Function unwritable_metadata 

pub fn unwritable_metadata(kind: &str, record: MetadataRecord) -> SourceError

The refusal a source answers a narrow metadata write with when it cannot make one on its own.

It reads the linear plugin cannot write a document's metadata on its own, naming the record. Spelled once beside unwritable_field for that function’s reason: the trait’s defaults and the engine’s refusal of a plugin whose handshake does not declare the write say the same thing in the same words.