[GHC] #914: Improve detection of unnecessary SOURCE imports

#914: Improve detection of unnecessary SOURCE imports
-------------------------+--------------------------------------------------
    Reporter:  simonmar  |        Owner:  simonmar        
        Type:  task      |       Status:  new             
    Priority:  normal    |    Milestone:  _|_             
   Component:  Compiler  |      Version:  6.5             
    Severity:  normal    |     Keywords:                  
          Os:  Unknown   |   Difficulty:  Moderate (1 day)
Architecture:  Unknown   |  
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Currently unnecessary SOURCE imports are detected by the renamer, but that
 is a slightly unsatisfactory solution because it is unpredictable whether
 we will have compiled the real module before we get to the unnecessary
 SOURCE import or not - it depends on the topological sort performed by the
 downsweep (or make).

 An improvement would be to look at the results of the strongly connected
 component analysis performed on the graph with SOURCE imports included,
 and report any SOURCE import edges ''between components'' as unnecessary,
 because replacing one of these with a non-SOURCE import cannot introduce a
 cycle.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/914>
GHC <http://www.haskell.org/ghc/>
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