         FnTrait::from_lang_item(self.db.lang_attr(trait_id.into())?)
     }

crates/hir-ty/src/tests/traits.rs

@@ -4903,3 +4903,30 @@ fn main() {
         "#]],
     );
 }
+
+#[test]
+fn async_fn_return_type() {
+    check_infer(
+        r#"
+//- minicore: async_fn
+fn foo<F: AsyncFn() -> R, R>(_: F) -> R {
+    loop {}
+}
+
+fn main() {
+    foo(async move || ());
+}
+    "#,
+        expect![[r#"
+            29..30 '_': F
+            40..55 '{     loop {} }': R
+            46..53 'loop {}': !
+            51..53 '{}': ()
+            67..97 '{     ...()); }': ()
+            73..76 'foo': fn foo<impl AsyncFn() -> impl Future<Output = ()>, ()>(impl AsyncFn() -> impl Future<Output = ()>)
+            73..94 'foo(as...|| ())': ()
+            77..93 'async ... || ()': impl AsyncFn() -> impl Future<Output = ()>
+            91..93 '()': ()
+        "#]],
+    );
+}

crates/hir-ty/src/traits.rs

@@ -291,4 +291,9 @@ impl FnTrait {
     pub fn get_id(self, db: &dyn HirDatabase, krate: Crate) -> Option<TraitId> {
         self.lang_item().resolve_trait(db, krate)
     }
+
+    #[inline]
+    pub(crate) fn is_async(self) -> bool {
+        matches!(self, FnTrait::AsyncFn | FnTrait::AsyncFnMut | FnTrait::AsyncFnOnce)
+    }
 }