Separate SCRIPT_LINES__ from ast.c

This patch suggests relocating the code dealing with `SCRIPT_LINES__` from ast.c to ruby_parser.c.

## Background

- I guess `AbstractSyntaxTree.of` method used to use `SCRIPT_LINES__` internally for some reason before
- However, now it appears `SCRIPT_LINES__` is no longer used meaningfully by the method
- As evidence of this, (and as my patch shows,) removing the function call of `rb_script_lines_for()` from `ast_s_of()` does not affect the result of `test/ruby/test_ast.rb`

Given the above, I think two possibilities can be considered:

- (A) `AbstractSyntaxTree.of` has not needed `SCRIPT_LINES__` already (I pick this)
- (B) We lack a test case of `AbstractSyntaxTree.of` that needs to use `SCRIPT_LINES__`

## Besides,

The current implementation causes strange behavior:

```console
ruby -e"SCRIPT_LINES__ = {__FILE__ => []}; puts RubyVM::AbstractSyntaxTree.of(->{ 1 + 2 }, keep_script_lines: true).script_lines"
=> `-e:1:in '<main>': undefined method 'script_lines' for nil (NoMethodError)`
```

I think this is a bug because `AbstractSyntaxTree.of` is not supposed to return `nil` even in this case.
This happens due to the ast.c's dependence on `SCRIPT_LINES__`.
And at the end of the `ast_s_of()`, `node_find()` can not find the target child node obviously because it doesn't make sense to look for a corresponding node made from the parameter of `AbstractSyntaxTree.of` in the AST tree made from the value of `{__FILE__ => []}`

## Solution

Since I think it's good enough `SCRIPT_LINES__` to be only referred by ruby.c, I chose the possibility "(A)" and wrote this patch which moves `rb_script_lines_for()` from ast.c to ruby_parser.c.

So as the result:

- `ast_s_of()` function no longer look up `SCRIPT_LINES__`
- Even so, this patched code passes the existing tests
- The strange behavior above no longer happens (I also added a test for it)

Please correct me if I miss something🙏

ast.c

@@ -183,29 +183,6 @@ node_find(VALUE self, const int node_id)
 
 extern VALUE rb_e_script;
 
-VALUE
-rb_script_lines_for(VALUE path, bool add)
-{
-    VALUE hash, lines;
-    ID script_lines;
-    CONST_ID(script_lines, "SCRIPT_LINES__");
-    if (!rb_const_defined_at(rb_cObject, script_lines)) return Qnil;
-    hash = rb_const_get_at(rb_cObject, script_lines);
-    if (!RB_TYPE_P(hash, T_HASH)) return Qnil;
-    if (add) {
-        rb_hash_aset(hash, path, lines = rb_ary_new());
-    }
-    else if (!RB_TYPE_P((lines = rb_hash_lookup(hash, path)), T_ARRAY)) {
-        return Qnil;
-    }
-    return lines;
-}
-static VALUE
-script_lines(VALUE path)
-{
-    return rb_script_lines_for(path, false);
-}
-
 static VALUE
 node_id_for_backtrace_location(rb_execution_context_t *ec, VALUE module, VALUE location)
 {
@@ -267,7 +244,7 @@ ast_s_of(rb_execution_context_t *ec, VALUE module, VALUE body, VALUE keep_script
         rb_raise(rb_eArgError, "cannot get AST for method defined in eval");
     }
 
-    if (!NIL_P(lines) || !NIL_P(lines = script_lines(path))) {
+    if (!NIL_P(lines)) {
         node = rb_ast_parse_array(lines, keep_script_lines, error_tolerant, keep_tokens);
     }
     else if (e_option) {

ruby.c

@@ -2581,7 +2581,7 @@ struct load_file_arg {
     VALUE f;
 };
 
-VALUE rb_script_lines_for(VALUE path, bool add);
+VALUE rb_script_lines_for(VALUE path);
 
 static VALUE
 load_file_internal(VALUE argp_v)
@@ -2686,7 +2686,7 @@ load_file_internal(VALUE argp_v)
     rb_parser_set_options(parser, opt->do_print, opt->do_loop,
                           opt->do_line, opt->do_split);
 
-    VALUE lines = rb_script_lines_for(orig_fname, true);
+    VALUE lines = rb_script_lines_for(orig_fname);
     if (!NIL_P(lines)) {
         rb_parser_set_script_lines(parser, lines);
     }

ruby_parser.c

@@ -994,3 +994,16 @@ rb_node_encoding_val(const NODE *node)
 {
     return rb_enc_from_encoding(RNODE_ENCODING(node)->enc);
 }
+
+VALUE
+rb_script_lines_for(VALUE path)
+{
+    VALUE hash, lines;
+    ID script_lines;
+    CONST_ID(script_lines, "SCRIPT_LINES__");
+    if (!rb_const_defined_at(rb_cObject, script_lines)) return Qnil;
+    hash = rb_const_get_at(rb_cObject, script_lines);
+    if (!RB_TYPE_P(hash, T_HASH)) return Qnil;
+    rb_hash_aset(hash, path, lines = rb_ary_new());
+    return lines;
+}

test/ruby/test_ast.rb

@@ -746,6 +746,19 @@ dummy
     assert_equal("def test_keep_script_lines_for_of\n", node_method.source.lines.first)
   end
 
+  def test_keep_script_lines_for_of_with_existing_SCRIPT_LINES__that_has__FILE__as_a_key
+    # This test confirms that the bug that previously occurred because of
+    # `AbstractSyntaxTree.of`s unnecessary dependence on SCRIPT_LINES__ does not reproduce.
+    # The bug occurred only if SCRIPT_LINES__ included __FILE__ as a key.
+    lines = [
+      "SCRIPT_LINES__ = {__FILE__ => []}",
+      "puts RubyVM::AbstractSyntaxTree.of(->{ 1 + 2 }, keep_script_lines: true).script_lines",
+      "p SCRIPT_LINES__"
+    ]
+    test_stdout = lines + ['{"-e"=>[]}']
+    assert_in_out_err(["-e", lines.join("\n")], "", test_stdout, [])
+  end
+
   def test_source_with_multibyte_characters
     ast = RubyVM::AbstractSyntaxTree.parse(%{a("\u00a7");b("\u00a9")}, keep_script_lines: true)
     a_fcall, b_fcall = ast.children[2].children