Ensure String bool functions do not fail on empty lists

Author: José Valim

lib/elixir/lib/string.ex

@@ -1275,6 +1275,10 @@ defmodule String do
     true
   end
 
+  def starts_with?(_string, []) do
+    false
+  end
+
   def starts_with?(string, prefix) when is_list(prefix) or is_binary(prefix) do
     Kernel.match?({0, _}, :binary.match(string, prefix))
   end
@@ -1366,6 +1370,10 @@ defmodule String do
     true
   end
 
+  def contains?(_string, []) do
+    false
+  end
+
   def contains?(string, contents) do
     :binary.match(string, contents) != :nomatch
   end

lib/elixir/test/elixir/string_test.exs

@@ -435,6 +435,7 @@ defmodule StringTest do
   test :starts_with? do
     assert String.starts_with? "hello", "he"
     assert String.starts_with? "hello", "hello"
+    refute String.starts_with? "hello", []
     assert String.starts_with? "hello", ["hellö", "hell"]
     assert String.starts_with? "エリクシア", "エリ"
     refute String.starts_with? "hello", "lo"
@@ -446,6 +447,7 @@ defmodule StringTest do
   test :ends_with? do
     assert String.ends_with? "hello", "lo"
     assert String.ends_with? "hello", "hello"
+    refute String.ends_with? "hello", []
     assert String.ends_with? "hello", ["hell", "lo", "xx"]
     assert String.ends_with? "hello", ["hellö", "lo"]
     assert String.ends_with? "エリクシア", "シア"
@@ -458,6 +460,7 @@ defmodule StringTest do
   test :contains? do
     assert String.contains? "elixir of life", "of"
     assert String.contains? "エリクシア", "シ"
+    refute String.contains? "elixir of life", []
     assert String.contains? "elixir of life", ["mercury", "life"]
     refute String.contains? "elixir of life", "death"
     refute String.contains? "エリクシア", "仙"