GHC incorrectly concludes rebuild unnecessary, even when ghc-options have changed

[edsko]

Steps:

1. Compile your program (using new-build). Realize it has a space leak.
2. Change cabal.project to:

package ingest
  ghc-options: -fno-full-laziness

1. Call new-build again:

# cabal new-build
In order, the following will be built (use -v for more details):
 - ingest-0.1.0.0 (lib) --enable-profiling (configuration changed)
 - ingest-0.1.0.0 (exe:ingest) --enable-profiling (configuration changed)
Configuring component lib from ingest-0.1.0.0
Preprocessing library ingest-0.1.0.0...
Configuring component exe:ingest from ingest-0.1.0.0
Preprocessing executable 'ingest' for ingest-0.1.0.0...

Note that although it detected "configuration changed", nothing actually got recompiled. The change only takes effect after I manually delete the dist-newstyle directory and rebuild again.

[ezyang]

Judging from the log output, it is likely we are invoking GHC, but GHC does not consider that recompilation is necessary. So technically this is an upstream bug. But that's not an excuse, we should do something here.

[edsko]

FWIW, with the traditional build this works just fine (and I use that all the time when debugging low level issues).

[ezyang]

OK, well, never mind then!

[edsko]

Hmmm, it seems I was wrong about ti working with traditional build. Not sure why I concluded that. It doesn't work with traditional build either :) Apologies.

[ezyang]

OK relabeled ;)