example.proto

syntax = "proto2";

package ext;

import "google/protobuf/descriptor.proto";

extend google.protobuf.FieldOptions {
  optional bool required = 30004;
}

message HelloWorld {
  optional string name = 1 [(ext.required) = true];
}

example.rs

pub struct RequiredExt {
}
impl ::prost::Extension for RequiredExt {
    type Value = bool;
    const NUMBER: i32 = 30004;
}
#[derive(Clone, PartialEq, ::prost::Message)]
pub struct HelloWorld {
    #[prost(string, optional, tag="1")]
    pub name: ::std::option::Option<std::string::String>,
}
impl HelloWorld {
    pub fn name_ext<E>() -> ::std::option::Option<E::Value> where E: ::prost::Extension {
        use ::prost::Message;
        match E::NUMBER {
            30004 => {
                let mut message = E::Value::decode_length_delimited::<&[u8]>(&[2, 8, 1]).ok()?;
                ::std::option::Option::Some(message)
            },
            _ => None
        }
    }
}

usage.rs

HelloWorld::name_ext<RequiredExt>() // Some(true)