fix: Fix return type of async closures.

(rebased onto 6dfd8ae)

crates/hir-def/src/body/lower.rs

@@ -499,6 +499,8 @@ fn maybe_collect_expr(&mut self, expr: ast::Expr) -> Option<ExprId> {
                         Movability::Movable
                     };
                     ClosureKind::Generator(movability)
+                } else if e.async_token().is_some() {
+                    ClosureKind::Async
                 } else {
                     ClosureKind::Closure
                 };

crates/hir-def/src/body/pretty.rs

@@ -375,10 +375,21 @@ fn print_expr(&mut self, expr: ExprId) {
                     }
                 }
                 w!(self, "|");
-                if let Some(ret_ty) = ret_type {
+                match (ret_type, closure_kind) {
+                    (Some(ret_ty), ClosureKind::Async) => {
+                        w!(self, " -> impl Future<Output = ");
+                        self.print_type_ref(ret_ty);
+                        w!(self, ">");
+                    }
+                    (Some(ret_ty), _) => {
                         w!(self, " -> ");
                         self.print_type_ref(ret_ty);
                     }
+                    (None, ClosureKind::Async) => {
+                        w!(self, " -> impl Future<Output = {{unknown}}>"); // FIXME(zachs18): {unknown} or ()?
+                    }
+                    (None, _) => {}
+                }
                 self.whitespace();
                 self.print_expr(*body);
             }

crates/hir-def/src/expr.rs

@@ -245,6 +245,7 @@ pub enum Expr {
 pub enum ClosureKind {
     Closure,
     Generator(Movability),
+    Async,
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]

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

@@ -286,7 +286,8 @@ fn infer_expr_inner(&mut self, tgt_expr: ExprId, expected: &Expectation) -> Ty {
                 })
                 .intern(Interner);
 
-                let (ty, resume_yield_tys) = if matches!(closure_kind, ClosureKind::Generator(_)) {
+                let (closure_id, ty, resume_yield_tys) = match closure_kind {
+                    ClosureKind::Generator(_) => {
                         // FIXME: report error when there are more than 1 parameter.
                         let resume_ty = match sig_tys.first() {
                             // When `sig_tys.len() == 1` the first type is the return type, not the
@@ -305,14 +306,18 @@ fn infer_expr_inner(&mut self, tgt_expr: ExprId, expected: &Expectation) -> Ty {
                         let generator_id = self.db.intern_generator((self.owner, tgt_expr)).into();
                         let generator_ty = TyKind::Generator(generator_id, subst).intern(Interner);
 
-                    (generator_ty, Some((resume_ty, yield_ty)))
+                        (None, generator_ty, Some((resume_ty, yield_ty)))
-                } else {
+                    }
+                    ClosureKind::Async | ClosureKind::Closure => {
                         let closure_id = self.db.intern_closure((self.owner, tgt_expr)).into();
-                    let closure_ty =
+                        let closure_ty = TyKind::Closure(
-                        TyKind::Closure(closure_id, Substitution::from1(Interner, sig_ty.clone()))
+                            closure_id,
+                            Substitution::from1(Interner, sig_ty.clone()),
+                        )
                         .intern(Interner);
 
-                    (closure_ty, None)
+                        (Some(closure_id), closure_ty, None)
+                    }
                 };
 
                 // Eagerly try to relate the closure type with the expected
@@ -321,7 +326,7 @@ fn infer_expr_inner(&mut self, tgt_expr: ExprId, expected: &Expectation) -> Ty {
                 self.deduce_closure_type_from_expectations(tgt_expr, &ty, &sig_ty, expected);
 
                 // Now go through the argument patterns
-                for (arg_pat, arg_ty) in args.iter().zip(sig_tys) {
+                for (arg_pat, arg_ty) in args.iter().zip(&sig_tys) {
                     self.infer_top_pat(*arg_pat, &arg_ty);
                 }
 
@@ -333,16 +338,47 @@ fn infer_expr_inner(&mut self, tgt_expr: ExprId, expected: &Expectation) -> Ty {
                 let prev_resume_yield_tys =
                     mem::replace(&mut self.resume_yield_tys, resume_yield_tys);
 
+                let (breaks, ()) =
                     self.with_breakable_ctx(BreakableKind::Border, None, None, |this| {
                         this.infer_return(*body);
                     });
 
+                let inner_ty = if matches!(closure_kind, ClosureKind::Async) {
+                    // Use the first type parameter as the output type of future.
+                    // existential type AsyncBlockImplTrait<InnerType>: Future<Output = InnerType>
+                    let impl_trait_id =
+                        crate::ImplTraitId::AsyncBlockTypeImplTrait(self.owner, *body);
+                    let opaque_ty_id = self.db.intern_impl_trait_id(impl_trait_id).into();
+                    TyKind::OpaqueType(opaque_ty_id, Substitution::from1(Interner, ret_ty.clone()))
+                        .intern(Interner)
+                } else {
+                    ret_ty.clone()
+                };
+
                 self.diverges = prev_diverges;
                 self.return_ty = prev_ret_ty;
                 self.return_coercion = prev_ret_coercion;
                 self.resume_yield_tys = prev_resume_yield_tys;
 
-                ty
+                sig_tys.pop();
+                sig_tys.push(inner_ty);
+
+                let sig_ty = TyKind::Function(FnPointer {
+                    num_binders: 0,
+                    sig: FnSig { abi: (), safety: chalk_ir::Safety::Safe, variadic: false },
+                    substitution: FnSubst(
+                        Substitution::from_iter(Interner, sig_tys.clone()).shifted_in(Interner),
+                    ),
+                })
+                .intern(Interner);
+
+                match closure_id {
+                    Some(closure_id) => {
+                        TyKind::Closure(closure_id, Substitution::from1(Interner, sig_ty.clone()))
+                            .intern(Interner)
+                    }
+                    None => ty,
+                }
             }
             Expr::Call { callee, args, .. } => {
                 let callee_ty = self.infer_expr(*callee, &Expectation::none());