fix ice in len_zero lint when type has no inherent impls at all

fixes #1336

Author: oli-obk

clippy_lints/src/len_zero.rs

@@ -194,11 +194,11 @@ fn has_is_empty(cx: &LateContext, expr: &Expr) -> bool {
 
     /// Check the inherent impl's items for an `is_empty(self)` method.
     fn has_is_empty_impl(cx: &LateContext, id: DefId) -> bool {
-        cx.tcx.inherent_impls.borrow()[&id].iter().any(|imp| {
+        cx.tcx.inherent_impls.borrow().get(&id).map_or(false, |impls| impls.iter().any(|imp| {
             cx.tcx.impl_or_trait_items(*imp).iter().any(|item| {
                 is_is_empty(&cx.tcx.impl_or_trait_item(*item))
             })
-        })
+        }))
     }
 
     let ty = &walk_ptrs_ty(cx.tcx.expr_ty(expr));

tests/ice_exacte_size.rs

@@ -0,0 +1,17 @@
+#![feature(plugin)]
+#![plugin(clippy)]
+#![deny(clippy)]
+
+#[allow(dead_code)]
+struct Foo;
+
+impl Iterator for Foo {
+    type Item = ();
+
+    fn next(&mut self) -> Option<()> {
+        let _ = self.len() == 0;
+        unimplemented!()
+    }
+}
+
+impl ExactSizeIterator for Foo { }