2018 edition - confusing error message when declaring unnamed parameters

[EpicatSupercell]

This code works correctly on nightly and stable

trait Trait {
    fn func(u8);
}

but not on 2018 edition, where it returns the error

error: expected one of `:` or `@`, found `)`
 --> src/main.rs:2:15
  |
2 |     fn func(u8);
  |               ^ expected one of `:` or `@` here

Changing it to func(_: u8) solves it.

Is this an intended change?
I couldn't find anything about it in the edition guide.

[jonas-schievink]

Yes, this was done in #53612

[EpicatSupercell]

So apparently it was part of RFC 1685. In that case we should at least fix the error message, since it doesn't explain the source of the error at all.