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

[GHC]

#5218: Add unpackCStringLen# to create Strings from string literals
-------------------------------------+-------------------------------------
        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:                    |
-------------------------------------+-------------------------------------

Comment (by bgamari):

 > But i fail to understand how can we provide `unsafeMkByteArray#` in an
 obvious way.

 This would essentially amount to making `ByteArray#` a sum internally. You
 would simply add a closure type which would carry a pointer to the payload
 instead of the VLA carried by `StgArrBytes` currently.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/5218#comment:64>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler