Take working directory into account in preprocessor hs-boot hack

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
 
 -- ------------------------------------------------------------
 

cabal-testsuite/PackageTests/HsBootHack/dep/dep.cabal

@@ -0,0 +1,15 @@
+cabal-version: 3.0
+name:          dep
+version:       0.1.0.0
+license:       BSD-3-Clause
+author:        sheaf
+maintainer:    sheaf
+category:      Testing
+build-type:    Simple
+description:   Testing the preprocessor hs-boot hack
+
+library
+  hs-source-dirs:   src
+  exposed-modules:  DepA, DepB, DepC
+  build-depends:    base
+  default-language: Haskell2010

cabal-testsuite/PackageTests/HsBootHack/dep/src/DepA.hs

@@ -0,0 +1,5 @@
+module DepA where
+
+import {-# SOURCE #-} DepB
+
+data A = A B

cabal-testsuite/PackageTests/HsBootHack/dep/src/DepB.hs-boot

@@ -0,0 +1,3 @@
+module DepB where
+
+data B

cabal-testsuite/PackageTests/HsBootHack/dep/src/DepB.hsc

@@ -0,0 +1,5 @@
+module DepB where
+
+import DepA
+
+data B = B A

cabal-testsuite/PackageTests/HsBootHack/dep/src/DepC.hs

@@ -0,0 +1,5 @@
+module DepC where
+
+import DepB
+
+data C = C B

cabal-testsuite/PackageTests/HsBootHack/setup.test.hs

@@ -0,0 +1,6 @@
+import Test.Cabal.Prelude
+
+-- Test the hs-boot hack respects working directory
+main = setupTest . recordMode DoNotRecord $ withDirectory "dep" $ do
+ void $ setup' "configure" []
+ void $ setup' "build" []

changelog.d/pr-10991.md

@@ -0,0 +1,15 @@
+---
+synopsis: "Take --working-dir into account in runPreProcessorWithHsBootHack"
+packages: [Cabal]
+prs: 10991
+issues: [11000]
+---
+
+The preprocessor hs-boot hack handles the situation in which a file to be
+preprocessed is supplied alongside an hs-boot file, e.g. Foo.x and Foo.hs-boot
+are given together.
+
+This code now respects the --working-dir Cabal global argument. This fixes
+build failures of the form:
+
+  attempting to use module `Foo` (Foo.hs-boot) which is not loaded