[GHC] #10420: "Care with plugin imports" is wrong / orphan RULE visibility

15 May 2015

      #10420: "Care with plugin imports" is wrong / orphan RULE visibility
-------------------------------------+-------------------------------------
              Reporter:  ezyang      |             Owner:  ezyang
                  Type:  bug         |            Status:  new
              Priority:  low         |         Milestone:
             Component:  Compiler    |           Version:  7.11
  (Type checker)                     |  Operating System:  Unknown/Multiple
              Keywords:              |   Type of failure:  None/Unknown
          Architecture:              |        Blocked By:
  Unknown/Multiple                   |   Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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 A module loaded by a plugin poisons the module cache, so we never load an
 orphan RULE or instance even if we legitimately should do so. The way to
 test this is a bit convoluted, but it goes something like this:

 plugins07.hs
 {{{
 module Main where

 import Plugins07a
 import RuleDefiningPlugin

 {-# NOINLINE x #-}
 x = "foo"

 main = putStrLn (show x)
 }}}

 Plugins07a.hs
 {{{
 --{-# OPTIONS_GHC -fplugin RuleDefiningPlugin #-}
 module Plugins07a where
 }}}

 RuleDefiningPlugin.hs, in ANOTHER PACKAGE (otherwise, EPT rules don't
 apply)
 {{{
 module RuleDefiningPlugin where

 import GhcPlugins

 {-# RULES "unsound" forall x. show x = "SHOWED" #-}

 plugin :: Plugin
 plugin = defaultPlugin
 }}}

 I'll commit the full test.

 Here's what happens:

  * Building `Plugins07a` results in `loadPluginInterface` on
 `RuleDefiningPlugin`. We load the `ModIface` but only add its types to the
 environment because of the "Care with plugin imports" special case.

  * Building `plugins07.hs` results in a normal source level import for
 `RuleDefiningPlugin`, but `ModIface` is already in the cache so we don't
 load anything. RULE is not loaded, disaster!

 Admittedly, actually triggering this bug requires a convoluted chain of
 events. But really this problem arose because the "Care with plugin
 imports" fix is just completely nonsense. Here's what we should do:

  * We should apply the same fix from #2182 on orphan instances to orphan
 rules too. This way, we can safely load RULEs into the EPS without
 accidentally bringing them into scope when they shouldn't be.

  * Loading an interface should unconditionally suck in the instances and
 rules.

 The result is more correct and simpler, so it seems worth fixing.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10420
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