rewriter: Enforce an empty project ID string

docs/markdown/Rewriter.md

@@ -87,7 +87,7 @@ Currently, only the following function types are supported:
 
 - dependency
 - target (any build target, the function ID is the target name/ID)
-- project (the function ID can be anything since project() can only be called once)
+- project (the function ID must be an empty string since project() can only be called once)
 
 For more information see the help output of the rewriter kwargs command.
 

mesonbuild/rewriter.py

@@ -479,6 +479,8 @@ class Rewriter:
         node = None
         arg_node = None
         if cmd['function'] == 'project':
+            if cmd['id'] != '':
+                mlog.error('The ID for the function type project must be an empty string --> skipping')
             node = self.interpreter.project_node
             arg_node = node.args
         elif cmd['function'] == 'target':