Take CWD into account in preprocessor hs-boot hack

This commit fixes an oversight in runPreProcessorWithHsBootHack, which
did not correctly take into account the working directory. This could
lead to situations in which we failed to apply the hack.

Recall that this hack is for situations when one has both an hs-boot
file and a file in need of preprocessing, such as:

  Foo.y
  Foo.hs-boot

or

  Bar.hsc
  Bar.hs-boot

Failing to apply the hack essentially meant that GHC would not be able
to see the hs-boot file, which caused build failures.

Author: sheaf

Cabal/src/Distribution/Simple/PreProcess.hs

@@ -345,8 +345,8 @@ preprocessFile mbWorkDir searchLoc buildLoc forSDist baseFile verbosity builtinS
           createDirectoryIfMissingVerbose verbosity True destDir
           runPreProcessorWithHsBootHack
             pp
-            (getSymbolicPath $ psrcLoc, getSymbolicPath $ psrcRelFile)
-            (getSymbolicPath $ buildLoc, srcStem <.> "hs")
+            (psrcLoc, getSymbolicPath $ psrcRelFile)
+            (buildLoc, srcStem <.> "hs")
   where
     i = interpretSymbolicPath mbWorkDir -- See Note [Symbolic paths] in Distribution.Utils.Path
     buildAsSrcLoc :: SymbolicPath Pkg (Dir Source)
@@ -361,20 +361,25 @@ preprocessFile mbWorkDir searchLoc buildLoc forSDist baseFile verbosity builtinS
       pp
       (inBaseDir, inRelativeFile)
       (outBaseDir, outRelativeFile) = do
+        -- Preprocessors are expected to take into account the working
+        -- directory, e.g. using runProgramCwd with a working directory
+        -- computed with mbWorkDirLBI.
+        -- Hence the use of 'getSymbolicPath' here.
         runPreProcessor
           pp
-          (inBaseDir, inRelativeFile)
-          (outBaseDir, outRelativeFile)
+          (getSymbolicPath $ inBaseDir, inRelativeFile)
+          (getSymbolicPath $ outBaseDir, outRelativeFile)
           verbosity
 
-        exists <- doesFileExist inBoot
-        when exists $ copyFileVerbose verbosity inBoot outBoot
-        where
+        -- Here we interact directly with the file system, so we must
+        -- interpret symbolic paths with respect to the working directory.
+        let
+          inFile = normalise (i inBaseDir </> inRelativeFile)
+          outFile = normalise (i outBaseDir </> outRelativeFile)
           inBoot = replaceExtension inFile "hs-boot"
           outBoot = replaceExtension outFile "hs-boot"
-
-          inFile = normalise (inBaseDir </> inRelativeFile)
-          outFile = normalise (outBaseDir </> outRelativeFile)
+        exists <- doesFileExist inBoot
+        when exists $ copyFileVerbose verbosity inBoot outBoot
 
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