lib.rs

pub trait Logger {
    fn log(&self, msg: &str);
}

pub struct LoggerImpl1;

pub struct LoggerImpl2;

impl LoggerImpl1 {
    pub fn new() -> LoggerImpl1 {
        return LoggerImpl1;
    }
}

impl LoggerImpl2 {
    pub fn new() -> LoggerImpl2 {
        return LoggerImpl2;
    }
}

impl Logger for LoggerImpl1 {
    fn log(&self, msg: &str) {
    }
}

impl Logger for LoggerImpl2 {
    fn log(&self, msg: &str) {
    }
}

main.rs

extern crate logging;

use std::rand;

fn main() {
    let logger: Box<logging::Logger> = match rand::random::<bool>() {
        true => box logging::LoggerImpl1::new(),
        false => box logging::LoggerImpl2::new(),
    };
    logger.log("text");
}

output

   Compiling logging v0.0.1 (file:///home/daboross/Projects/Rust/logging)
/home/daboross/Projects/Rust/logging/src/main.rs:6:40: 9:6 error: match arms have incompatible types: expected `Box<logging::LoggerImpl1>`, found `Box<logging::LoggerImpl2>` (expected struct logging::LoggerImpl1, found struct logging::LoggerImpl2)
/home/daboross/Projects/Rust/logging/src/main.rs:6     let logger: Box<logging::Logger> = match rand::random::<bool>() {
/home/daboross/Projects/Rust/logging/src/main.rs:7         true => box logging::LoggerImpl1::new(),
/home/daboross/Projects/Rust/logging/src/main.rs:8         false => box logging::LoggerImpl2::new(),
/home/daboross/Projects/Rust/logging/src/main.rs:9     };
/home/daboross/Projects/Rust/logging/src/main.rs:8:18: 8:49 note: match arm with an incompatible type
/home/daboross/Projects/Rust/logging/src/main.rs:8         false => box logging::LoggerImpl2::new(),
                                                                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
error: aborting due to previous error
Could not compile `logging`.

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