In 00f4e61 we attempted to share the filepaths in GHC's usage field.

Unfortunately the `mi_usages` is evaluated by `checkOldIface` before we have a chance to do the sharing,
resulting in them not actually being shared on the first load.

Solution is to share the usages before `checkOldIface` has a chance to inspect them, breaking the sharing.

ghcide/src/Development/IDE/Core/Compile.hs

@@ -36,6 +36,7 @@ module Development.IDE.Core.Compile
  , TypecheckHelpers(..)
  , sourceTypecheck
  , sourceParser
+ , shareUsages
  ) where
 
 import Control.Monad.IO.Class
@@ -468,6 +469,8 @@ filterUsages = id
 #endif
 
 -- | Mitigation for https://gitlab.haskell.org/ghc/ghc/-/issues/22744
+-- Important to do this immediately after reading the unit before
+-- anything else has a chance to read `mi_usages`
 shareUsages :: ModIface -> ModIface
 shareUsages iface = iface {mi_usages = usages}
  where usages = map go (mi_usages iface)
@@ -1479,11 +1482,23 @@ loadInterface session ms linkableNeeded RecompilationInfo{..} = do
  | source_version <= dest_version -> SourceUnmodified
  | otherwise -> SourceModified
 
+ old_iface <- case mb_old_iface of
+ Just iface -> pure (Just iface)
+ Nothing -> do
+ let ncu = hsc_NC sessionWithMsDynFlags
+ dflags = hsc_dflags sessionWithMsDynFlags
+ read_result <- readIface read_dflags ncu mod iface_file
+ case read_result of
+ Failed _err -> return Nothing
+ -- important to call `shareUsages` here before checkOldIface
+ -- consults `mi_usages`
+ Just iface -> return $ Just (shareUsages iface)
+
  -- If mb_old_iface is nothing then checkOldIface will load it for us
  -- given that the source is unmodified
  (recomp_iface_reqd, mb_checked_iface)
 #if MIN_VERSION_ghc(9,3,0)
- <- liftIO $ checkOldIface sessionWithMsDynFlags ms mb_old_iface >>= \case
+ <- liftIO $ checkOldIface sessionWithMsDynFlags ms old_iface >>= \case
  UpToDateItem x -> pure (UpToDate, Just x)
  OutOfDateItem reason x -> pure (NeedsRecompile reason, x)
 #else
@@ -1498,7 +1513,6 @@ loadInterface session ms linkableNeeded RecompilationInfo{..} = do
 
  case (mb_checked_iface, recomp_iface_reqd) of
  (Just iface', UpToDate) -> do
- let iface = shareUsages iface'
  details <- liftIO $ mkDetailsFromIface sessionWithMsDynFlags iface
  -- parse the runtime dependencies from the annotations
  let runtime_deps