[GHC] #1853: hpc mix files for Main modules overwrite each other

[GHC]

#1853: hpc mix files for Main modules overwrite each other
----------------------------------+--------------------------------------
        Reporter:  guest          |                Owner:  AndyGill
            Type:  bug            |               Status:  new
        Priority:  lowest         |            Milestone:  8.0.1
       Component:  Code Coverage  |              Version:  6.8.1
      Resolution:                 |             Keywords:
Operating System:  Linux          |         Architecture:  x86_64 (amd64)
 Type of failure:  None/Unknown   |            Test Case:
      Blocked By:                 |             Blocking:
 Related Tickets:                 |  Differential Rev(s):
----------------------------------+--------------------------------------

Comment (by mgsloan):

 This is also troublesome for using `hpc combine --union` to merge tix
 files which come from different programs.  My current hack is to
 preprocess the tix files and remove all of the executable modules, like
 this: https://gist.github.com/mgsloan/e44e8775516c50f10edf

 I think the ideal resolution would be to prefix all of the executable
 modules with some better identification.  How about having a new ghc flag
 like `--hpc-package-prefix prefix`, where the conventional value for
 prefix is the cabal `package:component`?  Then, the tix files will use
 this prefix on module names, and the mix files will get outputted to a
 subdirectory instead of the root of the hpcdir.

 This convention gets a little weird due to the usage of `:` when passing
 `--include` and `--exclude` to the hpc program (see
 https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/hpc.html).
 However, with care, `:` should be a valid thing to use in `PACKAGE`.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/1853#comment:18>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler