[GHC] #13753: Improve GHC's ghc package environment lookup logic

[GHC]

#13753: Improve GHC's ghc package environment lookup logic
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        Reporter:  hvr               |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D4690
       Wiki Page:                    |
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Comment (by Ben Gamari <ben@…>):

 In [changeset:"8f3c149d94814e4f278b08c562f06fc257eb3c43/ghc" 8f3c149/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="8f3c149d94814e4f278b08c562f06fc257eb3c43"
 Add support for opting out of package environments

 This implements the first part proposed in #13753:

 Define a special magic "null" environment, which instructs GHC to ignore
 any package environment files. To this end, I propose to use the name
 `-` (i.e. a single dash), as that is more portable than using the empty
 string for environment variables. In other words, a

 - `-package-env -` CLI flag, or a
 - `GHC_ENVIRONMENT=-` env var (unless a `-package-env` flag is present)

 would inhibit GHC from looking up and interpreting any package
 environment files from the filesystem; this is the equivalent of
 `-ignore-dot-ghci` for package environment files.

 Reviewers: bgamari

 Reviewed By: bgamari

 Subscribers: rwbarton, thomie, carter

 GHC Trac Issues: #13753

 Differential Revision: https://phabricator.haskell.org/D4690
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13753#comment:9>
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