cargo: Fix error when defining options on build-dependencies

We are not using them yet, but we should already at least declare its
options dictionary. This avoid crashing when generated code adds
features into that dict that was not yet declared.

mesonbuild/cargo/interpreter.py

@@ -437,6 +437,14 @@ def _create_project(cargo: Manifest, build: builder.Builder) -> T.List[mparser.B
     return [build.function('project', args, kwargs)]
 
 
+def _all_dependencies(cargo: Manifest) -> T.Iterable[T.Tuple[str, Dependency]]:
+    yield from cargo.dependencies.items()
+    yield from cargo.dev_dependencies.items()
+    yield from cargo.build_dependencies.items()
+    for deps in cargo.target.values():
+        yield from deps.items()
+
+
 def _process_feature(cargo: Manifest, feature: str) -> T.Tuple[T.Set[str], T.Dict[str, T.Set[str]], T.Set[str]]:
     # Set of features that must also be enabled if this feature is enabled.
     features: T.Set[str] = set()
@@ -474,7 +482,7 @@ def _create_features(cargo: Manifest, build: builder.Builder) -> T.List[mparser.
     # and one per dependency.
     ast: T.List[mparser.BaseNode] = []
     ast.append(build.assign(build.dict({}), 'features'))
-    for depname in itertools.chain.from_iterable([cargo.dependencies] + [deps for deps in cargo.target.values()]):
+    for depname, _ in _all_dependencies(cargo):
         ast.append(build.assign(build.dict({}), _options_varname(depname)))
 
     # Declare a dict that map required dependencies to true
@@ -769,13 +777,7 @@ def dependencies(source_dir: str) -> T.Dict[str, Dependency]:
     deps: T.Dict[str, Dependency] = {}
     manifests = _load_manifests(source_dir)
     for cargo in manifests.values():
-        deps_i = [
-            cargo.dependencies.values(),
-            cargo.dev_dependencies.values(),
-            cargo.build_dependencies.values(),
-        ]
-        deps_i += [deps.values() for deps in cargo.target.values()]
-        for dep in itertools.chain.from_iterable(deps_i):
+        for _, dep in _all_dependencies(cargo):
             depname = _dependency_name(dep.package, dep.api)
             deps[depname] = dep
     return deps