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

[GHC]

#1853: hpc mix files for Main modules overwrite each other
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        Reporter:  guest          |                Owner:
            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):
       Wiki Page:                 |
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Comment (by mgsloan):

 There's a pretty good reason not to vary `--hpcdir`.  You can only pass in
 one `--hpcdir` to the `hpc` program, and it's treated as a relative path
 to each of the `--srdir`s.  So, if you want to generate a report which
 spans multiple packages, `--hpcdir` needs to be kept constant.

 We might consider improving the hpc program's CLI.  This particular issue
 is discussed further in https://ghc.haskell.org/trac/ghc/ticket/10951

 The tricky thing about combining tix files together is that module names
 within the tix file are not sufficiently disambiguated.  For packages,
 they are prefixed by their package key / id.  So, you expect to find the
 mix file for `TixModule "cycli_6S2W1a0WjTe91AyXmPMq2j/Cyclic" ...` at some
 `HPCDIR/cycli_6S2W1a0WjTe91AyXmPMq2j/Cyclic.mix`.  However, for modules
 from the non-library components, there is no folder.  Instead it's just
 `TixModule "Main" ...`

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/1853#comment:25>
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