Function parse_serde_error_nested 

pub(crate) fn parse_serde_error_nested(
    meta: &ParseNestedMeta<'_>,
) -> Result<SerdeErrorSpec>

Parse the tail of a serde_error option inside parse_nested_meta. Only the option list (shape = .., tag = "...", skip(...), as_character(...), ...) is accepted; the bare flag and = true/false are rejected with SERDE_ERROR_BARE_HELP, since the serde path is on for every eligible error type under the serde feature.