Function heading 

pub fn heading(category: Category) -> Option<&'static str>

The heading a category’s changes are grouped under in the text report, or None for a category listed plainly. Five headings exist. “compatible on the wire, visible in source” is for a change that exits 0 but that a consumer sees in its source — an interface renamed on its number changes the generated identity-table names in both wire backends, and an interface leaving a service’s set stops the service.member addresses of that interface resolving under the service. “compatible on the wire, may change the catalog hash” is for a change that exits 0 but can change the catalog hash, which a generated face compares with its peer’s — a number ridl lock froze in place, which always changes it, and an enum or enum-set body reordered, which changes it when an interface reaches the enum or enum set (the hash covers only the declarations an interface reaches). “breaking under podbuf” is for a change whose verdict depends on the encodings the comparison is judged for (diff_refs_with): it exits 0 unless podbuf is among them. “breaking under flatbuffers” follows the same rule for flatbuffers, and “breaking under proto3” for proto3. Each such category’s --explain text states its own consequence; the JSON report carries the category word and no heading field.

Wildcard arms are denied for the reason category_word gives.