Implement the call argument checking order hack for closures

crates/ra_hir/src/ty/infer.rs

@@ -790,11 +790,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
         };
         self.unify(&expected_receiver_ty, &actual_receiver_ty);
 
-        let param_iter = param_tys.into_iter().chain(repeat(Ty::Unknown));
-        for (arg, param_ty) in args.iter().zip(param_iter) {
-            let param_ty = self.normalize_associated_types_in(param_ty);
-            self.infer_expr(*arg, &Expectation::has_type(param_ty));
-        }
+        self.check_call_arguments(args, &param_tys);
         let ret_ty = self.normalize_associated_types_in(ret_ty);
         ret_ty
     }
@@ -928,11 +924,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                     }
                 };
                 self.register_obligations_for_call(&callee_ty);
-                let param_iter = param_tys.into_iter().chain(repeat(Ty::Unknown));
-                for (arg, param_ty) in args.iter().zip(param_iter) {
-                    let param_ty = self.normalize_associated_types_in(param_ty);
-                    self.infer_expr(*arg, &Expectation::has_type(param_ty));
-                }
+                self.check_call_arguments(args, &param_tys);
                 let ret_ty = self.normalize_associated_types_in(ret_ty);
                 ret_ty
             }
@@ -1274,6 +1266,30 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
         ty
     }
 
+    fn check_call_arguments(&mut self, args: &[ExprId], param_tys: &[Ty]) {
+        // Quoting https://github.com/rust-lang/rust/blob/6ef275e6c3cb1384ec78128eceeb4963ff788dca/src/librustc_typeck/check/mod.rs#L3325 --
+        // We do this in a pretty awful way: first we type-check any arguments
+        // that are not closures, then we type-check the closures. This is so
+        // that we have more information about the types of arguments when we
+        // type-check the functions. This isn't really the right way to do this.
+        for &check_closures in &[false, true] {
+            let param_iter = param_tys.iter().cloned().chain(repeat(Ty::Unknown));
+            for (&arg, param_ty) in args.iter().zip(param_iter) {
+                let is_closure = match &self.body[arg] {
+                    Expr::Lambda { .. } => true,
+                    _ => false,
+                };
+
+                if is_closure != check_closures {
+                    continue;
+                }
+
+                let param_ty = self.normalize_associated_types_in(param_ty);
+                self.infer_expr(arg, &Expectation::has_type(param_ty));
+            }
+        }
+    }
+
     fn collect_const(&mut self, data: &ConstData) {
         self.return_ty = self.make_ty(data.type_ref());
     }

crates/ra_hir/src/ty/tests.rs

@@ -4078,6 +4078,86 @@ fn test<F: FnOnce(u32) -> u64>(f: F) {
     );
 }
 
+#[test]
+fn closure_as_argument_inference_order() {
+    assert_snapshot!(
+        infer(r#"
+#[lang = "fn_once"]
+trait FnOnce<Args> {
+    type Output;
+}
+
+fn foo1<T, U, F: FnOnce(T) -> U>(x: T, f: F) -> U {}
+fn foo2<T, U, F: FnOnce(T) -> U>(f: F, x: T) -> U {}
+
+struct S;
+impl S {
+    fn method(self) -> u64;
+
+    fn foo1<T, U, F: FnOnce(T) -> U>(self, x: T, f: F) -> U {}
+    fn foo2<T, U, F: FnOnce(T) -> U>(self, f: F, x: T) -> U {}
+}
+
+fn test() {
+    let x1 = foo1(S, |s| s.method());
+    let x2 = foo2(|s| s.method(), S);
+    let x3 = S.foo1(S, |s| s.method());
+    let x4 = S.foo2(|s| s.method(), S);
+}
+"#),
+        @r###"
+    [95; 96) 'x': T
+    [101; 102) 'f': F
+    [112; 114) '{}': ()
+    [148; 149) 'f': F
+    [154; 155) 'x': T
+    [165; 167) '{}': ()
+    [202; 206) 'self': S
+    [254; 258) 'self': S
+    [260; 261) 'x': T
+    [266; 267) 'f': F
+    [277; 279) '{}': ()
+    [317; 321) 'self': S
+    [323; 324) 'f': F
+    [329; 330) 'x': T
+    [340; 342) '{}': ()
+    [356; 515) '{     ... S); }': ()
+    [366; 368) 'x1': u64
+    [371; 375) 'foo1': fn foo1<S, u64, |S| -> u64>(T, F) -> U
+    [371; 394) 'foo1(S...hod())': u64
+    [376; 377) 'S': S
+    [379; 393) '|s| s.method()': |S| -> u64
+    [380; 381) 's': S
+    [383; 384) 's': S
+    [383; 393) 's.method()': u64
+    [404; 406) 'x2': u64
+    [409; 413) 'foo2': fn foo2<S, u64, |S| -> u64>(F, T) -> U
+    [409; 432) 'foo2(|...(), S)': u64
+    [414; 428) '|s| s.method()': |S| -> u64
+    [415; 416) 's': S
+    [418; 419) 's': S
+    [418; 428) 's.method()': u64
+    [430; 431) 'S': S
+    [442; 444) 'x3': u64
+    [447; 448) 'S': S
+    [447; 472) 'S.foo1...hod())': u64
+    [454; 455) 'S': S
+    [457; 471) '|s| s.method()': |S| -> u64
+    [458; 459) 's': S
+    [461; 462) 's': S
+    [461; 471) 's.method()': u64
+    [482; 484) 'x4': u64
+    [487; 488) 'S': S
+    [487; 512) 'S.foo2...(), S)': u64
+    [494; 508) '|s| s.method()': |S| -> u64
+    [495; 496) 's': S
+    [498; 499) 's': S
+    [498; 508) 's.method()': u64
+    [510; 511) 'S': S
+    "###
+    );
+}
+
 #[test]
 fn unselected_projection_in_trait_env_1() {
     let t = type_at(

crates/ra_hir/src/ty/traits/chalk.rs

@@ -406,8 +406,8 @@ where
 
         let ty: Ty = from_chalk(self.db, parameters[0].assert_ty_ref().clone());
         if let Ty::Apply(ApplicationTy { ctor: TypeCtor::Closure { def, expr }, .. }) = ty {
-            for fn_trait in
-                [super::FnTrait::FnOnce, super::FnTrait::FnMut, super::FnTrait::Fn].iter().copied()
+            for &fn_trait in
+                [super::FnTrait::FnOnce, super::FnTrait::FnMut, super::FnTrait::Fn].iter()
             {
                 if let Some(actual_trait) = get_fn_trait(self.db, self.krate, fn_trait) {
                     if trait_ == actual_trait {