Respect encoding option in Prism.lex and friends

utf-8 is the default for source files but can be overwritten via options

Author: Earlopain

ext/prism/extension.c

@@ -793,7 +793,7 @@ parse_lex_input(const uint8_t *input, size_t input_length, const pm_options_t *o
     parse_lex_data_t parse_lex_data = {
         .source = source,
         .tokens = rb_ary_new(),
-        .encoding = rb_utf8_encoding(),
+        .encoding = rb_enc_find(pm_parser_encoding_name(parser)),
         .freeze = pm_options_freeze(options),
     };
 

test/prism/lex_test.rb

@@ -47,6 +47,24 @@ def test_parse_lex_file
       end
     end
 
+    def test_lex_encoding
+      tokens = Prism.lex('"わたし"', encoding: Encoding::Windows_31J).value
+      tokens.each do |t|
+        assert_equal(Encoding::Windows_31J, t[0].value.encoding)
+      end
+
+      # Shebangs must appear on the first line. For these cases, the encoding
+      # comment may appear second, but it should still change encoding.
+      tokens = Prism.lex(<<~RUBY, encoding: Encoding::Windows_31J).value
+        #! /usr/bin/env ruby
+        # encoding: utf-8
+        "わたし"
+      RUBY
+      tokens.each do |t|
+        assert_equal(Encoding::UTF_8, t[0].value.encoding)
+      end
+    end
+
     if RUBY_VERSION >= "3.3"
       def test_lex_compat
         source = "foo bar"