I'm working on the implementation for https://github.com/bazelbuild/proposals/blob/main/designs/2021-06-15-improving-native.existing_rules.md (being reviewed by @brandjon offline).

The plan is to:

• add an experimental flag (--experimental_existing_rules_immutable_view --incompatible_existing_rules_immutable_view); if the flag is enabled, native.existing_rule and native.existing_rules will return immutable, lightweight dict-like view objects instead of mutable dicts;
• identify rule sets which currently rely on mutability or typename of the return value of existing_rule/s and migrate them so that they are compatible with both mutable and immutable behaviors of the functions;
• flip the flag.