Library SemVer considerations when it comes to default settings

How do you treat changes to default settings for your library's configuration? Are you strict and consider them part of your API or are you more lax and allow changes to the default settings within a major version? My concrete use case looks similar to this: pub trait Validator { fn validate(&self, _input: &[u8]) -> bool { false } } pub struct Validator1; pub struct Validator2; pub struct Validator3; impl Validator for Validator1 {} impl Validator for Validator2 {} impl Valida...