[GHC] #5218: Add unpackCStringLen# to create Strings from string literals

[GHC]

#5218: Add unpackCStringLen# to create Strings from string literals
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        Reporter:  tibbe             |                Owner:  thoughtpolice
            Type:  feature request   |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.0.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #5877 #10064      |  Differential Rev(s):  Phab:D2443
       Wiki Page:                    |
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Comment (by bgamari):

 > Fair enough, but why not just poke a hole in the `ByteArray#`
 abstraction in that case? Namely, provide a `unsafeMkByteArray# :: Addr#
 -> Int -> ByteArray#`.

 Although, thinking about this for a tad longer it's not necessarily clear
 that this is a direction which we'd want to go. Afterall, it would require
 the introduction of another closure type since the current representation
 requires that the bytes directly follow the closure header. I suspect this
 would introduce quite a bit of complexity in the RTS.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/5218#comment:62>
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