Function flatten_for_registry 

pub fn flatten_for_registry(
    fds: &FileDescriptorSet,
    root_fqn: &str,
) -> Result<DescriptorProto, FlattenError>

Flattens fds into a single self-contained DescriptorProto rooted at root_fqn, grafting the transitive closure of top-level ancestors and rewriting type references so a shallow, nested_type-only registry can resolve them. See the module documentation for the algorithm.

Errors

Returns a FlattenError if root_fqn is not a top-level message (FlattenError::RootNotFound/FlattenError::RootIsNested), if a referenced type is missing from the set (FlattenError::UnresolvedType - compile with protoc --include_imports), or if a grafted name cannot be disambiguated (FlattenError::NameCollision).

Panics

Does not panic in normal use: the worklist only ever contains type names already present in the index, which an internal assertion documents.