Code action to remove redundant imports is sometimes not created

[jhrcek]

Using hls 2.7.0.0 in vscode with ghc 9.4.8, I sometimes don't get "remove redundant imports" code action.

Here's a one-module reproducer (assuming you have a cabal project with postgresql-simple dependency):

module Main where

import Database.PostgreSQL.Simple.Types (In (..), PGArray (..), Values (..), fromIdentifier, fromQuery)

main :: IO ()
main = do
    let array = PGArray []
        in_ = In []
        values = Values [] []
    putStrLn "Hello, Haskell!"

Although warning is generated and displayed on hover:

The import of ‘Identifier(fromIdentifier), Query(fromQuery)’
from module ‘Database.PostgreSQL.Simple.Types’ is redundanttypecheck(-Wunused-imports)

The usual code action to remove unused import is not created.
Not a big deal, but it's breaking my flow, as now I have to focus on what the warning is exactly saying and fix it manually 😄

[jhrcek]

Hint for anyone looking into fixing this

This seems to happen when the unused field is a record field selector (for both data and newtypes). When these are unused, GHC warning actually mentions `RecordConstructor(recordFieldName)` in the diagnostic.

The place where this breaks down is in this function:

haskell-language-server/plugins/hls-refactor-plugin/src/Development/IDE/Plugin/CodeAction.hs

Lines 1942 to 1955 in 23005f8

	rangesForBinding' :: String -> LIE GhcPs -> [SrcSpan]
	rangesForBinding' b (L (locA -> l) (IEVar _ nm))
	| L _ (IEPattern _ (L _ b')) <- nm
	, T.unpack (printOutputable b') == b
	= [l]
	rangesForBinding' b (L (locA -> l) x@IEVar{})
	| T.unpack (printOutputable x) == b = [l]
	rangesForBinding' b (L (locA -> l) x@IEThingAbs{}) | T.unpack (printOutputable x) == b = [l]
	rangesForBinding' b (L (locA -> l) (IEThingAll _ x)) | T.unpack (printOutputable x) == b = [l]
	rangesForBinding' b (L (locA -> l) (IEThingWith _ thing _ inners))
	| T.unpack (printOutputable thing) == b = [l]
	| otherwise =
	[ locA l' | L l' x <- inners, T.unpack (printOutputable x) == b]
	rangesForBinding' _ _ = []

which gets passed b::String like RecordConstructor(recordFieldName), tries to compare it with printOutputable b' which at this point is just recordFieldName, which are not equal and so no code action is generated for these.