解答

use std::fmt::Display;
pub trait Logger {
    /// Log a message at the given verbosity level.
    fn log(&self, verbosity: u8, message: impl Display);
}
struct StderrLogger;
impl Logger for StderrLogger {
    fn log(&self, verbosity: u8, message: impl Display) {
        eprintln!("verbosity={verbosity}: {message}");
    }
}
fn do_things(logger: &impl Logger) {
    logger.log(5, "FYI");
    logger.log(2, "Uhoh");
}
/// Only log messages up to the given verbosity level.
struct VerbosityFilter {
    max_verbosity: u8,
    inner: StderrLogger,
}
impl Logger for VerbosityFilter {
    fn log(&self, verbosity: u8, message: impl Display) {
        if verbosity <= self.max_verbosity {
            self.inner.log(verbosity, message);
        }
    }
}
fn main() {
    let l = VerbosityFilter { max_verbosity: 3, inner: StderrLogger };
    do_things(&l);
}