For a long time (since 2019) git-annex has been progressively converting from FilePath to RawFilePath (aka ByteString).

The reason is mostly performance, also a simpler representation of filepaths that doesn't need encoding hacks to support non-UTF8 values.

Some commands like git-annex find use RawFilePath end-to-end. But this conversion is not yet complete. This is a todo to keep track of the status.

• The Abstract FilePath proposal (AFPP) has been implemented, and so a number of libraries like unix and directory now have versions that operate on OSPath. That could be used in git-annex eg for things like getDirectoryContents, when built against those versions. (But OSPath uses ShortByteString, while RawFilePath is ByteString, so conversion still entails a copy.)
• withFile remains to be converted, and is used in several important code paths, including Annex.Journal and Annex.Link. There is a RawFilePath version in file-io library, but that is not currently a git-annex dependency. (withFile is in base, and base is unlikely to convert to AFPP soon)

---

The following patch can be useful to find points where conversions are done. Especially useful to identify cases where a value is converted FilePath -> RawFilePath -> FilePath.

diff --git a/Utility/FileSystemEncoding.hs b/Utility/FileSystemEncoding.hs
index 2a1dc81bc1..03e6986f6e 100644
--- a/Utility/FileSystemEncoding.hs
+++ b/Utility/FileSystemEncoding.hs
@@ -84,6 +84,9 @@ encodeBL = L.fromStrict . encodeBS
 encodeBL = L8.fromString
 #endif

+debugConversions :: String -> IO ()
+debugConversions s = hPutStrLn stderr ("conversion: " ++ s)
+
 decodeBS :: S.ByteString -> FilePath
 #ifndef mingw32_HOST_OS
 -- This does the same thing as System.FilePath.ByteString.decodeFilePath,
@@ -92,6 +95,7 @@ decodeBS :: S.ByteString -> FilePath
 -- something other than a unix filepath.
 {-# NOINLINE decodeBS #-}
 decodeBS b = unsafePerformIO $ do
+   debugConversions (show b)
    enc <- Encoding.getFileSystemEncoding
    S.useAsCStringLen b (GHC.peekCStringLen enc)
 #else
@@ -106,17 +110,19 @@ encodeBS :: FilePath -> S.ByteString
 -- something other than a unix filepath.
 {-# NOINLINE encodeBS #-}
 encodeBS f = unsafePerformIO $ do
+   debugConversions f
    enc <- Encoding.getFileSystemEncoding
-   GHC.newCStringLen enc f >>= unsafePackMallocCStringLen
+   b <- GHC.newCStringLen enc f >>= unsafePackMallocCStringLen
+   return b
 #else
 encodeBS = S8.fromString
 #endif

 fromRawFilePath :: RawFilePath -> FilePath
-fromRawFilePath = decodeFilePath
+fromRawFilePath = decodeBS -- decodeFilePath

 toRawFilePath :: FilePath -> RawFilePath
-toRawFilePath = encodeFilePath
+toRawFilePath = encodeBS -- encodeFilePath

 {- Truncates a FilePath to the given number of bytes (or less),
  - as represented on disk.