2 files changed, 161 insertions, 1 deletions

diff --git a/crates/ra_hir/src/ty/infer.rs b/crates/ra_hir/src/ty/infer.rs
index 76b4b6faa..746b07a05 100644
--- a/crates/ra_hir/src/ty/infer.rs
+++ b/crates/ra_hir/src/ty/infer.rs
@@ -806,6 +806,47 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
         }
     }
 
+    fn unify_with_autoderef(&mut self, from_ty: &Ty, to_ty: &Ty) -> bool {
+        macro_rules! ty_app {
+            ($ctor:pat, $param:pat) => {
+                Ty::Apply(ApplicationTy { ctor: $ctor, parameters: $param })
+            };
+        }
+
+        // If given type and expected type are compatible reference,
+        // trigger auto-deref.
+        let (_to_mut, from_ty, to_ty) =
+            match (&*self.resolve_ty_shallow(&from_ty), &*self.resolve_ty_shallow(&to_ty)) {
+                (
+                    ty_app!(TypeCtor::Ref(from_mut), from_param),
+                    ty_app!(TypeCtor::Ref(to_mut), to_param),
+                ) if *from_mut == Mutability::Mut || from_mut == to_mut => {
+                    (to_mut, from_param[0].clone(), to_param[0].clone())
+                }
+                _ => {
+                    // Otherwise, just unify
+                    return self.unify(&from_ty, &to_ty);
+                }
+            };
+
+        let canonicalized = self.canonicalizer().canonicalize_ty(from_ty);
+        // FIXME: Auto DerefMut
+        for derefed_ty in
+            autoderef::autoderef(self.db, &self.resolver.clone(), canonicalized.value.clone())
+        {
+            let derefed_ty = canonicalized.decanonicalize_ty(derefed_ty.value);
+            match (&*self.resolve_ty_shallow(&derefed_ty), &*self.resolve_ty_shallow(&to_ty)) {
+                // Unify when constructor matches.
+                (ty_app!(from_ctor, _), ty_app!(to_ctor, _)) if from_ctor == to_ctor => {
+                    return self.unify(&derefed_ty, &to_ty);
+                }
+                _ => {}
+            }
+        }
+
+        false
+    }
+
     fn infer_expr(&mut self, tgt_expr: ExprId, expected: &Expectation) -> Ty {
         let ty = self.infer_expr_inner(tgt_expr, expected);
         let could_unify = self.unify(&ty, &expected.ty);
@@ -1285,7 +1326,8 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                 }
 
                 let param_ty = self.normalize_associated_types_in(param_ty);
-                self.infer_expr(arg, &Expectation::has_type(param_ty));
+                let arg_ty = self.infer_expr_inner(arg, &Expectation::has_type(param_ty.clone()));
+                self.unify_with_autoderef(&arg_ty, &param_ty);
             }
         }
     }
diff --git a/crates/ra_hir/src/ty/tests.rs b/crates/ra_hir/src/ty/tests.rs
index f4f63ca93..6076e4025 100644
--- a/crates/ra_hir/src/ty/tests.rs
+++ b/crates/ra_hir/src/ty/tests.rs
@@ -801,6 +801,124 @@ fn test2(a1: *const A, a2: *mut A) {
 }
 
 #[test]
+fn infer_argument_autoderef() {
+    assert_snapshot!(
+        infer(r#"
+#[lang = "deref"]
+pub trait Deref {
+    type Target: ?Sized;
+    fn deref(&self) -> &Self::Target;
+}
+
+struct A<T>(T);
+
+impl<T: Copy> A<T> {
+    fn foo(&self) -> T {
+        self.0
+    }
+}
+
+struct B<T>(T);
+
+impl<T> Deref for B<T> {
+    type Target = T;
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+fn test() {
+    A::foo(&&B(B(A(42))));
+}
+"#),
+        @r###"
+    [76; 80) 'self': &Self
+    [153; 157) 'self': &A<T>
+    [164; 186) '{     ...     }': T
+    [174; 178) 'self': &A<T>
+    [174; 180) 'self.0': T
+    [267; 271) 'self': &B<T>
+    [290; 313) '{     ...     }': &T
+    [300; 307) '&self.0': &T
+    [301; 305) 'self': &B<T>
+    [301; 307) 'self.0': T
+    [327; 357) '{     ...))); }': ()
+    [333; 339) 'A::foo': fn foo<i32>(&A<T>) -> T
+    [333; 354) 'A::foo...42))))': i32
+    [340; 353) '&&B(B(A(42)))': &&B<B<A<i32>>>
+    [341; 353) '&B(B(A(42)))': &B<B<A<i32>>>
+    [342; 343) 'B': B<B<A<i32>>>(T) -> B<T>
+    [342; 353) 'B(B(A(42)))': B<B<A<i32>>>
+    [344; 345) 'B': B<A<i32>>(T) -> B<T>
+    [344; 352) 'B(A(42))': B<A<i32>>
+    [346; 347) 'A': A<i32>(T) -> A<T>
+    [346; 351) 'A(42)': A<i32>
+    [348; 350) '42': i32
+"###
+    );
+}
+
+#[test]
+fn infer_method_argument_autoderef() {
+    assert_snapshot!(
+        infer(r#"
+#[lang = "deref"]
+pub trait Deref {
+    type Target: ?Sized;
+    fn deref(&self) -> &Self::Target;
+}
+
+struct A<T>(*mut T);
+
+impl<T: Copy> A<T> {
+    fn foo(&self, x: &A<T>) -> T {
+        x
+    }
+}
+
+struct B<T>(T);
+
+impl<T> Deref for B<T> {
+    type Target = T;
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+fn test(a: A<i32>) {
+    A(0 as *mut _).foo(&&B(B(a)));
+}
+"#),
+        @r###"
+    [76; 80) 'self': &Self
+    [158; 162) 'self': &A<T>
+    [164; 165) 'x': &A<T>
+    [179; 196) '{     ...     }': &A<T>
+    [189; 190) 'x': &A<T>
+    [277; 281) 'self': &B<T>
+    [300; 323) '{     ...     }': &T
+    [310; 317) '&self.0': &T
+    [311; 315) 'self': &B<T>
+    [311; 317) 'self.0': T
+    [335; 336) 'a': A<i32>
+    [346; 384) '{     ...))); }': ()
+    [352; 353) 'A': A<i32>(*mut T) -> A<T>
+    [352; 366) 'A(0 as *mut _)': A<i32>
+    [352; 381) 'A(0 as...B(a)))': i32
+    [354; 355) '0': i32
+    [354; 365) '0 as *mut _': *mut i32
+    [371; 380) '&&B(B(a))': &&B<B<A<i32>>>
+    [372; 380) '&B(B(a))': &B<B<A<i32>>>
+    [373; 374) 'B': B<B<A<i32>>>(T) -> B<T>
+    [373; 380) 'B(B(a))': B<B<A<i32>>>
+    [375; 376) 'B': B<A<i32>>(T) -> B<T>
+    [375; 379) 'B(a)': B<A<i32>>
+    [377; 378) 'a': A<i32>
+"###
+    );
+}
+
+#[test]
 fn bug_484() {
     assert_snapshot!(
         infer(r#"