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.

Author: wz1000

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
+            read_dflags = hsc_dflags sessionWithMsDynFlags
+        read_result <- liftIO $ readIface read_dflags ncu mod iface_file
+        case read_result of
+          Util.Failed{} -> return Nothing
+          -- important to call `shareUsages` here before checkOldIface
+          -- consults `mi_usages`
+          Util.Succeeded 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
@@ -1497,8 +1512,7 @@ loadInterface session ms linkableNeeded RecompilationInfo{..} = do
           regenerate linkableNeeded
 
     case (mb_checked_iface, recomp_iface_reqd) of
-      (Just iface', UpToDate) -> do
-             let iface = shareUsages iface'
+      (Just iface, UpToDate) -> do
              details <- liftIO $ mkDetailsFromIface sessionWithMsDynFlags iface
              -- parse the runtime dependencies from the annotations
              let runtime_deps

ghcide/src/Development/IDE/GHC/Compat/Core.hs

@@ -39,6 +39,7 @@ module Development.IDE.GHC.Compat.Core (
     lookupType,
     needWiredInHomeIface,
     loadWiredInHomeIface,
+    readIface,
     loadSysInterface,
     importDecl,
 #if MIN_VERSION_ghc(8,8,0)