[GHC] #16253: Offer a shorthand for `--skip=_build/stage$n/compiler/.dependencies.mk`

[GHC]

#16253: Offer a shorthand for `--skip=_build/stage$n/compiler/.dependencies.mk`
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        Reporter:  sgraf             |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Build System      |              Version:  8.6.3
  (Hadrian)                          |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by snowleopard):

 Looking at the documentation of `ghc -M`, it looks like the current per-
 package approach is due to a limitation of GHC's dependency analysis.
 Quoting from
 https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/separate_compilation.html
 #makefile-dependencies

 > In general, `ghc -M Foo` does the following. For each module `M` in the
 set `Foo` plus all its imports (transitively), it adds to the Makefile
 [...]

 That is, GHC always does **transitive** dependency analysis, which means
 invoking it separately on each file would be rather inefficient (each time
 it will likely traverse almost the whole dependency graph). This is why
 Make and Hadrian choose to perform the analysis just once but for the
 whole package.

 Perhaps, it's not too difficult to add a more fine-grain dependency
 analysis to GHC, i.e. produce only the list of immediate dependencies of a
 specified module.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16253#comment:4>
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