Debug.Trace should not use %s for format string

The current implementation of traceIO in Debug.Trace is as follows:

traceIO :: String -> IO ()
traceIO msg = do
    withCString "%s\n" $ \cfmt ->
     withCString msg  $ \cmsg ->
      debugBelch cfmt cmsg

This is bad news: it means if the String has a null in it, the string will be silently truncated. For example, in GHC, this means that you cannot pretty-print FastString uniques, since the default printing algorithm often results in a null.

Probably the proper thing to do here is to also pass a length indicator for the CString. We should also fix unique pretty-printing to never generate nulls. A good test case would be:

main = trace "\0foo" (return ())

which should have non-empty output.

Trac metadata

Trac field	Value
Version	7.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
Test case
Differential revisions
BlockedBy
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Blocking
CC	ekmett, hvr
Operating system
Architecture