build.rs

extern crate protobuf_codegen_pure;

fn main() {
    protobuf_codegen_pure::run(protobuf_codegen_pure::Args {
        out_dir: ".",
        input: &["./user_data.proto"],
        includes: &["."],
        customize: protobuf_codegen_pure::Customize::default(),
    }).expect("protoc");
}

Cargo.toml

[package]
name = "pb"
version = "0.1.0"
authors = ["Matt Brubeck <mbrubeck@limpet.net>"]
edition = "2018"

[[bin]]
name = "pb"
path = "main.rs"

[dependencies]
protobuf = "2.4"

[build-dependencies]
protobuf-codegen-pure = "2.4"

main.rs

mod user_data;

use protobuf::Message;

fn main() {
    let mut rmsg = user_data::user_data::new();
    rmsg.set_id("1234".into());
    rmsg.set_nick("test".into());

    let msg = rmsg.write_to_bytes().unwrap();
    println!("{:?}", msg);

    let test: user_data::user_data = protobuf::parse_from_bytes(&msg).unwrap();
    println!("serialized: {:?}\noriginal: {:?}", test, rmsg);
}

user_data.proto

message user_data {
        required string id           = 1;
        optional string nick         = 2;
        optional string theme        = 3;
        optional string admin_id     = 4;
        optional string lang         = 5;
        optional double credit       = 6;
        optional double bonus_credit = 7;
};