cargo: Handle the case default-members is not defined

It should fallback to root package, then all members:
https://doc.rust-lang.org/cargo/reference/workspaces.html#the-default-members-field

Author: xclaesse

mesonbuild/cargo/interpreter.py

@@ -151,8 +151,14 @@ def interpret_workspace(self, workspace: Workspace, build: builder.Builder, subd
         self.environment.wrap_resolver.load_and_merge(subprojects_dir, T.cast('SubProject', name))
         ast: T.List[mparser.BaseNode] = []
         if not ws.required_members:
-            for member in ws.workspace.default_members:
-                self._require_workspace_member(ws, member)
+            if ws.workspace.default_members:
+                for member in ws.workspace.default_members:
+                    self._require_workspace_member(ws, member)
+            elif ws.workspace.root_package:
+                self._require_workspace_member(ws, '.')
+            else:
+                for member in ws.workspace.members:
+                    self._require_workspace_member(ws, member)
 
         # Call subdir() for each required member of the workspace. The order is
         # important, if a member depends on another member, that member must be