[GHC] #15860: Hadrian build fails on FreeBSD

[GHC]

#15860: Hadrian build fails on FreeBSD
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        Reporter:  raichoo           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Build System      |              Version:  8.7
  (Hadrian)                          |
      Resolution:                    |             Keywords:
Operating System:  FreeBSD           |         Architecture:  x86_64
 Type of failure:  Compile-time      |  (amd64)
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by alpmestan):

 `-B` is used to point a given ghc executable to its associated "topdir" in
 GHC parlance, which corresponds to the `lib/` dir, which contains the
 `settings` file, the package db and all that. See
 [https://github.com/ghc/ghc/blob/master/compiler/main/SysTools/BaseDir.hs#L51
 this note] for more.

 You can see that the error is thrown by
 [https://github.com/ghc/ghc/blob/master/compiler/main/SysTools/BaseDir.hs#L125
 this piece of code], which tries to locate the topdir by looking up an env
 var and then by calling `getBaseDir`, whose implementations uses
 `getExecutablePath` on darwin/linux, does something else on Windows but
 returns `Nothing` the rest of the time. To put it another way, any GHC you
 build needs to be passed its topdir with `-B` (or to be used through a
 wrapper script), I think.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15860#comment:8>
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