2 files changed, 37 insertions, 2 deletions

diff --git a/crates/ra_hir_ty/src/method_resolution.rs b/crates/ra_hir_ty/src/method_resolution.rs
index df5901835..7b0ff8161 100644
--- a/crates/ra_hir_ty/src/method_resolution.rs
+++ b/crates/ra_hir_ty/src/method_resolution.rs
@@ -20,7 +20,7 @@ use crate::{
     db::HirDatabase,
     primitive::{FloatBitness, Uncertain},
     utils::all_super_traits,
-    Canonical, InEnvironment, TraitEnvironment, TraitRef, Ty, TypeCtor, TypeWalk,
+    ApplicationTy, Canonical, InEnvironment, TraitEnvironment, TraitRef, Ty, TypeCtor, TypeWalk,
 };
 
 /// This is used as a key for indexing impls.
@@ -214,7 +214,7 @@ pub fn iterate_method_candidates<T>(
             // the methods by autoderef order of *receiver types*, not *self
             // types*.
 
-            let deref_chain: Vec<_> = autoderef::autoderef(db, Some(krate), ty).collect();
+            let deref_chain = autoderef_method_receiver(db, krate, ty);
             for i in 0..deref_chain.len() {
                 if let Some(result) = iterate_method_candidates_with_autoref(
                     &deref_chain[i..],
@@ -548,3 +548,20 @@ fn generic_implements_goal(
     let obligation = super::Obligation::Trait(trait_ref);
     Canonical { num_vars, value: InEnvironment::new(env, obligation) }
 }
+
+fn autoderef_method_receiver(
+    db: &impl HirDatabase,
+    krate: CrateId,
+    ty: InEnvironment<Canonical<Ty>>,
+) -> Vec<Canonical<Ty>> {
+    let mut deref_chain: Vec<_> = autoderef::autoderef(db, Some(krate), ty).collect();
+    // As a last step, we can do array unsizing (that's the only unsizing that rustc does for method receivers!)
+    if let Some(Ty::Apply(ApplicationTy { ctor: TypeCtor::Array, parameters })) =
+        deref_chain.last().map(|ty| &ty.value)
+    {
+        let num_vars = deref_chain.last().unwrap().num_vars;
+        let unsized_ty = Ty::apply(TypeCtor::Slice, parameters.clone());
+        deref_chain.push(Canonical { value: unsized_ty, num_vars })
+    }
+    deref_chain
+}
diff --git a/crates/ra_hir_ty/src/tests/method_resolution.rs b/crates/ra_hir_ty/src/tests/method_resolution.rs
index 644d59e17..f9b394f05 100644
--- a/crates/ra_hir_ty/src/tests/method_resolution.rs
+++ b/crates/ra_hir_ty/src/tests/method_resolution.rs
@@ -839,6 +839,24 @@ fn test() { (&S).foo()<|>; }
 }
 
 #[test]
+fn method_resolution_unsize_array() {
+    let t = type_at(
+        r#"
+//- /main.rs
+#[lang = "slice"]
+impl<T> [T] {
+    fn len(&self) -> usize { loop {} }
+}
+fn test() {
+    let a = [1, 2, 3];
+    a.len()<|>;
+}
+"#,
+    );
+    assert_eq!(t, "usize");
+}
+
+#[test]
 fn method_resolution_trait_from_prelude() {
     let (db, pos) = TestDB::with_position(
         r#"