Constant SERDE_ERROR_BARE_HELP 

pub(crate) const SERDE_ERROR_BARE_HELP: &str = "`serde_error` is not a switch: with the `serde` \
    feature every `Result<T, E>` whose `E: Serialize + Display` is already classed from its \
    serde shape. The attribute only carries options; write `serde_error(shape = ENGINE)`, \
    options such as `serde_error(prefix = \"..\", skip(..), as_character(..))`, or drop it";

The bare flag / boolean forms: the serde path is not something to switch on per function.