From c9519af39acc5d157d1d7f5acc75cad1f02791d1 Mon Sep 17 00:00:00 2001
From: wz1000 <zubin.duggal@gmail.com>
Date: Tue, 25 Jul 2023 20:02:03 +0530
Subject: [PATCH] In 00f4e61da6c07f9a632e58dd57c73a79a1640fa3 we attempted to
 share the filepaths in GHC's usage field. (#3713)

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.

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
---
 ghcide/src/Development/IDE/Core/Compile.hs    | 25 ++++++++++++++++---
 ghcide/src/Development/IDE/GHC/Compat/Core.hs |  1 +
 2 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/ghcide/src/Development/IDE/Core/Compile.hs b/ghcide/src/Development/IDE/Core/Compile.hs
index 801942fc71..e34c5323f9 100644
--- a/ghcide/src/Development/IDE/Core/Compile.hs
+++ b/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
@@ -472,6 +473,8 @@ 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)
@@ -1490,11 +1493,28 @@ 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
+#if MIN_VERSION_ghc(9,3,0)
+        read_result <- liftIO $ readIface read_dflags ncu mod iface_file
+#else
+        read_result <- liftIO $ initIfaceCheck (text "readIface") sessionWithMsDynFlags
+                              $ readIface mod iface_file
+#endif
+        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
@@ -1508,8 +1528,7 @@ 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
diff --git a/ghcide/src/Development/IDE/GHC/Compat/Core.hs b/ghcide/src/Development/IDE/GHC/Compat/Core.hs
index be4d72beb3..3b516c6f40 100644
--- a/ghcide/src/Development/IDE/GHC/Compat/Core.hs
+++ b/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)