[GHC] #15858: Enabling a pure Plugin with existing build output never triggers recompilation

[GHC]

#15858: Enabling a pure Plugin with existing build output never triggers
recompilation
-------------------------------------+-------------------------------------
           Reporter:  DanielG        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.7
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Other
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Consider the following compilation session:


 {{{
 $ ghc -fplugin ImpurePlugin Foo.hs
 <Compiles Foo.hs>
 $ ghc Foo.hs
 }}}

 where ImpurePlugin is a `Plugin` which returns `ForceRecompile` in
 `pluginRecompile`.

 The second invocation of GHC doesn't recompile even though `ImpurePlugin`
 could have changed the generated build output which would now be out of
 date.

 The same thing happens when switching from an impure/fingerprinted plugin
 to a pure plugin.

 The problem is that `pluginRecompileToRecompileRequired` just assumes no
 compilation is required when the current plugin stack is pure, that is
 either only pure plugins or no plugins at all are registered:

 {{{
 pluginRecompileToRecompileRequired :: Fingerprint -> PluginRecompile ->
 RecompileRequired
 pluginRecompileToRecompileRequired old_fp pr =
   case pr of
     NoForceRecompile -> UpToDate
     ForceRecompile   -> RecompBecause "Plugin forced recompilation"
     MaybeRecompile fp  -> if fp == old_fp then UpToDate

 }}}

 note the `UpToDate` in the `NoForceRecompile` case.


 Furthermore I would argue that even switching from having no plugin at all
 active to having a pure plugin active should trigger recompilation, even
 though the plugin might be pure it could still choose to affect the build
 output after all.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15858>
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