sandbox hashcode in dist directory

[Tim Docker]

I'm attempting to use ghc-mod and emacs to get on the fly compiling 
working again. This is always pleasing when it works,
but it seems a never ending challenge to get it to find all my 
dependences :-(

Several of my cabal packages require autogenerated code which used to 
end up in dist/build/autogen. ghc-mod is configured to
search this directory, as per:

https://github.com/kazu-yamamoto/ghc-mod/blob/169069f6232c617f06f24b9caa40f8f967c1cbf5/Language/Haskell/GhcMod/CabalApi.hs#L71

However since starting to use cabal sandboxes, the generated code ends 
up in a directory like this:

     dist/dist-sandbox-c4c8b1d4/build/autogen/

What is this hashcode for? Is it always generated when using a sandbox? 
If I have code (like ghc-mod) that needs to access the
build directory, how do I work out the value of the hashcode?

Thanks,

Tim