Dedupe include dirs inherited from dependencies

[niteria]

If you build a big number of packages, all with
the same extra -I flags, the flags get inherited
by the dependent packages and duplicated.
For big dependency trees it can exceed the
maximum command line length on some systems,
this happened to me with Linux and hoogle 5.

This patch decreases the redundancy by
dropping all but the first occurrence of
an include dir, preserving the semantics,
as they are processed left to right.

[mention-bot]

@niteria, thanks for your PR! By analyzing the history of the files in this pull request, we identified @dcoutts, @ezyang and @ttuegel to be potential reviewers.

[dcoutts]

Good catch. Just a minor suggestion about code layout.

[dcoutts]

Personally I'd keep it inline here rather than pulling it out. e.g.

++ [ "-I" ++ dir
   , dir <- ordNub [ dir | dep <- deps
                         , dir <- Installed.includeDirs dep ]
            -- dedupe include dirs of dependencies
            -- to prevent quadratic blow-up
   ]

[niteria]

I've updated to accommodate the suggestion with a minor formatting change to respect 80-col limit.

[23Skidoo]

LGTM. Feel free to merge once Travis is green.

[niteria]

Travis timed out, I've tried restarting the job but it didn't seem to have done anything.

[23Skidoo]

I restarted it myself, seems to have worked.

[23Skidoo]

Merged, thanks!

[nh2]

@niteria I have a question about this. In the commit you say:

This patch decreases the redundancy by
dropping all but the first occurrence of
an include dir, preserving the semantics,
as they are processed left to right.

But if they are processed left to right (and thus I assume later ones override earlier ones) shouldn't you keep the last one instead the first?

E.g. if you have dirs a/ and b/ each including file.h, then -Ia/file.h -Ib/file.h -Ia/file.h would be different from -Ia/file.h -Ib/file.h, as in the former it's picked from a/ and the latter from b/?

[niteria]

@nh2: Here's the documentation: https://gcc.gnu.org/onlinedocs/cpp/Search-Path.html

The relevant part:

You can add to this list with the -Idir command-line option. All the directories named by -I are searched, in left-to-right order, before the default directories. The only exception is when dir is already searched by default. In this case, the option is ignored and the search order for system directories remains unchanged.

My understanding is that it searches left to right and takes the first one that matches.
So in your example both situations would have the same behavior.

[nh2]

OK, if that's how it works, then that's OK.