想问下一，实现 ToString trait 的 to_string 方法，为啥前面要加个 default 关键字？作用是啥

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: fmt::Display + ?Sized> ToString for T {
    #[inline]
    default fn to_string(&self) -> String {
        use fmt::Write;
        let mut buf = String::new();
        buf.write_fmt(format_args!("{}", self))
           .expect("a Display implementation returned an error unexpectedly");
        buf.shrink_to_fit();
        buf
    }
}
``