Support generating HIE files

Adds a `-fenable-ide-info` flag which instructs GHC to generate `.hie`
files (see the wiki page:
https://ghc.haskell.org/trac/ghc/wiki/HIEFiles).

This is a rebased version of Zubin Duggal's (@wz1000) GHC changes for
his GSOC project, as posted here:
https://gist.github.com/wz1000/5ed4ddd0d3e96d6bc75e095cef95363d.

Test Plan: ./validate

Reviewers: bgamari, gershomb, nomeata, alanz, sjakobi

Reviewed By: alanz, sjakobi

Subscribers: alanz, hvr, sjakobi, rwbarton, wz1000, carter

Differential Revision: https://phabricator.haskell.org/D5239

1 files changed, 19 insertions, 1 deletions

diff --git a/compiler/iface/MkIface.hs b/compiler/iface/MkIface.hs
index 7b66472d7a..c23f577bba 100644
--- a/compiler/iface/MkIface.hs
+++ b/compiler/iface/MkIface.hs
@@ -1291,6 +1291,8 @@ checkVersions hsc_env mod_summary iface
        ; if recompileRequired recomp then return (recomp, Nothing) else do {
        ; recomp <- checkHsig mod_summary iface
        ; if recompileRequired recomp then return (recomp, Nothing) else do {
+       ; recomp <- checkHie mod_summary
+       ; if recompileRequired recomp then return (recomp, Nothing) else do {
        ; recomp <- checkDependencies hsc_env mod_summary iface
        ; if recompileRequired recomp then return (recomp, Just iface) else do {
        ; recomp <- checkPlugins hsc_env iface
@@ -1313,7 +1315,7 @@ checkVersions hsc_env mod_summary iface
        ; updateEps_ $ \eps  -> eps { eps_is_boot = mod_deps }
        ; recomp <- checkList [checkModUsage this_pkg u | u <- mi_usages iface]
        ; return (recomp, Just iface)
-    }}}}}}}}}
+    }}}}}}}}}}
   where
     this_pkg = thisPackage (hsc_dflags hsc_env)
     -- This is a bit of a hack really
@@ -1365,6 +1367,22 @@ checkHsig mod_summary iface = do
         True -> up_to_date (text "implementing module unchanged")
         False -> return (RecompBecause "implementing module changed")
 
+-- | Check if @.hie@ file is out of date or missing.
+checkHie :: ModSummary -> IfG RecompileRequired
+checkHie mod_summary = do
+    dflags <- getDynFlags
+    let hie_date_opt = ms_hie_date mod_summary
+        hs_date = ms_hs_date mod_summary
+    pure $ case gopt Opt_WriteHie dflags of
+               False -> UpToDate
+               True -> case hie_date_opt of
+                           Nothing -> RecompBecause "HIE file is missing"
+                           Just hie_date
+                               | hie_date < hs_date
+                               -> RecompBecause "HIE file is out of date"
+                               | otherwise
+                               -> UpToDate
+
 -- | Check the flags haven't changed
 checkFlagHash :: HscEnv -> ModIface -> IfG RecompileRequired
 checkFlagHash hsc_env iface = do