Nice! Although I'm seeing this false positive:

value = { 'foo' => 'bar' }
{ key: value }.deep_symbolize_keys

For deep_* methods, we'd need to check that all nested values are literals.

Good catch! Thank you.

Another tricky false positive: the deep_* methods (surprisingly) recursively drill into Array values, so this cop would need to do the same.

{ a: [ { 'b' => 1 } ] }.deep_symbolize_keys
# => { a: [ { b: 1 } ] }

Lastly, if we wanted to be really fancy, we could also detect interpolated keys: ("name_#{index}" => ...", :"name_#{index}" => ...) and explicitly converted keys (foo.to_sym => ..., foo.to_s => ...) but that might be a stretch. 😛

Fixed for nested arrays and deep_ methods 👍.

Lastly, if we wanted to be really fancy, we could also detect interpolated keys

Do not want to be fancy - the code is already pretty complex and the usecase is pretty rare, imo.

Last false positive in our repo: a combination of the previous two cases (nested arrays + non-literal hash value)

value = { 'foo' => 'bar' }
{ a: [{ value: value }]  }.deep_symbolize_keys # ⚠️ Redundant hash keys conversion, all the keys have the required type.
# => { a: [{ foo: 'bar' }] }

Fixed that too 😅