Do array unsizing for method receivers

It turns out rustc actually only unsizes array method receivers, so we don't
need to do any trait solving for this (at least for now).

Fixes #2670.

crates/ra_hir_ty/src/tests/method_resolution.rs

@@ -838,6 +838,24 @@ fn test() { (&S).foo()<|>; }
     assert_eq!(t, "u128");
 }
 
+#[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(