diff --git a/cabal-install-solver/src/Distribution/Solver/Types/ProjectConfigPath.hs b/cabal-install-solver/src/Distribution/Solver/Types/ProjectConfigPath.hs
index 84375b0f4de..65fcfe0691e 100644
--- a/cabal-install-solver/src/Distribution/Solver/Types/ProjectConfigPath.hs
+++ b/cabal-install-solver/src/Distribution/Solver/Types/ProjectConfigPath.hs
@@ -30,12 +30,16 @@ import Data.Coerce (coerce)
 import Data.List.NonEmpty ((<|))
 import Network.URI (parseURI, parseAbsoluteURI)
 import System.Directory
-import System.FilePath
+import System.FilePath hiding (splitPath)
+import qualified System.FilePath as FP (splitPath)
+import qualified System.FilePath.Posix as Posix
+import qualified System.FilePath.Windows as Windows
 import qualified Data.List.NonEmpty as NE
 import Distribution.Solver.Modular.Version (VR)
 import Distribution.Pretty (prettyShow)
 import Text.PrettyPrint
 import Distribution.Simple.Utils (ordNub)
+import Distribution.System (OS(Windows), buildOS)
 
 -- | Path to a configuration file, either a singleton project root, or a longer
 -- list representing a path to an import.  The path is a non-empty list that we
@@ -58,6 +62,14 @@ newtype ProjectConfigPath = ProjectConfigPath (NonEmpty FilePath)
 -- The project itself, a single element root path, compared to any of the
 -- configuration paths it imports, should always sort first. Comparing one
 -- project root path against another is done lexically.
+--
+-- For comparison purposes, path separators are normalized to the @buildOS@
+-- platform's path separator.
+--
+-- >>> let abFwd = ProjectConfigPath $ "a/b.config" :| []
+-- >>> let abBwd = ProjectConfigPath $ "a\\b.config" :| []
+-- >>> compare abFwd abBwd
+-- EQ
 instance Ord ProjectConfigPath where
     compare pa@(ProjectConfigPath (NE.toList -> as)) pb@(ProjectConfigPath (NE.toList -> bs)) =
         case (as, bs) of
@@ -66,7 +78,7 @@ instance Ord ProjectConfigPath where
             -- this though, do a comparison anyway when both sides have length
             -- 1.  The root path, the project itself, should always be the first
             -- path in a sorted listing.
-            ([a], [b]) -> compare a b
+            ([a], [b]) -> compare (splitPath a) (splitPath b)
             ([_], _) -> LT
             (_, [_]) -> GT
 
@@ -80,6 +92,16 @@ instance Ord ProjectConfigPath where
                 P.<> compare (length aPaths) (length bPaths)
                 P.<> compare aPaths bPaths
         where
+            splitPath = FP.splitPath . normSep where
+                normSep p =
+                    if buildOS == Windows
+                        then
+                            Windows.joinPath $ Windows.splitDirectories
+                            [if Posix.isPathSeparator c then Windows.pathSeparator else c| c <- p]
+                        else
+                            Posix.joinPath $ Posix.splitDirectories
+                            [if Windows.isPathSeparator c then Posix.pathSeparator else c| c <- p]
+
             aPaths = splitPath <$> as
             bPaths = splitPath <$> bs
             aImporters = snd $ unconsProjectConfigPath pa
diff --git a/cabal-testsuite/PackageTests/BuildDeps/InternalLibrary2/setup.test.hs b/cabal-testsuite/PackageTests/BuildDeps/InternalLibrary2/setup.test.hs
index e36e33823d2..6b6da17f116 100644
--- a/cabal-testsuite/PackageTests/BuildDeps/InternalLibrary2/setup.test.hs
+++ b/cabal-testsuite/PackageTests/BuildDeps/InternalLibrary2/setup.test.hs
@@ -6,4 +6,4 @@ main = setupAndCabalTest . withPackageDb $ do
     assertEqual
         ("executable should have linked with the internal library")
         ("foo foo myLibFunc internal")
-        (concatOutput (resultOutput r))
+        (lineBreaksToSpaces $ resultOutput r)
diff --git a/cabal-testsuite/PackageTests/BuildDeps/InternalLibrary3/setup.test.hs b/cabal-testsuite/PackageTests/BuildDeps/InternalLibrary3/setup.test.hs
index 549e8bf8bb4..ac05c394383 100644
--- a/cabal-testsuite/PackageTests/BuildDeps/InternalLibrary3/setup.test.hs
+++ b/cabal-testsuite/PackageTests/BuildDeps/InternalLibrary3/setup.test.hs
@@ -8,4 +8,4 @@ main = setupAndCabalTest . withPackageDb $ do
     assertEqual
         ("executable should have linked with the internal library")
         ("foo foo myLibFunc internal")
-        (concatOutput (resultOutput r))
+        (lineBreaksToSpaces $ resultOutput r)
diff --git a/cabal-testsuite/PackageTests/CheckSetup/setup.test.hs b/cabal-testsuite/PackageTests/CheckSetup/setup.test.hs
index 96ed4395785..67997e7e21b 100644
--- a/cabal-testsuite/PackageTests/CheckSetup/setup.test.hs
+++ b/cabal-testsuite/PackageTests/CheckSetup/setup.test.hs
@@ -13,8 +13,12 @@ main = cabalTest $ do
               "The dependency 'setup-depends: 'base' does not specify "
               ++ "an upper bound on the version number"
 
+        -- Replace line breaks with spaces in the haystack so that we can search
+        -- for a string that wraps lines.
+        let lineBreakBlind = needleHaystack{txHaystack = txContainsId{txFwd = lineBreaksToSpaces}}
+
         -- Asserts for the desired check messages after configure.
-        assertOutputContains libError1 checkResult
-        assertOutputContains libError2 checkResult
+        assertOn lineBreakBlind libError1 checkResult
+        assertOn lineBreakBlind libError2 checkResult
 
         return ()
diff --git a/cabal-testsuite/PackageTests/ConditionalAndImport/cabal.test.hs b/cabal-testsuite/PackageTests/ConditionalAndImport/cabal.test.hs
index 67118d362c0..d0abb33de2d 100644
--- a/cabal-testsuite/PackageTests/ConditionalAndImport/cabal.test.hs
+++ b/cabal-testsuite/PackageTests/ConditionalAndImport/cabal.test.hs
@@ -1,7 +1,6 @@
 import Test.Cabal.Prelude
-
-normalizeWindowsOutput :: String -> String
-normalizeWindowsOutput = if isWindows then map (\x -> case x of '/' -> '\\'; _ -> x) else id
+import Test.Cabal.OutputNormalizer
+import Data.Function ((&))
 
 main = cabalTest . withRepo "repo" . recordMode RecordMarked $ do
   let log = recordHeader . pure
@@ -111,89 +110,65 @@ main = cabalTest . withRepo "repo" . recordMode RecordMarked $ do
   --          +-- hops/hops-9.config (no further imports so not cyclical)
   log "checking that imports work skipping into a subfolder and then back out again and again"
   hopping <- cabal' "v2-build" [ "--project-file=hops-0.project" ]
-  assertOutputContains "Configuration is affected by the following files" hopping
-  assertOutputContains "- hops-0.project" hopping
-
-  assertOutputContains
-    (normalizeWindowsOutput "- hops-2.config \
-    \    imported by: hops/hops-1.config \
-    \    imported by: hops-0.project")
-    hopping
-
-  assertOutputContains
-    (normalizeWindowsOutput "- hops-4.config \
-    \    imported by: hops/hops-3.config \
-    \    imported by: hops-2.config \
-    \    imported by: hops/hops-1.config \
-    \    imported by: hops-0.project")
-    hopping
-
-  assertOutputContains
-    (normalizeWindowsOutput "- hops-6.config \
-    \    imported by: hops/hops-5.config \
-    \    imported by: hops-4.config \
-    \    imported by: hops/hops-3.config \
-    \    imported by: hops-2.config \
-    \    imported by: hops/hops-1.config \
-    \    imported by: hops-0.project")
-    hopping
-
-  assertOutputContains
-    (normalizeWindowsOutput "- hops-8.config \
-    \    imported by: hops/hops-7.config \
-    \    imported by: hops-6.config \
-    \    imported by: hops/hops-5.config \
-    \    imported by: hops-4.config \
-    \    imported by: hops/hops-3.config \
-    \    imported by: hops-2.config \
-    \    imported by: hops/hops-1.config \
-    \    imported by: hops-0.project")
-    hopping
 
-  assertOutputContains
-    (normalizeWindowsOutput "- hops/hops-1.config \
-    \    imported by: hops-0.project")
-    hopping
-
-  assertOutputContains
-    (normalizeWindowsOutput "- hops/hops-3.config \
-    \    imported by: hops-2.config \
-    \    imported by: hops/hops-1.config \
-    \    imported by: hops-0.project")
-    hopping
-
-  assertOutputContains
-    (normalizeWindowsOutput "- hops/hops-5.config \
-    \    imported by: hops-4.config \
-    \    imported by: hops/hops-3.config \
-    \    imported by: hops-2.config \
-    \    imported by: hops/hops-1.config \
-    \    imported by: hops-0.project")
-    hopping
-
-  assertOutputContains
-    (normalizeWindowsOutput "- hops/hops-7.config \
-    \    imported by: hops-6.config \
-    \    imported by: hops/hops-5.config \
-    \    imported by: hops-4.config \
-    \    imported by: hops/hops-3.config \
-    \    imported by: hops-2.config \
-    \    imported by: hops/hops-1.config \
-    \    imported by: hops-0.project")
-    hopping
-
-  assertOutputContains
-    (normalizeWindowsOutput "- hops/hops-9.config \
-    \    imported by: hops-8.config \
-    \    imported by: hops/hops-7.config \
-    \    imported by: hops-6.config \
-    \    imported by: hops/hops-5.config \
-    \    imported by: hops-4.config \
-    \    imported by: hops/hops-3.config \
-    \    imported by: hops-2.config \
-    \    imported by: hops/hops-1.config \
-    \    imported by: hops-0.project")
-    hopping
+  "Configuration is affected by the following files:\n\
+  \- hops-0.project\n\
+  \- hops-2.config\n\
+  \    imported by: hops/hops-1.config\n\
+  \    imported by: hops-0.project\n\
+  \- hops-4.config\n\
+  \    imported by: hops/hops-3.config\n\
+  \    imported by: hops-2.config\n\
+  \    imported by: hops/hops-1.config\n\
+  \    imported by: hops-0.project\n\
+  \- hops-6.config\n\
+  \    imported by: hops/hops-5.config\n\
+  \    imported by: hops-4.config\n\
+  \    imported by: hops/hops-3.config\n\
+  \    imported by: hops-2.config\n\
+  \    imported by: hops/hops-1.config\n\
+  \    imported by: hops-0.project\n\
+  \- hops-8.config\n\
+  \    imported by: hops/hops-7.config\n\
+  \    imported by: hops-6.config\n\
+  \    imported by: hops/hops-5.config\n\
+  \    imported by: hops-4.config\n\
+  \    imported by: hops/hops-3.config\n\
+  \    imported by: hops-2.config\n\
+  \    imported by: hops/hops-1.config\n\
+  \    imported by: hops-0.project\n\
+  \- hops/hops-1.config\n\
+  \    imported by: hops-0.project\n\
+  \- hops/hops-3.config\n\
+  \    imported by: hops-2.config\n\
+  \    imported by: hops/hops-1.config\n\
+  \    imported by: hops-0.project\n\
+  \- hops/hops-5.config\n\
+  \    imported by: hops-4.config\n\
+  \    imported by: hops/hops-3.config\n\
+  \    imported by: hops-2.config\n\
+  \    imported by: hops/hops-1.config\n\
+  \    imported by: hops-0.project\n\
+  \- hops/hops-7.config\n\
+  \    imported by: hops-6.config\n\
+  \    imported by: hops/hops-5.config\n\
+  \    imported by: hops-4.config\n\
+  \    imported by: hops/hops-3.config\n\
+  \    imported by: hops-2.config\n\
+  \    imported by: hops/hops-1.config\n\
+  \    imported by: hops-0.project\n\
+  \- hops/hops-9.config\n\
+  \    imported by: hops-8.config\n\
+  \    imported by: hops/hops-7.config\n\
+  \    imported by: hops-6.config\n\
+  \    imported by: hops/hops-5.config\n\
+  \    imported by: hops-4.config\n\
+  \    imported by: hops/hops-3.config\n\
+  \    imported by: hops-2.config\n\
+  \    imported by: hops/hops-1.config\n\
+  \    imported by: hops-0.project"
+    & normalizeWindowsOutput
+    & flip (assertOn multilineNeedleHaystack) hopping
 
   -- The project is named oops as it is like hops but has conflicting constraints.
   -- +-- oops-0.project
@@ -208,22 +183,25 @@ main = cabalTest . withRepo "repo" . recordMode RecordMarked $ do
   --          +-- oops/oops-9.config (has conflicting constraints)
   log "checking conflicting constraints skipping into a subfolder and then back out again and again"
   oopsing <- fails $ cabal' "v2-build" [ "all", "--project-file=oops-0.project" ]
-  assertOutputContains "rejecting: hashable-1.4.2.0" oopsing
-  assertOutputContains "rejecting: hashable-1.4.3.0" oopsing
-  assertOutputContains "(constraint from oops-0.project requires ==1.4.3.0)" oopsing
 
-  assertOutputContains
-    (normalizeWindowsOutput "      (constraint from oops/oops-9.config requires ==1.4.2.0) \
-    \        imported by: oops-8.config \
-    \        imported by: oops/oops-7.config \
-    \        imported by: oops-6.config \
-    \        imported by: oops/oops-5.config \
-    \        imported by: oops-4.config \
-    \        imported by: oops/oops-3.config \
-    \        imported by: oops-2.config \
-    \        imported by: oops/oops-1.config \
-    \        imported by: oops-0.project")
-    oopsing
+  "Could not resolve dependencies:\n\
+  \[__0] trying: oops-0.1 (user goal)\n\
+  \[__1] next goal: hashable (dependency of oops)\n\
+  \[__1] rejecting: hashable-1.4.3.0\n\
+  \      (constraint from oops/oops-9.config requires ==1.4.2.0)\n\
+  \        imported by: oops-8.config\n\
+  \        imported by: oops/oops-7.config\n\
+  \        imported by: oops-6.config\n\
+  \        imported by: oops/oops-5.config\n\
+  \        imported by: oops-4.config\n\
+  \        imported by: oops/oops-3.config\n\
+  \        imported by: oops-2.config\n\
+  \        imported by: oops/oops-1.config\n\
+  \        imported by: oops-0.project\n\
+  \[__1] rejecting: hashable-1.4.2.0\n\
+  \      (constraint from oops-0.project requires ==1.4.3.0)"
+    & normalizeWindowsOutput
+    & flip (assertOn multilineNeedleHaystack) oopsing
 
   -- The project is named yops as it is like hops but with y's for forks.
   -- +-- yops-0.project
@@ -264,13 +242,14 @@ main = cabalTest . withRepo "repo" . recordMode RecordMarked $ do
 
   log "checking that missing package message lists configuration provenance"
   missing <- fails $ cabal' "v2-build" [ "--project-file=cabal-missing-package.project" ]
-  assertOutputContains
-    (normalizeWindowsOutput "When using configuration from: \
-    \  - cabal-missing-package.project \
-    \  - missing/pkgs.config \
-    \  - missing/pkgs/default.config \
-    \The following errors occurred: \
-    \  - The package location 'pkg-doesnt-exist' does not exist.")
-    missing
+
+  "When using configuration from:\n\
+  \  - cabal-missing-package.project\n\
+  \  - missing/pkgs.config\n\
+  \  - missing/pkgs/default.config\n\
+  \The following errors occurred:\n\
+  \  - The package location 'pkg-doesnt-exist' does not exist."
+    & normalizeWindowsOutput
+    & flip (assertOn multilineNeedleHaystack) missing
 
   return ()
diff --git a/cabal-testsuite/PackageTests/NewBuild/T4288/cabal.test.hs b/cabal-testsuite/PackageTests/NewBuild/T4288/cabal.test.hs
index 3e3b8de853e..3313f596546 100644
--- a/cabal-testsuite/PackageTests/NewBuild/T4288/cabal.test.hs
+++ b/cabal-testsuite/PackageTests/NewBuild/T4288/cabal.test.hs
@@ -1,4 +1,5 @@
 import Test.Cabal.Prelude
+import Data.Function ((&))
 
 -- This test is similar to the simplified example in issue #4288. The package's
 -- setup script only depends on base and setup-helper. setup-helper exposes a
@@ -10,8 +11,7 @@ main = cabalTest $ do
   skipUnless "no v2-build compatible boot-Cabal" =<< hasNewBuildCompatBootCabal
   r <- recordMode DoNotRecord $ cabal' "v2-build" ["T4288"]
   assertOutputContains "This is setup-helper-1.0." r
-  assertOutputContains
-      ("In order, the following will be built: "
-       ++ " - setup-helper-1.0 (lib:setup-helper) (first run) "
-       ++ " - T4288-1.0 (lib:T4288) (first run)")
-      r
+  "In order, the following will be built:\n\
+  \ - setup-helper-1.0 (lib:setup-helper) (first run)\n\
+  \ - T4288-1.0 (lib:T4288) (first run)"
+    & flip (assertOn multilineNeedleHaystack) r
diff --git a/cabal-testsuite/PackageTests/ProjectImport/DedupUsingConfigFromComplex/cabal.test.hs b/cabal-testsuite/PackageTests/ProjectImport/DedupUsingConfigFromComplex/cabal.test.hs
index e354b356d7f..0c65ff68c60 100644
--- a/cabal-testsuite/PackageTests/ProjectImport/DedupUsingConfigFromComplex/cabal.test.hs
+++ b/cabal-testsuite/PackageTests/ProjectImport/DedupUsingConfigFromComplex/cabal.test.hs
@@ -1,4 +1,5 @@
 import Test.Cabal.Prelude
+import Data.Function ((&))
 
 main = cabalTest . recordMode RecordMarked $ do
   let log = recordHeader . pure
@@ -31,12 +32,12 @@ main = cabalTest . recordMode RecordMarked $ do
     out
 
   log "checking that package directories and locations are reported in order"
-  assertOutputContains
-    "The following errors occurred: \
-    \  - The package directory 'no-pkg-1' does not contain any .cabal file. \
-    \  - The package location 'no-pkg-2-dir' does not exist. \
-    \  - The package directory 'no-pkg-3' does not contain any .cabal file. \
-    \  - The package location 'no-pkg-4-dir' does not exist."
-    out
+
+  "The following errors occurred:\n\
+  \  - The package directory 'no-pkg-1' does not contain any .cabal file.\n\
+  \  - The package location 'no-pkg-2-dir' does not exist.\n\
+  \  - The package directory 'no-pkg-3' does not contain any .cabal file.\n\
+  \  - The package location 'no-pkg-4-dir' does not exist."
+    & flip (assertOn multilineNeedleHaystack) out
 
   return ()
diff --git a/cabal-testsuite/PackageTests/ProjectImport/ParseErrorProvenance/cabal.out b/cabal-testsuite/PackageTests/ProjectImport/ParseErrorProvenance/cabal.out
new file mode 100644
index 00000000000..7a5d5d5c6f0
--- /dev/null
+++ b/cabal-testsuite/PackageTests/ProjectImport/ParseErrorProvenance/cabal.out
@@ -0,0 +1,10 @@
+# cabal v2-build
+Warning: <ROOT>/else.project, else.project: Unrecognized section '_' on line 3
+# Multiline string marking:
+# ^When using configuration from:$
+# ^  - else.project$
+# ^  - dir-else/else.config$
+# ^The following errors occurred:$
+# ^  - The package location 'no-pkg-here' does not exist.$
+# Pseudo multiline string marking:
+# ^When using configuration from:  - else.project  - dir-else/else.configThe following errors occurred:  - The package location 'no-pkg-here' does not exist.$
diff --git a/cabal-testsuite/PackageTests/ProjectImport/ParseErrorProvenance/cabal.test.hs b/cabal-testsuite/PackageTests/ProjectImport/ParseErrorProvenance/cabal.test.hs
new file mode 100644
index 00000000000..f4b48678158
--- /dev/null
+++ b/cabal-testsuite/PackageTests/ProjectImport/ParseErrorProvenance/cabal.test.hs
@@ -0,0 +1,29 @@
+import Test.Cabal.Prelude
+
+main = cabalTest . recordMode RecordMarked $ do
+  let log = recordHeader . pure
+
+  let expectMulti =
+        "When using configuration from:\n\
+        \  - else.project\n\
+        \  - dir-else/else.config\n\
+        \The following errors occurred:\n\
+        \  - The package location 'no-pkg-here' does not exist."
+
+  outElse <- fails $ cabal' "v2-build" [ "all", "--dry-run", "--project-file=else.project" ]
+
+  let expectSingle = filter (/= '\n') expectMulti
+
+  log "Multiline string marking:"
+  mapM_ log (lines . decodeLfMarkLines $ encodeLf expectMulti)
+
+  log "Pseudo multiline string marking:"
+  mapM_ log (lines . decodeLfMarkLines $ encodeLf expectSingle)
+
+  assertOn multilineNeedleHaystack expectMulti outElse
+  assertOn multilineNeedleHaystack{expectNeedleInHaystack = False} expectSingle outElse
+
+  assertOutputDoesNotContain expectMulti outElse
+  assertOutputDoesNotContain expectSingle outElse
+
+  return ()
diff --git a/cabal-testsuite/PackageTests/ProjectImport/ParseErrorProvenance/dir-else/else.config b/cabal-testsuite/PackageTests/ProjectImport/ParseErrorProvenance/dir-else/else.config
new file mode 100644
index 00000000000..f9c44e63d5b
--- /dev/null
+++ b/cabal-testsuite/PackageTests/ProjectImport/ParseErrorProvenance/dir-else/else.config
@@ -0,0 +1,4 @@
+if false
+else
+    _
+    packages: no-pkg-here
diff --git a/cabal-testsuite/PackageTests/ProjectImport/ParseErrorProvenance/else.project b/cabal-testsuite/PackageTests/ProjectImport/ParseErrorProvenance/else.project
new file mode 100644
index 00000000000..959c40f5660
--- /dev/null
+++ b/cabal-testsuite/PackageTests/ProjectImport/ParseErrorProvenance/else.project
@@ -0,0 +1 @@
+import: dir-else/else.config
diff --git a/cabal-testsuite/PackageTests/Regression/T5409/use-different-versions-of-dependency-for-library-and-build-tool.test.hs b/cabal-testsuite/PackageTests/Regression/T5409/use-different-versions-of-dependency-for-library-and-build-tool.test.hs
index 2a3eb3c093c..4bbb8b91a9b 100644
--- a/cabal-testsuite/PackageTests/Regression/T5409/use-different-versions-of-dependency-for-library-and-build-tool.test.hs
+++ b/cabal-testsuite/PackageTests/Regression/T5409/use-different-versions-of-dependency-for-library-and-build-tool.test.hs
@@ -1,4 +1,5 @@
 import Test.Cabal.Prelude
+import Data.Function ((&))
 
 -- The local package, pkg-1.0, depends on build-tool-pkg-1 as a library and
 -- build-tool-pkg-2 as a build-tool.  This test checks that cabal uses the
@@ -16,16 +17,13 @@ main = cabalTest $ withShorterPathForNewBuildStore $ do
       r1 <- recordMode DoNotRecord $
             cabal' "v2-build" ["pkg:my-exe"]
 
-      let msg = concat
-              [ "In order, the following will be built:"
-              , "  - build-tool-pkg-1 (lib) (requires build)"
-              , "  - build-tool-pkg-2 (lib) (requires build)"
-              , "  - build-tool-pkg-2 (exe:build-tool-exe) (requires build)"
-              , "  - pkg-1.0 (exe:my-exe) (first run)"
-              ]
+      "In order, the following will be built:\n\
+      \ - build-tool-pkg-1 (lib) (requires build)\n\
+      \ - build-tool-pkg-2 (lib) (requires build)\n\
+      \ - build-tool-pkg-2 (exe:build-tool-exe) (requires build)\n\
+      \ - pkg-1.0 (exe:my-exe) (first run)"
+        & flip (assertOn multilineNeedleHaystack) r1
 
-      assertOutputContains msg r1
       withPlan $ do
         r2 <- runPlanExe' "pkg" "my-exe" []
-        assertOutputContains
-            "build-tool library version: 1, build-tool exe version: 2" r2
+        assertOn multilineNeedleHaystack "build-tool library version: 1,\nbuild-tool exe version: 2" r2
diff --git a/cabal-testsuite/src/Test/Cabal/Prelude.hs b/cabal-testsuite/src/Test/Cabal/Prelude.hs
index d8cee954d83..656bd41e95b 100644
--- a/cabal-testsuite/src/Test/Cabal/Prelude.hs
+++ b/cabal-testsuite/src/Test/Cabal/Prelude.hs
@@ -1,8 +1,10 @@
-{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE CPP #-}
+{-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE NamedFieldPuns #-}
 {-# LANGUAGE NondecreasingIndentation #-}
-{-# LANGUAGE FlexibleContexts #-}
-{-# LANGUAGE CPP #-}
+{-# LANGUAGE RecordWildCards #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE ViewPatterns #-}
 
 -- | Generally useful definitions that we expect most test scripts
 -- to use.
@@ -795,19 +797,68 @@ recordMode mode = withReaderT (\env -> env {
     testRecordUserMode = Just mode
     })
 
-assertOutputContains :: MonadIO m => WithCallStack (String -> Result -> m ())
-assertOutputContains needle result =
+-- | Transformations for the search strings and the text to search in.
+data TxContains =
+    TxContains
+        {
+            -- | Reverse conversion for display, applied to the forward converted value.
+            txBwd :: (String -> String),
+            -- | Forward conversion for comparison.
+            txFwd :: (String -> String)
+        }
+
+txContainsId :: TxContains
+txContainsId = TxContains id id
+
+-- | Conversions of the needle and haystack strings, the seach string and the
+-- text to search in.
+data NeedleHaystack =
+    NeedleHaystack
+        {
+            expectNeedleInHaystack :: Bool,
+            displayHaystack :: Bool,
+            txNeedle :: TxContains,
+            txHaystack :: TxContains
+        }
+
+-- | Symmetric needle and haystack functions, the same conversion for each going
+-- forward and the same coversion for each going backward.
+symNeedleHaystack :: (String -> String) -> (String -> String) -> NeedleHaystack
+symNeedleHaystack bwd fwd = let tx = TxContains bwd fwd in NeedleHaystack True False tx tx
+
+multilineNeedleHaystack :: NeedleHaystack
+multilineNeedleHaystack = symNeedleHaystack decodeLfMarkLines encodeLf
+
+-- | Needle and haystack functions that do not change the strings. Set up for
+-- finding the needle in the haystack and not displaying the line-delimited
+-- haystack.
+needleHaystack :: NeedleHaystack
+needleHaystack = NeedleHaystack True False txContainsId txContainsId
+
+assertOn :: MonadIO m => WithCallStack (NeedleHaystack -> String -> Result -> m ())
+assertOn NeedleHaystack{..} (txFwd txNeedle -> needle) (txFwd txHaystack. resultOutput -> output) =
     withFrozenCallStack $
-    unless (needle `isInfixOf` (concatOutput output)) $
-    assertFailure $ " expected: " ++ needle
-  where output = resultOutput result
+    if expectNeedleInHaystack
+        then unless (needle `isInfixOf` output)
+            $ assertFailure $ "expected:\n" ++ (txBwd txNeedle needle) ++
+            if displayHaystack
+                then "\nin output:\n" ++ (txBwd txHaystack output)
+                else ""
+        else when (needle `isInfixOf` output)
+            $ assertFailure $ "unexpected:\n" ++ (txBwd txNeedle needle) ++
+            if displayHaystack
+                then "\nin output:\n" ++ (txBwd txHaystack output)
+                else ""
+
+assertOutputContains :: MonadIO m => WithCallStack (String -> Result -> m ())
+assertOutputContains = assertOn needleHaystack{txHaystack = TxContains{txBwd = decodeLfMarkLines, txFwd = encodeLf}}
 
 assertOutputDoesNotContain :: MonadIO m => WithCallStack (String -> Result -> m ())
-assertOutputDoesNotContain needle result =
-    withFrozenCallStack $
-    when (needle `isInfixOf` (concatOutput output)) $
-    assertFailure $ "unexpected: " ++ needle
-  where output = resultOutput result
+assertOutputDoesNotContain = assertOn
+    needleHaystack
+        { expectNeedleInHaystack = False
+        , txHaystack = TxContains{txBwd = decodeLfMarkLines, txFwd = encodeLf}
+        }
 
 assertFindInFile :: MonadIO m => WithCallStack (String -> FilePath -> m ())
 assertFindInFile needle path =
@@ -862,8 +913,64 @@ assertNoFileContains paths needle =
           assertFileDoesNotContain path needle
 
 -- | Replace line breaks with spaces, correctly handling "\r\n".
-concatOutput :: String -> String
-concatOutput = unwords . lines . filter ((/=) '\r')
+--
+-- >>> lineBreaksToSpaces "foo\nbar\r\nbaz"
+-- "foo bar baz"
+--
+-- >>> lineBreaksToSpaces "foo\nbar\r\nbaz\n"
+-- "foo bar baz"
+--
+-- >>> lineBreaksToSpaces "\nfoo\nbar\r\nbaz\n"
+-- " foo bar baz"
+lineBreaksToSpaces :: String -> String
+lineBreaksToSpaces = unwords . lines . filter ((/=) '\r')
+
+-- | Replace line breaks with <EOL>, correctly handling "\r\n".
+--
+-- >>> encodeLf "foo\nbar\r\nbaz"
+-- "foo<EOL>bar<EOL>baz"
+--
+-- >>> encodeLf "foo\nbar\r\nbaz\n"
+-- "foo<EOL>bar<EOL>baz"
+--
+-- >>> encodeLf "\nfoo\nbar\r\nbaz\n"
+-- "<EOL>foo<EOL>bar<EOL>baz"
+encodeLf :: String -> String
+encodeLf =
+    (\s -> if "<EOL>" `isPrefixOf` s then drop 5 s else s)
+    . concat
+    . (fmap ("<EOL>" ++))
+    . lines
+    . filter ((/=) '\r')
+
+-- | Replace <LF> markers with line breaks and wrap lines with ^ and $ markers
+-- for the start and end.
+--
+-- >>> decodeLfMarkLines "foo<EOL>bar<EOL>baz"
+-- "^foo$\n^bar$\n^baz$\n"
+--
+-- >>> decodeLfMarkLines "<EOL>foo<EOL>bar<EOL>baz"
+-- "^foo$\n^bar$\n^baz$\n"
+decodeLfMarkLines:: String -> String
+decodeLfMarkLines output =
+    (\xs -> case reverse $ lines xs of
+        [] -> xs
+        [line0] -> line0 ++ "$"
+        lineN : ys ->
+            let lineN' = lineN ++ "$"
+            in unlines $ reverse (lineN' : ys))
+    . unlines
+    . (fmap ('^' :))
+    . lines
+    . (\s -> if "<EOL>" `isPrefixOf` s then drop 5 s else s)
+    $ foldr
+            (\c acc -> c :
+                if ("<EOL>" `isPrefixOf` acc)
+                    then "$\n" ++ drop 5 acc
+                    else acc
+            )
+            ""
+    output
 
 -- | The directory where script build artifacts are expected to be cached
 getScriptCacheDirectory :: FilePath -> TestM FilePath
@@ -1093,6 +1200,9 @@ flakyIfCI ticket m = do
 flakyIfWindows :: IssueID -> TestM a -> TestM a
 flakyIfWindows ticket m = flakyIf isWindows ticket m
 
+normalizeWindowsOutput :: String -> String
+normalizeWindowsOutput = if isWindows then map (\x -> case x of '/' -> '\\'; _ -> x) else id
+
 getOpenFilesLimit :: TestM (Maybe Integer)
 #ifdef mingw32_HOST_OS
 -- No MS-specified limit, was determined experimentally on Windows 10 Pro x64,
diff --git a/changelog.d/pr-10646 b/changelog.d/pr-10646
new file mode 100644
index 00000000000..ab1366afaa9
--- /dev/null
+++ b/changelog.d/pr-10646
@@ -0,0 +1,34 @@
+---
+synopsis: Deduplicate path separator duplicates
+packages: [cabal-install-solver]
+prs: 10646
+issues: 10645
+---
+
+The "using configuration from" message no longer has duplicates on Windows when
+the `cabal.project` uses forward slashes for its imports but the message reports
+imports with backslashes.
+
+```diff
+$ cat cabal.project
+import: dir-a/b.config
+
+$ cabal build all --dry-run
+...
+When using configuration from:
+-   - dir-a/b.config
+    - dir-a\b.config
+    - cabal.project
+```
+
+## Ord ProjectConfigPath Instance Changes
+
+For comparison purposes, path separators are normalized to the @buildOS@
+platform's path separator.
+
+```haskell
+-- >>> let abFwd = ProjectConfigPath $ "a/b.config" :| []
+-- >>> let abBwd = ProjectConfigPath $ "a\\b.config" :| []
+-- >>> compare abFwd abBwd
+-- EQ
+```