Evolve converts/validates again not changed fields

Here's the usecase:

def decrypt_secret(value):
    return ...

@attrs.frozen
class Settings:
    user: str
    pwd: str = attrs.field(converter=decrypt_secret)

sett = Settings(user="name", pwd=<encrypted_value>)
sett.pwd  # already decrypted value

attrs.evolve(sett, user="name2")

This will fail, because it tries to decrypt already decrypted value, although the field pwd not changed at all.

Probably it is a wrong behaviour to convert/validate not changed fields.

Didn't find how to avoid converting/validating in this case.

The only entry point named is attrs.evolve; start by tracing its implementation and the existing tests around evolve, then reproduce the frozen Settings example. Done means evolving only user does not reconvert or revalidate unchanged pwd, with regression coverage for that behavior.