Add str to types considered by len_zero

src/len_zero.rs

@@ -198,7 +198,7 @@ fn has_is_empty(cx: &LateContext, expr: &Expr) -> bool {
         }
         ty::TyProjection(_) => ty.ty_to_def_id().map_or(false, |id| has_is_empty_impl(cx, &id)),
         ty::TyEnum(ref id, _) | ty::TyStruct(ref id, _) => has_is_empty_impl(cx, &id.did),
-        ty::TyArray(..) => true,
+        ty::TyArray(..) | ty::TyStr => true,
         _ => false,
     }
 }

tests/compile-fail/len_zero.rs

@@ -76,6 +76,12 @@ fn main() {
         println!("This should not happen!");
     }
 
+    if "".len() == 0 {
+        //~^ERROR length comparison to zero
+        //~|HELP consider using `is_empty`
+        //~|SUGGESTION "".is_empty()
+    }
+
     let y = One;
     if y.len()  == 0 { //no error because One does not have .is_empty()
         println!("This should not happen either!");