Unnecessary recompilation when using the CPP language extension

[asr]

Using the files below I got the following behaviour:

$ cabal install --enable-tests
...

$ cabal install
...
Preprocessing library Foo-0.1.0.0...
[1 of 1] Compiling Foo              ( Foo.hs, dist/build/Foo.o ) [dist/build/autogen/cabal_macros.h changed]

Cabal correctly changes cabal.macros.h because the dependencies of the library and the test-suite are different.

Why the module Foo.hs is recompiled?

If I comment-out {-# LANGUAGE CPP #-} in Foo.hs, this module isn't recompiled.

Files: Foo.cabal.txt, Foo.hs.txt and Test.hs.txt.

Cabal and GHC versions:

$ caball --version
cabal-install version 1.24.0.0
compiled using version 1.24.0.0 of the Cabal library

$ ghc --numeric-version
8.0.1

[phadej]

The is no way to check whether symbols from cabal_macros.h aren't used in Foo.hs, most likely if you have {-# LANGUAGE CPP #-}, then you use it.

solution to #3545 will probably fix this as well, as libraries and test-suite cabal_macros.h will be actually different.

[asr]

Thanks for the reference to #3545.

[ezyang]

Yes, this is fixed in master.

BTW, there is always an implicit include of cabal_macros.h. Without CPP this can never affect the module, but with it we have to recompile. Arguably GHC should be smarter about not recompiling when preprocessor output is the same, but that is a GHC bug.

[ezyang]

PS the real ticket is #1893 and the fixing commit is 90e908b

[asr]

I couldn't reproduce the problem using the current master (commit 0260415). Thanks!