Shared via Rust Playground

playground.rs

// The traits in question.

trait Read {
    fn read(&self);
}
trait Write {
    fn write(&mut self);
}

// Only those types may implement ReadWrite
// that implement both Read and Write.
trait ReadWrite: Read + Write {}

// Our data type.
struct SomeData;

// Trait implementations.

impl Read for SomeData {
    fn read(&self) {
        println!("read called");
    }
}

impl Write for SomeData {
    fn write(&mut self) {
        println!("write called");
    }
}

impl ReadWrite for SomeData {}

fn read<A:Read>(x:Box<A>) -> () { x.read(); }

fn write<A:Write>(mut x:Box<A>) -> () { x.write(); }

fn main() {
    // Get trait object.
    let mut trait_object: Box<ReadWrite> = Box::new(SomeData);
    // Implicit cast to Read.
    read(trait_object);
    // Implicit cast to Write.
    write(trait_object);
}