Solution

// Copyright 2025 Google LLC
// SPDX-License-Identifier: Apache-2.0

pub trait Logger {
    /// Log a message at the given verbosity level.
    fn log(&self, verbosity: u8, message: &str);
}

struct StderrLogger;

impl Logger for StderrLogger {
    fn log(&self, verbosity: u8, message: &str) {
        eprintln!("[stderr] verbosity={verbosity}: {message}");
    }
}

struct StdoutLogger;

impl Logger for StdoutLogger {
    fn log(&self, verbosity: u8, message: &str) {
        println!("[stdout] verbosity={verbosity}: {message}");
    }
}

/// Only log messages matching a filtering predicate.
struct Filter<L, P> {
    inner: L,
    predicate: P,
}

impl<L, P> Filter<L, P>
where
    P: Fn(u8, &str) -> bool,
{
    fn new(inner: L, predicate: P) -> Self {
        Self { inner, predicate }
    }
}

impl<L, P> Logger for Filter<L, P>
where
    L: Logger,
    P: Fn(u8, &str) -> bool,
{
    fn log(&self, verbosity: u8, message: &str) {
        if (self.predicate)(verbosity, message) {
            self.inner.log(verbosity, message);
        }
    }
}

fn main() {
    let prefix = "yikes";
    let max_verbosity = 3;

    // Logger that writes to stderr and filters based on message contents.
    let logger = Filter::new(StderrLogger, |_verbosity, msg| msg.contains(prefix));
    logger.log(5, "FYI");
    logger.log(1, "yikes, something went wrong");
    logger.log(2, "uhoh");

    // Logger that writes to stdout and filters based on verbosity.
    let logger =
        Filter::new(StdoutLogger, |verbosity, _msg| verbosity <= max_verbosity);
    logger.log(5, "FYI");
    logger.log(1, "yikes, something went wrong");
    logger.log(2, "uhoh");
}

• Storing Closures: To store a closure in a struct, we use a generic type parameter (here P). This is because every closure in Rust has a unique, anonymous type generated by the compiler.
• Fn Trait Bound: The bound P: Fn(u8, &str) -> bool tells the compiler that P can be called as a function with the specified arguments and return type. We use Fn (instead of FnMut or FnOnce) because log takes &self, so we can only access the predicate immutably.
• Calling fields: We invoke the closure using (self.predicate)(...). The parentheses around self.predicate are necessary to disambiguate between calling a method named predicate and calling the field itself.