Support for namespaced module generation

[samuela]

Right now if you try to include a generated rust capnp module outside the root module, you run into some problems:

 // In src/newworld/mod.rs
pub mod filetree_capnp {
  include!(concat!(env!("OUT_DIR"), "/src/newworld/filetree_capnp.rs"));
}

// In build.rs
fn main() {
  ::capnpc::CompilerCommand::new()
    .file("src/newworld/filetree.capnp")
    .run()
    .unwrap();
}

We get a bunch of errors like:

error[E0433]: failed to resolve: unresolved import
 --> /opt/nuvemfs/cli/cargo-target-dir/debug/build/nuvemfs-cli-c1dfeda28eaf92b2/out/src/newworld/filetree_capnp.rs:9:67
  |
9 |   pub type HeartbeatParams<> = ::capnp::capability::Params<crate::filetree_capnp::peer::heartbeat_params::Owned>;
  |                                                                   ^^^^^^^^^^^^^^
  |                                                                   |
  |                                                                   unresolved import
  |                                                                   help: a similar path exists: `crate::newworld::filetree_capnp`

[dwrensha]

Hi! Did you try using the Rust.parentModule annotation? It was added just a few days ago. See #16.

[samuela]

I tried following along with the example, so now I have

@0xa850d6aba0560467;

using Rust = import "rust.capnp";
$Rust.parentModule("newworld");

interface Peer {
  heartbeat @0 ();
}

but I'm getting a capnp compilation error:

src/newworld/filetree.capnp:3:21-33: error: Import failed: rust.capnp
src/newworld/filetree.capnp:4:2-6: error: Not defined: Rust

[dwrensha]

You'll need to have rust.capnp in the include path somehow. I recommend just copying it over to wherever the rest of your schema files are.

[samuela]

Hmm, I see... Is there any way that capnpc::CompilerCommand could inject rust.capnp into the include path at build time?

[dwrensha]

I think the ideal solution would involve something like this: rust-lang/cargo#3544

Another thing we might try is to get rust.capnp included alongside c++.capnp in the binary packaging of the capnp tool.

If we're really ambitious, we could rewrite the capnp tool in Rust and inject rust.capnp there, but that would be a lot of work.

Aside from those approaches, I think the best we can do for now is to copy rust.capnp into a temporary directory and to pass that directory as a -I parameter to the capnp tool. I'm a bit reluctant to do that, though, as it seems hacky.

[samuela]

Aside from those approaches, I think the best we can do for now is to copy rust.capnp into a temporary directory and to pass that directory as a -I parameter to the capnp tool. I'm a bit reluctant to do that, though, as it seems hacky.

Yeah it does seem a bit hacky... It's not ideal but I'm ok with having things at the root of my project for now, so I think I'll revisit this once it get too unwieldy.

[dwrensha]

Closing, because parentModule addresses this issue. If you have other, more specific problems, please open new issues for them.

I've added a few doc comments: 998510c.