linecode/dustr

Dart - rust - Flutter compatibility

DUSTR

With dustr, you can call this rust code:

#[derive(FFIShim)]
struct User {
    name: String,
}

#[ffishim_function]
fn hello(u: User) -> String {
    format!("Hello, {}!", u.name)
}

from dart:

import 'package:hello/hello.dart';

void main() {
    var greeting = hello(User.build("fred"));
    print("${greeting}");
}

dustr is a binary that parses rust code to generate its dart bindings. The rust code must be marked using the ffishim_derive procedural macros from the ffishim library.

These procedural macros generate an FFI-compatible API around the original data structure/function. This is necessary because many basic rust types (String, Option, Vec, ...) do not respect the C ABI.

Install & usage

Now, suppose we want to reproduce our first example. We need make, cargo and the dart sdk. We install dustr using cargo:

export PATH=$PATH:$HOME/.cargo/bin
cargo install dustr

We also create our hello crate which we will mark as C dynamic library (cdylib, to generate a .so shared object in the rusthello/target/debug directory.)

cargo new --lib rusthello --name hello
cat >>rusthello/Cargo.toml <<EOF
ffishim = "^0"
ffishim_derive = "^0"

[lib]
crate-type = ["cdylib"]
EOF
cat >rusthello/src/lib.rs <<EOF
#[macro_use]
extern crate ffishim_derive;

#[derive(FFIShim)]
struct User {
    name: String,
}

#[ffishim_function]
fn hello(u: User) -> String {
    format!("Hello, {}!", u.name)
}
EOF
cargo build --manifest-path=rusthello/Cargo.toml
ls rusthello/target/debug/libhello.so

We took the opportunity to add the code (with a healthy dose more plumbing this time) and build the library. Now let's step into the dart side...

dustr --dest darthello --name hello rusthello/
cd darthello; pub get; cd -

The dustr command will create the dart package containing the bindings to the rusthello library. pub get pulls in any dependencies. We'll now set up the dart app which will use our bindings:

mkdir -p dartapp/bin
cat >dartapp/bin/main.dart <<EOF
import 'package:hello/hello.dart';

void main() {
    var greeting = hello(User.build("fred"));
    print("\${greeting}");
}
EOF
cat >dartapp/pubspec.yaml <<EOF
---
name: app
dependencies:
  hello:
    path: ../darthello
environment:
  sdk: ">=2.0.0 <3.0.0"
EOF
cd dartapp; pub get; cd -

Now we can run the app. Don't forget to provide the rust library:

LD_LIBRARY_PATH=rusthello/target/debug dart dartapp/bin/main.dart

Examples

You can find examples of dustr's behavior by looking at the tests folder. The structure of the tests are as follow:

• src/lib.rs: the rust library to expose
• Cargo.toml: manifest of the rust library (defines cdylib, etc..)
• pubspec.yaml: manifest of dart binary (defines dep on generated bindings)
• bin/main.dart: the dart code that uses this rust library
• expected_output: contains the output expected from running the C program

Every test folder is a stand-alone app. For example, you could:

• Run make to test all of them
• Inspect the generated bindings code in the target/bindings hierarchy
• Tweak a test or two to try out behavior and rerun make to test things out
• Use them as templates to create your own working rust <-> dart integration

C ABI Disclaimer

Because dart ffi support is still in alpha, it cannot quite consume the C ABI just yet. For example, it does not support nested structs, and structures cannot be passed by value to functions. For this reason, the ffishim crate we use does not generate C-ABI code exactly, but a bastard version consumable by the dart ffi.

TODO/Limitations

This crate is still in beta. It is not fit for production use yet.

Bugs

• Fix leaking of returned strings when not inside struct/enum
• Fix leaking of returned options when not inside struct/enum

Features

• Adapt to potential name mangling in ffishim (to avoid duplicates)
• Find equivalent dart type for usize/isize/char
• Implement DateTime behavior

Testing

• Add a "complete" test/example situation
• Re-organize tests by domain (find taxonomy)
• Benchmark some use-cases

Documentation

• Write README.md introduction on what this crate does
• Where to write binary documentation in doc.rs?