Fix #9243 so recompilation avoidance works with -fno-code

Summary:
Where we track timestamps of object files, also track timestamps
for interface files.  When -fno-code -fwrite-interface is enabled, use
the interface file timestamp as an extra check to see if the files are
up-to-date.  We had to apply this logic to one-shot and make modes.

This fix would be good to merge into 7.10; it makes using -fno-code
-fwrite-interface for flywheel type checking usable.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Test Plan: validate and new test cases

Reviewers: austin

Subscribers: carter, thomie

Differential Revision: https://phabricator.haskell.org/D596

GHC Trac Issues: #9243

1 files changed, 15 insertions, 4 deletions

diff --git a/compiler/main/DriverPipeline.hs b/compiler/main/DriverPipeline.hs
index e8be29759b..6d597f9437 100644
--- a/compiler/main/DriverPipeline.hs
+++ b/compiler/main/DriverPipeline.hs
@@ -30,7 +30,7 @@ module DriverPipeline (
    runPhase, exeFileName,
    mkExtraObjToLinkIntoBinary, mkNoteObjsToLinkIntoBinary,
    maybeCreateManifest, runPhase_MoveBinary,
-   linkingNeeded, checkLinkInfo
+   linkingNeeded, checkLinkInfo, writeInterfaceOnlyMode
   ) where
 
 #include "HsVersions.h"
@@ -935,6 +935,11 @@ runPhase (RealPhase (Hsc src_flavour)) input_fn dflags0
         location <- getLocation src_flavour mod_name
 
         let o_file = ml_obj_file location -- The real object file
+            hi_file = ml_hi_file location
+            dest_file | writeInterfaceOnlyMode dflags
+                            = hi_file
+                      | otherwise
+                            = o_file
 
   -- Figure out if the source has changed, for recompilation avoidance.
   --
@@ -952,10 +957,10 @@ runPhase (RealPhase (Hsc src_flavour)) input_fn dflags0
                 --      (b) we aren't going all the way to .o file (e.g. ghc -S)
              then return SourceModified
                 -- Otherwise look at file modification dates
-             else do o_file_exists <- doesFileExist o_file
-                     if not o_file_exists
+             else do dest_file_exists <- doesFileExist dest_file
+                     if not dest_file_exists
                         then return SourceModified       -- Need to recompile
-                        else do t2 <- getModificationUTCTime o_file
+                        else do t2 <- getModificationUTCTime dest_file
                                 if t2 > src_timestamp
                                   then return SourceUnmodified
                                   else return SourceModified
@@ -975,6 +980,7 @@ runPhase (RealPhase (Hsc src_flavour)) input_fn dflags0
                                         ms_location  = location,
                                         ms_hs_date   = src_timestamp,
                                         ms_obj_date  = Nothing,
+                                        ms_iface_date   = Nothing,
                                         ms_textual_imps = imps,
                                         ms_srcimps      = src_imps }
 
@@ -2248,6 +2254,11 @@ joinObjectFiles dflags o_files output_fn = do
 -- -----------------------------------------------------------------------------
 -- Misc.
 
+writeInterfaceOnlyMode :: DynFlags -> Bool
+writeInterfaceOnlyMode dflags =
+ gopt Opt_WriteInterface dflags &&
+ HscNothing == hscTarget dflags
+
 -- | What phase to run after one of the backend code generators has run
 hscPostBackendPhase :: DynFlags -> HscSource -> HscTarget -> Phase
 hscPostBackendPhase _ HsBootFile _    =  StopLn