[GHC] #16295: Enable cached builds with Hadrian

[GHC]

#16295: Enable cached builds with Hadrian
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        Reporter:  snowleopard       |                Owner:  (none)
            Type:  bug               |               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 Marge Bot <ben+marge-bot@…>):

 In [changeset:"1dad4fc27ea128a11ba0077f459494c2a1ca0d5c/ghc"
 1dad4fc2/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="1dad4fc27ea128a11ba0077f459494c2a1ca0d5c"
 Hadrian: Fix untracked dependencies

 This is a preparation for #16295:
 https://ghc.haskell.org/trac/ghc/ticket/16295

 This commit mostly focuses on getting rid of untracked dependencies,
 which prevent Shake's new `--shared` feature from appropriately caching
 build rules.

 There are three different solutions to untracked dependencies:

 * Track them! This is the obvious and the best approach, but in some
   situations we cannot use it, for example, because a build rule creates
   files whose names are not known statically and hence cannot be
   specified as the rule's outputs.

 * Use Shake's `produces` to record outputs dynamically, within the rule.

 * Use Shake's `historyDisable` to disable caching for a particular build
   rule. We currently use this approach only for `ghc-pkg` which mutates
   the package database and the file `package.cache`.

 These two tickets are fixed as the result:

 Ticket #16271: ​https://ghc.haskell.org/trac/ghc/ticket/16271

 Ticket #16272: ​https://ghc.haskell.org/trac/ghc/ticket/16272 (this one
 is fixed only partially: we correctly record the dependency, but we
 still copy files into the RTS build tree).
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16295#comment:2>
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