rust/src/librustc_error_codes/error_codes/E0551.md

An invalid meta-item was used inside an attribute.

Erroneous code example:

#[deprecated(note)] // error!
fn i_am_deprecated() {}

Meta items are the key-value pairs inside of an attribute. To fix this issue, you need to give a value to the note key. Example:

#[deprecated(note = "because")] // ok!
fn i_am_deprecated() {}