How is the __LINE__ and __FILE__ constants implemented in Ruby?

Question

It seems the __FILE__ and __LINE__ constants are dynamically updated with the current file and line numbers under execution, I am wondering how is the behaviour implemented in Ruby?

I've greped the source code but there are too many noises for __LINE__ and __FILE__ appearance, I am wonder anyone could help me point to the source code and provide a clue to understand its behaviour.

Explanation in either Rubinis or MRI will be fine.

Answer 1

Both __FILE__ and __LINE__ get replaced by literals directly in the parser:

case keyword__FILE__:
    return NEW_STR(rb_external_str_new_with_enc(ruby_sourcefile, strlen(ruby_sourcefile),
                                                rb_filesystem_encoding()));
case keyword__LINE__:
    return NEW_LIT(INT2FIX(tokline));

In other words, they behave exactly as if you had typed in the resulting string or number yourself.

Note that for __LINE__, this doesn't always behave how you'd expect.