Poor error message when using attributes without `derive`

[mwillsey]

The following code doesn't compile on 1.23:

#[macro_use]
extern crate serde_derive;
extern crate serde;

#[serde(untagged)]
enum CellIndex {
    Auto,
    Index(u32),
}

The error message is:

error: The attribute `serde` is currently unknown to the compiler and may have meaning added to it in the future (see issue #29642)
  --> src/lib.rs:20:1
   |
20 | #[serde(untagged)]
   | ^^^^^^^^^^^^^^^^^^

When you add #[derive(Serialize, Deserialize)] to the enum, it compiles, but the error message was quite confusing. It sent me on a feature-flag-enabling goose hunt, and it took someone who knew what they were doing (h/t @dtolnay) to set me straight. A better error message would be great!

[dtolnay]

Thanks! This isn't something we can fix on our end. I filed rust-lang/rust#47608 to follow up in the compiler.