[GHC] #12617: Make the interface of traceTc and traceRn the same

[GHC]

#12617: Make the interface of traceTc and traceRn the same
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        Reporter:  mpickering        |                Owner:
            Type:  task              |               Status:  new
        Priority:  lowest            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      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 mpickering):

 Modifying the `traceRn` interface to the `traceTc` interface causes 3 of
 the performance tests to fail due to the strings being floated to the top
 level (by `-ffull-laziness`) and hence allocated. Previously these strings
 were not allocated as there is a rule which optimises `text "abc"` to
 `ptext (Ptr p)`.

 Here is the diff where I tried various things to fix this:
 https://phabricator.haskell.org/D2586

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12617#comment:2>
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