Fix for rust-lang#17497 - Invalid RA diagnostic error: expected 2 arg…

…uments, found 1

Also fixes rust-lang#17233

The issue occurs because in some configurations of traits where one of them has Deref as a supertrait, RA's type inference algorithm fails to fully resolve a trait's self type, and instead uses a TyKind::BoundVar. This then erroneously matches with types that do not implement the trait. In other words, unconnected types appear to inherit the trait's methods.

The fix is to insert a heuristic when invoking lookup_method() that allows the caller to determine if the returned method is a good match. If the returned method would result in diagnostic errors, the caller instructs the algorithm to continue iterating the possible methods to see if a better one exists (i.e. one that won't result in diagnostic errors).

Includes a unit test that only passes after applying the fixes in this commit.

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

@@ -378,6 +378,7 @@ impl InferenceContext<'_> {
                     &param_tys,
                     &indices_to_skip,
                     is_varargs,
+                    false,
                 );
                 self.normalize_associated_types_in(ret_ty)
             }
@@ -1615,14 +1616,22 @@ impl InferenceContext<'_> {
     ) -> Ty {
         let receiver_ty = self.infer_expr_inner(receiver, &Expectation::none());
         let canonicalized_receiver = self.canonicalize(receiver_ty.clone());
+        let traits_in_scope = self.get_traits_in_scope().left_or_else(|it| it.clone());
 
-        let resolved = method_resolution::lookup_method(
+        let resolved = method_resolution::lookup_method_using_heuristic(
             self.db,
             &canonicalized_receiver,
             self.table.trait_env.clone(),
-            self.get_traits_in_scope().as_ref().left_or_else(|&it| it),
+            &traits_in_scope,
             VisibleFromModule::Filter(self.resolver.module()),
             method_name,
+            |adjust, func| {
+                let (receiver_ty, _) = adjust.apply(&mut self.table, receiver_ty.clone());
+                let generics = generics(self.db.upcast(), func.into());
+                let substs = self.substs_for_method_call(generics, generic_args);
+                let method_ty = self.db.value_ty(func.into()).unwrap();
+                self.try_method_call(tgt_expr, args, method_ty, substs, receiver_ty, expected)
+            }
         );
         let (receiver_ty, method_ty, substs) = match resolved {
             Some((adjust, func, visible)) => {
@@ -1684,6 +1693,21 @@ impl InferenceContext<'_> {
         self.check_method_call(tgt_expr, args, method_ty, substs, receiver_ty, expected)
     }
 
+    #[inline]
+    fn try_method_call(
+        &mut self,
+        tgt_expr: ExprId,
+        args: &[ExprId],
+        method_ty: Binders<Ty>,
+        substs: Substitution,
+        receiver_ty: Ty,
+        expected: &Expectation,
+    ) -> bool {
+        self.check_method_call_inner(tgt_expr, args, method_ty, substs, receiver_ty, expected, true)
+            .is_ok()
+    }
+
+    #[inline]
     fn check_method_call(
         &mut self,
         tgt_expr: ExprId,
@@ -1693,6 +1717,20 @@ impl InferenceContext<'_> {
         receiver_ty: Ty,
         expected: &Expectation,
     ) -> Ty {
+        self.check_method_call_inner(tgt_expr, args, method_ty, substs, receiver_ty, expected, false)
+            .unwrap()
+    }
+
+    fn check_method_call_inner(
+        &mut self,
+        tgt_expr: ExprId,
+        args: &[ExprId],
+        method_ty: Binders<Ty>,
+        substs: Substitution,
+        receiver_ty: Ty,
+        expected: &Expectation,
+        is_dryrun: bool,
+    ) -> Result<Ty,()> {
         let method_ty = method_ty.substitute(Interner, &substs);
         self.register_obligations_for_call(&method_ty);
         let ((formal_receiver_ty, param_tys), ret_ty, is_varargs) =
@@ -1713,8 +1751,8 @@ impl InferenceContext<'_> {
         let expected_inputs =
             self.expected_inputs_for_expected_output(expected, ret_ty.clone(), param_tys.clone());
 
-        self.check_call_arguments(tgt_expr, args, &expected_inputs, &param_tys, &[], is_varargs);
-        self.normalize_associated_types_in(ret_ty)
+        if !self.check_call_arguments(tgt_expr, args, &expected_inputs, &param_tys, &[], is_varargs, is_dryrun) && is_dryrun { return Err(()); }
+        Ok(self.normalize_associated_types_in(ret_ty))
     }
 
     fn expected_inputs_for_expected_output(
@@ -1752,8 +1790,12 @@ impl InferenceContext<'_> {
         param_tys: &[Ty],
         skip_indices: &[u32],
         is_varargs: bool,
-    ) {
+        is_dryrun: bool,
+    ) -> bool {
+        let mut is_ok = true;
         if args.len() != param_tys.len() + skip_indices.len() && !is_varargs {
+            if is_dryrun { return false; } // Fail fast
+            is_ok = false;
             self.push_diagnostic(InferenceDiagnostic::MismatchedArgCount {
                 call_expr: expr,
                 expected: param_tys.len() + skip_indices.len(),
@@ -1814,13 +1856,16 @@ impl InferenceContext<'_> {
                 // type vars here to avoid type mismatch false positive.
                 let coercion_target = self.insert_type_vars(coercion_target);
                 if self.coerce(Some(arg), &ty, &coercion_target).is_err() {
+                    if is_dryrun { return false; } // Fail fast
+                    is_ok = false;
                     self.result.type_mismatches.insert(
                         arg.into(),
                         TypeMismatch { expected: coercion_target, actual: ty.clone() },
                     );
                 }
             }
         }
+        is_ok
     }
 
     fn substs_for_method_call(

crates/hir-ty/src/method_resolution.rs

@@ -437,15 +437,38 @@ pub fn def_crates(
 }
 
 /// Look up the method with the given name.
+#[inline]
 pub(crate) fn lookup_method(
     db: &dyn HirDatabase,
     ty: &Canonical<Ty>,
     env: Arc<TraitEnvironment>,
     traits_in_scope: &FxHashSet<TraitId>,
     visible_from_module: VisibleFromModule,
     name: &Name,
+) -> Option<(ReceiverAdjustments, FunctionId, bool)> {
+    lookup_method_using_heuristic(
+        db,
+        ty,
+        env,
+        traits_in_scope,
+        visible_from_module,
+        name,
+        |_,_| true,
+    )
+}
+
+/// Look up the method with the given name, using a callback to evaluate matches.
+pub(crate) fn lookup_method_using_heuristic(
+    db: &dyn HirDatabase,
+    ty: &Canonical<Ty>,
+    env: Arc<TraitEnvironment>,
+    traits_in_scope: &FxHashSet<TraitId>,
+    visible_from_module: VisibleFromModule,
+    name: &Name,
+    mut is_best_match: impl FnMut(&ReceiverAdjustments, FunctionId) -> bool,
 ) -> Option<(ReceiverAdjustments, FunctionId, bool)> {
     let mut not_visible = None;
+    let mut not_best_match = None;
     let res = iterate_method_candidates(
         ty,
         db,
@@ -455,15 +478,24 @@ pub(crate) fn lookup_method(
         Some(name),
         LookupMode::MethodCall,
         |adjustments, f, visible| match f {
-            AssocItemId::FunctionId(f) if visible => Some((adjustments, f, true)),
+            AssocItemId::FunctionId(f) if visible => {
+                if is_best_match(&adjustments, f) {
+                    Some((adjustments, f, true))
+                } else {
+                    if not_best_match.is_none() {
+                        not_best_match = Some((adjustments, f, true));
+                    }
+                    None
+                }
+            },
             AssocItemId::FunctionId(f) if not_visible.is_none() => {
                 not_visible = Some((adjustments, f, false));
                 None
             }
             _ => None,
         },
     );
-    res.or(not_visible)
+    res.or(not_best_match).or(not_visible)
 }
 
 /// Whether we're looking up a dotted method call (like `v.len()`) or a path

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

@@ -2050,3 +2050,42 @@ fn test() {
 "#,
     );
 }
+
+#[test]
+fn mismatched_args_due_to_subtraits_with_deref() {
+    check_no_mismatches(
+        r#"
+//- minicore: deref
+use core::ops::Deref;
+
+trait Trait1 {
+    type Assoc: Deref<Target = String>;
+}
+
+trait Trait2: Trait1 {
+}
+
+trait Trait3 {
+    type T1: Trait1;
+    type T2: Trait2;
+    fn bar(&self, x: bool, y: bool);
+}
+
+struct Foo;
+
+impl Foo {
+    fn bar(&mut self, _: &'static str) {}
+}
+
+impl Deref for Foo {
+    type Target = u32;
+    fn deref(&self) -> &Self::Target { &0 }
+}
+
+fn problem_method<T: Trait3>() {
+    let mut foo = Foo;
+    foo.bar("hello"); // Rustc ok, RA errors (mismatched args)
+}
+"#,
+    );
+}