Formatting source with Unicode characters causes misaligned indexes

[osyvokon]

When the source code contains Unicode characters, the indexes static-ls returns for textDocument/formatting are incorrect. This leads to code corruption in the editor.

This issue occurs when using either fourmolu or ormolu as the formatters. It doesn't happen with hls.

Failing example

$ static-ls --fourmoluCommand=fourmolu

File before formatting:

main :: IO ()
main = do
    let x = read @Int "42" -- привіт
    let y =    read @Double "3.14"

File after formatting:

main :: IO ()
main = do
    let x = read @Int "42" -- привіт
    le =    read @Double "3.14"
----- ^^^^^^

LSP Logs (note the incorrect start/end characters positions):

// Send:
{"jsonrpc":"2.0","id":77,"method":"textDocument/formatting","params":{"textDocument":{"uri":"file:///home/silver/develop/playground/haskell-test/app/Main.hs"},"options":{"tabSize":4,"insertSpaces":true,"trimTrailingWhitespace":true,"insertFinalNewline":true,"trimFinalNewlines":true}}}

// Receive:
{"id":77,"jsonrpc":"2.0","result":[{"newText":"","range":{"end":{"character":9,"line":3},"start":{"character":6,"line":3}}}]}

Expected

If you replace the Unicode string with an ASCII string of the same length, the problem disappears:

Before:

main :: IO ()
main = do
    let x = read @Int "42" -- Hello!
    let y =    read @Double "3.14"

After:

main :: IO ()
main = do
    let x = read @Int "42" -- Hello!
    let y = read @Double "3.14"

LSP logs (start/end characters are now correct)

// Send:
{"jsonrpc":"2.0","id":102,"method":"textDocument/formatting","params":{"textDocument":{"uri":"file:///home/silver/develop/playground/haskell-test/app/Main.hs"},"options":{"tabSize":4,"insertSpaces":true,"trimTrailingWhitespace":true,"insertFinalNewline":true,"trimFinalNewlines":true}}}

// Receive:
{"id":102,"jsonrpc":"2.0","result":[{"newText":"","range":{"end":{"character":15,"line":3},"start":{"character":12,"line":3}}}]}

I came across this while investigating zed-industries/zed#35048

[josephsumabat]

Thank you I’ll look at fixing this!