Add unpackCStringLen# to create Strings from string literals

GHC insert calls to unpackCString# to convert string literals to Strings. Libraries like bytestring use rewrite rules to match on this call to optimize code like pack (unpackCString# s).

If GHC would instead use a version of unpackCString#, say unpackCStringLen#, that includes the (statically known) length, creating ByteStrings from literals could be a constant time operation instead of a linear time operation.

Another use case, which motivated this ticket, is appending string literals to builders (e.g. using Data.Binary.Builder.fromByteString). For small strings the most efficient way to append a string to the builder is to copy the statically allocated string directly into the builder's output buffer. If the string length was known statically, we could do this efficiently using memcpy or even using a small unrolled loop.

Trac metadata

Trac field	Value
Version	7.0.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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