Bug 1370296 - Ensure rust libraries are fully in the compile graph. r=gps

This was not strictly necessary because they would appear as
dependencies of other things in the compile graph, but it makes it
easier to iterate the compile graph for all the node.

Incidentally, this makes the rust libraries on par with rust programs.

--HG--
extra : rebase_source : 3c672308478d3ef3dd38b9036f7b1862abcf3f97

python/mozbuild/mozbuild/backend/recursivemake.py

@@ -615,6 +615,10 @@ class RecursiveMakeBackend(CommonBackend):
             # 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]
+
         elif isinstance(obj, SharedLibrary):
             self._process_shared_library(obj, backend_file)
             self._process_linked_libraries(obj, backend_file)