AFAICT, the hashable is used for semi-persistent, low-rate hashing. The hashable is not designed for that:

localFileNameForRemoteTarball :: URI -> FilePath
localFileNameForRemoteTarball uri =
  mangleName uri
    ++ "-"
    ++ showHex locationHash ""
  where
    mangleName =
      truncateString 10
        . dropExtension
        . dropExtension
        . takeFileName
        . dropTrailingPathSeparator
        . uriPath

    locationHash :: Word
    locationHash = fromIntegral (Hashable.hash (uriToString id uri ""))

You'd be better of to use e.g. https://hackage.haskell.org/package/cryptohash-sha256 on which cabal-install depends through hackage-security anyway.

Clarification: the usage is persistent, if the user happens to have even the same cabal-install version compiled against the different hashable versions; they won't find the same local files / caches. Don't use hashable for persistence.