Corrected version of ring agreement usage

Cargo.toml

[package]
name = "ring-pk-example"
version = "0.1.0"
authors = ["Krishna Kumar <krishnakumar4a4@gmail.com>"]

[dependencies]
ring = "^0.12"
untrusted = "^0.5"

main.rs

extern crate ring;
extern crate untrusted;

use ring::{agreement, rand};
use untrusted::*;

fn main() {
    println!("Hello, world!");

    let rng = rand::SystemRandom::new();

    let my_private_key =
        agreement::EphemeralPrivateKey::generate(&agreement::X25519, &rng).unwrap();

    // Make `my_public_key` a byte slice containing my public key. In a real
    // application, this would be sent to the peer in an encoded protocol
    // message.
    let mut my_public_key = [0u8; agreement::PUBLIC_KEY_MAX_LEN];
    let my_public_key =
        &mut my_public_key[..my_private_key.public_key_len()];
    my_private_key.compute_public_key(my_public_key).unwrap();

    // In a real application, the peer public key would be parsed out of a
    // protocol message. Here we just generate one.
    let mut peer_public_key_buf = [0u8; agreement::PUBLIC_KEY_MAX_LEN];
    let peer_public_key;
    {
        let peer_private_key =
            agreement::EphemeralPrivateKey::generate(&agreement::X25519, &rng).unwrap();
        peer_public_key =
            &mut peer_public_key_buf[..peer_private_key.public_key_len()];
        peer_private_key.compute_public_key(peer_public_key).unwrap();
    }
    let peer_public_key = untrusted::Input::from(peer_public_key);

    // In a real application, the protocol specifies how to determine what
    // algorithm was used to generate the peer's private key. Here, we know it
    // is X25519 since we just generated it.
    let peer_public_key_alg = &agreement::X25519;

    agreement::agree_ephemeral(my_private_key, peer_public_key_alg,
                               peer_public_key, ring::error::Unspecified,
                               |_key_material| {
                                   // In a real application, we'd apply a KDF to the key material and the
                                   // public keys (as recommended in RFC 7748) and then derive session
                                   // keys from the result. We omit all that here.
                                   Ok(())
                               });
}

Reference: https://briansmith.org/rustdoc/ring/agreement/index.html