syntax_data: {"ast_node_type":"other","tokens":[{"end":71,"kind":"keyword","start":0,"text":"external_src:"}]}
source_snippet:             external_src: FreezeLock::frozen(ExternalSource::Unneeded),
context_before:             // a new SourceFile, filling in `external_src` properly
context_after:             lines: FreezeLock::new(lines),