Nix-style builds need to install data files for inplace builds

[dagit]

Currently, new-build does not install data-files (possibly also extra-files). A sufficient condition for this bug is an "inplace" build. It may happen in other cases as well.

[ezyang]

To be more specific, when we new-build an inplace package, we configure some bogus install paths for data files which get baked into Paths_pkg.hs. However, a new-build of an inplace package never actually installs into those directories, so if you actually try to run an inplace executable it won't find anything.

One way to fix this is to configure install paths into the dist-newstyle directory (at the cost of accumulating technical debt for when we eventually want relocatable build directories) and then installing the files into that location. Another possibility is to teach Paths to be able to pick up local data files prior to installation (so we don't have to go copy things), although this may be difficult if the install layout is different than the source layout. @dcoutts is likely to have Opinions(TM).

[ezyang]

Known workarounds:

1. If the data-files don't change much, run an old style "cabal install" which will stick the install files where the bogus paths are looking
2. Set environment variables to where the data-files are (note that your in-tree data files need to have the same layout as when they are installed, which may not necessarily be the case)

[fgaz]

your in-tree data files need to have the same layout as when they are installed

Is it reasonable to assume this? What could change it?

Old-run also does this. Is it fine if i replicate this behaviour in new-run (i.e. not copying the data files anywhere and simply setting the env var)?

[23Skidoo]

What could change it?

I can't think of anything except maybe custom setup shenaningans (it should be ok to ignore those). You should remember to take data-dir into account, though.

[23Skidoo]

NB: This is not fixed in the general case (running the inplace-built exe from dist-newstyle/... manually), only for new-run.