The governance canister also does this:
https://github.com/dfinity/ic/blob/c48280bbeec347a6b73160199566c612bd8ccbf7/rs/nns/governance/canister/canister.rs#L944
And see this thread for more:
https://forum.dfinity.org/t/automatically-generate-candid-from-rust-sources/5924/5?u=ori