Bug 1241022 - Do not handle HAS_MISC_RULE with affected_tiers. r=gps

This initiates a move off affected_tiers in VARIABLES definition to
explicit in-backend handling, which will hopfully make things clearer.

HAS_MISC_RULE is currently used to opt-in to the misc tier in a few
directories with a misc:: rule.  It is, in fact, mostly used for custom
xpi creation, which will be separately addressed in bug 1240676, so it
will eventually go away entirely, but in the meantime, we send it as a
throwaway passthru.

python/mozbuild/mozbuild/backend/recursivemake.py

@@ -497,6 +497,9 @@ class RecursiveMakeBackend(CommonBackend):
         elif isinstance(obj, VariablePassthru):
             # Sorted so output is consistent and we don't bump mtimes.
             for k, v in sorted(obj.variables.items()):
+                if k == 'HAS_MISC_RULE':
+                    self._no_skip['misc'].add(backend_file.relobjdir)
+                    continue
                 if isinstance(v, list):
                     for item in v:
                         backend_file.write(

python/mozbuild/mozbuild/frontend/context.py

@@ -1029,7 +1029,7 @@ VARIABLES = {
         Please note that converting ``libs`` rules to the ``misc`` tier must
         be done with care, as there are many implicit dependencies that can
         break the build in subtle ways.
-        """, 'misc'),
+        """, None),
 
     'FINAL_TARGET_FILES': (ContextDerivedTypedHierarchicalStringList(Path), list,
         """List of files to be installed into the application directory.

python/mozbuild/mozbuild/frontend/emitter.py

@@ -595,6 +595,7 @@ class TreeMetadataEmitter(LoggingMixin):
             'LD_VERSION_SCRIPT',
             'USE_EXTENSION_MANIFEST',
             'NO_JS_MANIFEST',
+            'HAS_MISC_RULE',
         ]
         for v in varlist:
             if v in context and context[v]:

python/mozbuild/mozbuild/test/frontend/data/traversal-simple/bar/moz.build

@@ -1 +0,0 @@
-HAS_MISC_RULE = True

python/mozbuild/mozbuild/test/frontend/test_emitter.py

@@ -101,8 +101,6 @@ class TestEmitterBasic(unittest.TestCase):
         reldirs = [o.relativedir for o in objs]
         self.assertEqual(reldirs, ['', 'foo', 'foo/biz', 'bar'])
 
-        self.assertEqual(objs[3].affected_tiers, {'misc'})
-
         dirs = [[d.full_path for d in o.dirs] for o in objs]
         self.assertEqual(dirs, [
             [