Fix non-existing atom exception when accidentally matching on map lit…

…eral

This fixes a crash that looks like this:

```
     ** (ArgumentError) errors were found at the given arguments:

       * 1st argument: not an already existing atom
```

...which was previously occurring when you used the literal map matcher on a map with arbitrary string keys that don't have a corresponding atom defined. For me, this occurred when I forgot to wrap my map matcher in a `superset/1`. So, for instance, what I *meant* was:

```elixir
assert %{"foo" => "bar", "baz" => "bop"} ~> superset(%{"foo" => "bar"})
```

...but what I accidentally wrote was just:

```elixir
assert %{"foo" => "bar", "baz" => "bop"} ~> %{"foo" => "bar"}
```

Because the error was so generic, this was pretty difficult to debug. With this change applied, though, you now get a nice "Unexpected key" error.

lib/machete/matchers/literal_map_matcher.ex

@@ -31,9 +31,7 @@ defmodule Machete.LiteralMapMatcher do
 
       string_keys =
         extra_keys
-        |> Enum.filter(
-          &(is_binary(&1) && MapSet.member?(missing_keys, String.to_existing_atom(&1)))
-        )
+        |> Enum.filter(&(is_binary(&1) && MapSet.member?(missing_keys, safe_existing_atom(&1))))
         |> MapSet.new()
 
       extra_keys =
@@ -43,8 +41,8 @@ defmodule Machete.LiteralMapMatcher do
 
       missing_keys =
         missing_keys
-        |> MapSet.difference(atom_keys |> Enum.map(&Kernel.to_string/1) |> MapSet.new())
-        |> MapSet.difference(string_keys |> Enum.map(&String.to_existing_atom/1) |> MapSet.new())
+        |> MapSet.difference(MapSet.new(atom_keys, &Kernel.to_string/1))
+        |> MapSet.difference(MapSet.new(string_keys, &safe_existing_atom/1))
 
       Enum.flat_map(extra_keys, &mismatch("Unexpected key", &1)) ++
         Enum.flat_map(missing_keys, &mismatch("Missing key", &1)) ++
@@ -60,5 +58,11 @@ defmodule Machete.LiteralMapMatcher do
         _ -> []
       end)
     end
+
+    defp safe_existing_atom(str) do
+      String.to_existing_atom(str)
+    rescue
+      ArgumentError -> {:error, :machete_no_existing_atom}
+    end
   end
 end

test/machete/matchers/literal_map_matcher_test.exs

@@ -33,6 +33,19 @@ defmodule LiteralMapMatcherTest do
     assert 1 ~>> %{} ~> mismatch("Value is not a map")
   end
 
+  test "produces useful mismatches on non-existing atoms" do
+    assert %{"abcdefghijkl" => 1, "mnopqrstuvwxyz" => 2}
+           ~>> %{"abcdefghijkl" => 1}
+           ~> mismatch("Unexpected key", "mnopqrstuvwxyz")
+
+    assert %{"abcdefghijkl" => 1, "mnopqrstuvwxyz" => 2, "zyxwvutsrq" => 3}
+           ~>> %{"abcdefghijkl" => 1}
+           ~> [
+             %Machete.Mismatch{message: "Unexpected key", path: ["mnopqrstuvwxyz"]},
+             %Machete.Mismatch{message: "Unexpected key", path: ["zyxwvutsrq"]}
+           ]
+  end
+
   describe "nested matchers" do
     test "matches when nested matcher returns empty list" do
       assert %{a: 1} ~>> %{a: test_matcher(behaviour: :match_returning_list)} ~> []

test/machete/matchers/superset_matcher_test.exs

@@ -17,4 +17,8 @@ defmodule SupersetMatcherTest do
   test "produces a useful mismatch for when value is not a map" do
     assert 1 ~>> superset(%{a: 1}) ~> mismatch("1 is not a map")
   end
+
+  test "handles non-existing atoms" do
+    assert %{"abcdefghijkl" => 1, "mnopqrstuvwxyz" => 2} ~> superset(%{"abcdefghijkl" => 1})
+  end
 end