driver: Improve -Wunused-packages error message (and simplify implementation)

In the past I improved the part of -Wunused-packages which found which
packages were used. Now I improve the part which detects which ones were
specified. The key innovation is to use the explicitUnits field from
UnitState which has the result of resolving the package flags, so we
don't need to mess about with the flag arguments from DynFlags anymore.

The output now always includes the package name and version (and the
flag which exposed it).

```
    The following packages were specified via -package or -package-id flags,
    but were not needed for compilation:
      - bytestring-0.11.2.0 (exposed by flag -package bytestring)
      - ghc-9.3 (exposed by flag -package ghc)
      - process-1.6.13.2 (exposed by flag -package process)
```

Fixes #21307

1 files changed, 12 insertions, 27 deletions

diff --git a/compiler/GHC/Driver/Make.hs b/compiler/GHC/Driver/Make.hs
index c4858d04b0..de3401455f 100644
--- a/compiler/GHC/Driver/Make.hs
+++ b/compiler/GHC/Driver/Make.hs
@@ -264,7 +264,7 @@ instantiationNodes uid unit_state = InstantiationNode uid <$> iuids_to_check
   where
     iuids_to_check :: [InstantiatedUnit]
     iuids_to_check =
-      nubSort $ concatMap goUnitId (explicitUnits unit_state)
+      nubSort $ concatMap (goUnitId . fst) (explicitUnits unit_state)
      where
       goUnitId uid =
         [ recur
@@ -460,11 +460,18 @@ warnUnusedPackages us dflags mod_graph =
             $ concatMap ms_imps (
               filter (\ms -> homeUnitId_ dflags == ms_unitid ms) (mgModSummaries mod_graph))
 
-        requestedArgs = mapMaybe packageArg (packageFlags dflags)
+        used_args = Set.fromList $ map unitId loadedPackages
 
-        unusedArgs
-          = filter (\arg -> not $ any (matching us arg) loadedPackages)
-                   requestedArgs
+        resolve (u,mflag) = do
+                  -- The units which we depend on via the command line explicitly
+                  flag <- mflag
+                  -- Which we can find the UnitInfo for (should be all of them)
+                  ui <- lookupUnit us u
+                  -- Which are not explicitly used
+                  guard (Set.notMember (unitId ui) used_args)
+                  return (unitId ui, unitPackageName ui, unitPackageVersion ui, flag)
+
+        unusedArgs = mapMaybe resolve (explicitUnits us)
 
         warn = singleMessage $ mkPlainMsgEnvelope diag_opts noSrcSpan (DriverUnusedPackages unusedArgs)
 
@@ -472,28 +479,6 @@ warnUnusedPackages us dflags mod_graph =
         then emptyMessages
         else warn
 
-    where
-        packageArg (ExposePackage _ arg _) = Just arg
-        packageArg _ = Nothing
-
-        matchingStr :: String -> UnitInfo -> Bool
-        matchingStr str p
-                =  str == unitPackageIdString p
-                || str == unitPackageNameString p
-
-        matching :: UnitState -> PackageArg -> UnitInfo -> Bool
-        matching _ (PackageArg str) p = matchingStr str p
-        matching state (UnitIdArg uid) p = uid == realUnit state p
-
-        -- For wired-in packages, we have to unwire their id,
-        -- otherwise they won't match package flags
-        realUnit :: UnitState -> UnitInfo -> Unit
-        realUnit state
-          = unwireUnit state
-          . RealUnit
-          . Definite
-          . unitId
-
 
 -- | A ModuleGraphNode which also has a hs-boot file, and the list of nodes on any
 -- path from module to its boot file.