Function field_check 

async fn field_check<T>(
    services: &Services,
    body: &Ruma<T>,
    field: &ProfileFieldName,
) -> Result
where
    T: Sync,

Refuses a profile field change withheld by profile.enable_set_displayname or profile.enable_set_avatar_url.

may_set_profile_field decides which fields are gated and who is exempt.