Bug 1336153 - Skip RustLibrary init on artifact builds. r=froydnj,nalexander

Artifact builds were relying on disabling Rust source file inclusion.
The RustLibrary mozbuild class wants to know cargo's output directory
when initializing itself, but when no compile environment is available
rust.configure isn't included and the corresponding config keys
aren't available.

Skip inializing the dependent fields in that case. Since the artifact
build never tries to compile any of the rust libraries, leaving
these properties undefined works ok.

MozReview-Commit-ID: 8IzTsweSygn

--HG--
extra : rebase_source : a59fc01483fbc85766ff4445c5db7ddb1e49b87c

python/mozbuild/mozbuild/frontend/data.py

@@ -543,12 +543,19 @@ class RustLibrary(StaticLibrary):
                                      basename.replace('-', '_'),
                                      context.config.rust_lib_suffix)
         self.dependencies = dependencies
+        self.features = features
+        self.target_dir = target_dir
+        # Skip setting properties below which depend on cargo
+        # when we don't have a compile environment. The required
+        # config keys won't be available, but the instance variables
+        # that we don't set should never be accessed by the actual
+        # build in that case.
+        if not context.config.substs.get('COMPILE_ENVIRONMENT'):
+            return
         build_dir = mozpath.join(target_dir,
                                  cargo_output_directory(context, self.TARGET_SUBST_VAR))
         self.import_name = mozpath.join(build_dir, self.lib_name)
         self.deps_path = mozpath.join(build_dir, 'deps')
-        self.features = features
-        self.target_dir = target_dir
 
 
 class SharedLibrary(Library):