Recompilation avoidance doesn't work for -fno-code/-fwrite-interface
With the latest
-fwrite-interface enhancements, a build directory can contain interface files independently of object files. In cases like this, we would like to have recompilation avoidance avoid typechecking files whose interface files are up-to-date.
However, this does not currently work: we always re-typecheck:
t-edyang@cam-05-unx:/5playpen/t-edyang/sandbox/q$ /5playpen/t-edyang/ghc-backpack/inplace/bin/ghc-stage2 --make A.hs -fno-code -fwrite-interface [1 of 1] Compiling A ( A.hs, nothing ) t-edyang@cam-05-unx:/5playpen/t-edyang/sandbox/q$ /5playpen/t-edyang/ghc-backpack/inplace/bin/ghc-stage2 --make A.hs -fno-code -fwrite-interface [1 of 1] Compiling A ( A.hs, nothing )
The reason for this is that recompilation avoidance logic in
DriverPipeline.hs) seems to rely exclusively on "linkables" in order to figure out if source has been modified or not. We never generate an object file with
-fwrite-interface, so the compiler always concludes that we need to retypecheck.
However, recompilation avoidance for hs-boot does work. The reason for this is because we create a dummy o-boot linkable. We can't use this strategy for
-fno-code, because the dummy object file would imply that we actually compiled the file (which we didn't).
The upshot is that to fix this problem, it looks like we might have to rejigger all of the logic in
compileOne, which why I gave up on this for now.
Related to https://github.com/haskell/cabal/issues/1179