Failures when building with rust 2018

[SecurityInsanity]

Enabling rust 2018 on a prost project will cause failures in certain generated messages. Specifically, those generated messages who end up generating modules. Specifically you might see error messages like:

error: cannot find derive macro `Oneof` in this scope
  --> /Users/vagrant/git/target/debug/build/gwiz-rust-427dfcfefeb48ff9/out/grpc.slack.rs:82:21
   |
82 |     #[derive(Clone, Oneof, PartialEq)]
   |                     ^^^^^

Or maybe even some messages like:

error[E0658]: The attribute `prost` is currently unknown to the compiler and may have meaning added to it in the future (see issue #29642)
  --> /Users/vagrant/git/target/debug/build/gwiz-rust-427dfcfefeb48ff9/out/grpc.slack.rs:90:9
   |
90 |         #[prost(string, tag="4")]
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^

These are caused by internal modules not including the types from: prost_derive. (Specifically the generated macro types).

All we should need is an: "use prost_derive::*" in these generated modules, however we probably want to also turn it into a feature for only 2018, so this crate doesn't force 2018 edition code on others. Or maybe since it's an include that just won't get used it's fine since it'll be optimized away?

[leeola]

Possibly related?

error[E0277]: the size for values of type `dyn std::fmt::Debug` cannot be known at compilation time
 --> /.../target/debug/build/prost-test-b14c221d72aa9018/out/foo.rs:1:28
  |
1 | #[derive(Clone, PartialEq, Message)]
  |                            ^^^^^^^ doesn't have a size known at compile-time
  |

via 2018 version. After switching to nightly, this went away. .. though, I'm still running into import failures via #142

[vorner]

I've worked around this with:

mod proto {
    use prost_derive::{Enumeration, Message};
    include!(concat!(env!("OUT_DIR"), "/proto.rs"));
}