Bug 1560442 - Allow to build rust code in a separate tier. r=froydnj

When the `MOZ_RUST_TIER` environment variable is set, we enable a separate
tier that builds rust code only. This is useful to measure build times for
rust code separately from other compilations.

Differential Revision: https://phabricator.services.mozilla.com/D35501

--HG--
extra : moz-landing-system : lando

config/baseconfig.mk

@@ -42,7 +42,7 @@ endif # WINNT
 ifndef INCLUDED_AUTOCONF_MK
 default::
 else
-TIERS := $(if $(MOZ_ARTIFACT_BUILDS),artifact )$(if $(MOZ_EME_WIN32_ARTIFACT),win32-artifact )$(if $(MOZ_ANDROID_FAT_AAR_ARCHITECTURES),android-fat-aar-artifact )pre-export export $(if $(COMPILE_ENVIRONMENT),compile )misc libs tools$(if $(filter check recurse_check,$(MAKECMDGOALS)), check)
+TIERS := $(if $(MOZ_ARTIFACT_BUILDS),artifact )$(if $(MOZ_EME_WIN32_ARTIFACT),win32-artifact )$(if $(MOZ_ANDROID_FAT_AAR_ARCHITECTURES),android-fat-aar-artifact )pre-export export $(if $(COMPILE_ENVIRONMENT),$(if $(MOZ_RUST_TIER),rust )compile )misc libs tools$(if $(filter check recurse_check,$(MAKECMDGOALS)), check)
 endif
 
 # These defines are used to support the twin-topsrcdir model for comm-central.

python/mozbuild/mozbuild/backend/recursivemake.py

@@ -401,7 +401,7 @@ class RecursiveMakeBackend(CommonBackend):
 
         self._traversal = RecursiveMakeTraversal()
         self._compile_graph = OrderedDefaultDict(set)
-        self._rust_dirs = set()
+        self._rust_targets = set()
 
         self._no_skip = {
             'export': set(),
@@ -632,17 +632,17 @@ class RecursiveMakeBackend(CommonBackend):
 
         elif isinstance(obj, RustProgram):
             self._process_rust_program(obj, backend_file)
-            self._rust_dirs.add(obj.relobjdir)
             # Hook the program into the compile graph.
             build_target = self._build_target_for_obj(obj)
             self._compile_graph[build_target]
+            self._rust_targets.add(build_target)
 
         elif isinstance(obj, HostRustProgram):
             self._process_host_rust_program(obj, backend_file)
-            self._rust_dirs.add(obj.relobjdir)
             # Hook the program into the compile graph.
             build_target = self._build_target_for_obj(obj)
             self._compile_graph[build_target]
+            self._rust_targets.add(build_target)
 
         elif isinstance(obj, RustTests):
             self._process_rust_tests(obj, backend_file)
@@ -680,13 +680,13 @@ class RecursiveMakeBackend(CommonBackend):
         elif isinstance(obj, RustLibrary):
             self.backend_input_files.add(obj.cargo_file)
             self._process_rust_library(obj, backend_file)
-            self._rust_dirs.add(obj.relobjdir)
             # No need to call _process_linked_libraries, because Rust
             # libraries are self-contained objects at this point.
 
             # Hook the library into the compile graph.
             build_target = self._build_target_for_obj(obj)
             self._compile_graph[build_target]
+            self._rust_targets.add(build_target)
 
         elif isinstance(obj, SharedLibrary):
             self._process_shared_library(obj, backend_file)
@@ -820,10 +820,12 @@ class RecursiveMakeBackend(CommonBackend):
             # Directories containing rust compilations don't generally depend
             # on other directories in the tree, so putting them first here will
             # start them earlier in the build.
-            rule.add_dependencies(
-                chain((r for r in roots if mozpath.dirname(r) in self._rust_dirs),
-                      (r for r in roots if mozpath.dirname(r) not in self._rust_dirs))
-                )
+            rust_roots = [r for r in roots if r in self._rust_targets]
+            if category == 'compile' and rust_roots:
+                rust_rule = root_deps_mk.create_rule(['recurse_rust'])
+                rust_rule.add_dependencies(rust_roots)
+
+            rule.add_dependencies(chain(rust_roots, roots))
             for target, deps in sorted(graph.items()):
                 if deps:
                     rule = root_deps_mk.create_rule([target])