Shouldn't the "use of unstable library feature" error have an error number?

[carols10cents]

I thought every error was supposed to have a number that one could look up in the error index, run rustc --explain with, etc... but if I run this code with stable rust 1.23.0:

fn testl() {
    ::std::u128::MAX;
}

I get:

error: use of unstable library feature 'i128' (see issue #35118)
 --> src/lib.rs:2:5
  |
2 |     ::std::u128::MAX;
  |     ^^^^^^^^^^^^^^^^

Rather than error[E0644] or something.

Feel free to close if this was a deliberate decision.

[GuillaumeGomez]

Every error should have an error number! I'm adding it.