Bug 1229233 - Skip building XPIDL when there aren't any in the FasterMake backend. r=gps

The test I'm about to add doesn't have XPIDL files, and that currently avoids
the FasterMake backend to run properly. Also, in the future, when the FasterMake
grows the ability to build C++ files, it should be possible to build Spidermonkey
with the FasterMake backend, but it doesn't have XPIDL files either.

config/faster/rules.mk

@@ -45,8 +45,10 @@ default: $(addprefix install-,$(INSTALL_MANIFESTS))
 default: $(addprefix $(TOPOBJDIR)/,$(MANIFEST_TARGETS))
 default: $(TOPOBJDIR)/dist/bin/platform.ini
 
+ifndef NO_XPIDL
 # Targets from the recursive make backend to be built for a default build
 default: $(TOPOBJDIR)/config/makefiles/xpidl/xpidl
+endif
 
 ifeq (cocoa,$(MOZ_WIDGET_TOOLKIT))
 # Mac builds require to copy things in dist/bin/*.app

python/mozbuild/mozbuild/backend/fastermake.py

@@ -16,6 +16,7 @@ from mozbuild.frontend.data import (
     JsPreferenceFile,
     Resources,
     VariablePassthru,
+    XPIDLFile,
 )
 from mozbuild.jar import JarManifestParser
 from mozbuild.makeutil import Makefile
@@ -42,6 +43,8 @@ class FasterMakeBackend(CommonBackend):
 
         self._dependencies = OrderedDefaultDict(list)
 
+        self._has_xpidl = False
+
     def _add_preprocess(self, obj, path, dest, target=None, **kwargs):
         if target is None:
             target = mozpath.basename(path)
@@ -168,6 +171,14 @@ class FasterMakeBackend(CommonBackend):
                     self._manifest_entries[top_level].append(entry)
             self._manifest_entries[obj.path].append(str(obj.entry))
 
+        elif isinstance(obj, XPIDLFile):
+            self._has_xpidl = True
+            # XPIDL are emitted before Defines, which breaks the assert in the
+            # branch for Defines. OTOH, we don't actually care about the
+            # XPIDLFile objects just yet, so we can just pretend we didn't see
+            # an object in the directory yet.
+            return True
+
         else:
             # We currently ignore a lot of object types, so just acknowledge
             # everything.
@@ -286,6 +297,8 @@ class FasterMakeBackend(CommonBackend):
         mk.add_statement('TOPSRCDIR = %s' % self.environment.topsrcdir)
         mk.add_statement('TOPOBJDIR = %s' % self.environment.topobjdir)
         mk.add_statement('BACKEND = %s' % self._backend_output_list_file)
+        if not self._has_xpidl:
+            mk.add_statement('NO_XPIDL = 1')
 
         # Add a few necessary variables inherited from configure
         for var in (