Use File::path for FFI

lib/prism/ffi.rb

@@ -217,12 +217,12 @@ def self.with_string(string)
 
       # Yields a PrismSource to the given block, backed by a pm_source_t.
       def self.with_file(filepath, options)
-        unless filepath.is_a?(String)
-          filepath = filepath.to_path if filepath.respond_to?(:to_path)
-          filepath = filepath.to_str if filepath.respond_to?(:to_str)
-        end
+        # File.path applies the same coercion and validation as the C
+        # extension's rb_get_path: it accepts to_path/to_str objects and
+        # rejects paths containing a NUL byte, which would otherwise truncate
+        # the path when it is handed to C as a NUL-terminated string.
+        filepath = File.path(filepath)
         options[:filepath] = filepath
-        raise TypeError unless filepath.is_a?(String)
 
         # On Windows and Mac, it's expected that filepaths will be encoded in
         # UTF-8. If they are not, we need to convert them to UTF-8 before

test/prism/api/parse_test.rb

@@ -61,6 +61,14 @@ def test_parse_file
       end
     end
 
+    def test_parse_file_null_byte
+      Dir.mktmpdir do |dir|
+        path = File.join(dir, "target.txt")
+        File.write(path, "# secret")
+        assert_raise(ArgumentError) { Prism.parse_file("#{path}\0.rb") }
+      end
+    end
+
     def test_parse_tempfile
       Tempfile.create(["test_parse_tempfile", ".rb"]) do |t|
         t.puts ["begin\n", " end\n"]