[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:  new
        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):
       Wiki Page:                    |
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Comment (by osa1):

 I believe pre-unboxed-sums unarise pass could be made type-safe (I
 actually had a patch that moved unarise pass to Core and I don't recall
 any type safety issues), but now that we have sums moving unarise to Core
 means adding `unsafeCoerce#`s everywhere. One of my initial unboxed sums
 patches was doing that, but no one liked it (SPJ reviewed it on Phab)
 because of `unsafeCoerce#`s in Core.

 So yeah unarise can't be made type safe without `unsafeCoerce#`s at the
 moment, it needs to stay as a STG pass.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/5218#comment:51>
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