Function schema_major 

pub fn schema_major(schema_version: &str) -> &str

Major component of a v<major>.<minor> schema_version. Used to decide whether a received event is wire-compatible.

Today: every Wire schema is v3.x; major == “v3”. A 0.5.12 binary might start emitting v4.0 events; older 0.5.x binaries see major=v4 and bail instead of attempting to decode an incompatible shape.