(diagnostic
  (schema slovo.diagnostic)
  (version 1)
  (severity error)
  (code UnsupportedUnitSignatureType)
  (message "function return type `unit` is unsupported")
  (file "<fixture>")
  (span
    (bytes 31 35)
    (range 4 16 4 20)
  )
  (expected "non-unit function return type")
  (found "unit")
  (hint "`unit` is reserved for compiler/runtime unit-producing forms")
)