Maybe settings should have min and max value?

[TCeason]

Now the setting value has these items.

#[derive(Clone, Debug)]
pub struct SettingValue {
    // Default value of this setting.
    default_value: UserSettingValue,
    user_setting: UserSetting,
    level: ScopeLevel,
    desc: &'static str,
    possible_values: Option<Vec<&'static str>>,
}

#[derive(Serialize, Deserialize, Clone, PartialEq, Eq)]
pub enum UserSettingValue {
    UInt64(u64),

    // TO BE COMPATIBLE WITH old version: `String<Vec<u8>>`
    #[serde(deserialize_with = "deser_str_from_vu8")]
    #[serde(serialize_with = "str_vu8")]
    String(String),
}

But now we have a settings max_thread.

If we set it to 0, it does not have any sense.

mysql> set global max_threads=0;
Query OK, 0 rows affected (0.03 sec)

mysql> select 1;
ERROR 1105 (HY000): Code: 1068, displayText = Cannot join handle from context's runtime, cause: task 58 panicked.

Maybe we can add a min, max item in SettingValue or UserSettingValue?