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It would be at least as important to be able to call Java from Rust. Does anyone know if that's hard?

There's a library[1] that wraps the jni, allowing just that.

I haven't tried it, but it looks featureful enough to be useful.

[1] https://github.com/Monnoroch/RustJni

I think the continuation of this would be a neat project. The lib starts up a JVM, it would be interesting to see a macro [1] (or will probably have to be a compiler plugin [2]) to model Java objects and make it clear the ownership of memory.

1 - http://doc.rust-lang.org/0.12.0/guide.html#macros 2 - http://doc.rust-lang.org/0.12.0/guide-plugin.html

It's probably possible to create a situation where a JVM process calls Rust code which then calls back to some Java function, if you're inclined to spend enough time hacking with JNI (is it possible to pass a function pointer out of JVM land?). But if you started as a native Rust process, there's no JVM runtime to call into, you'd need start that up first.

Can't Rust just use the OS to start a JVM?

It's definitely possible to bootstrap the JVM in-process but there are quite some steps to it and it requires a bunch of platform dependent code to locate and load the library via LoadLibrary(jvm.dll) / dlopen(libjvm.so). IIRC the JNI functions all have C linkage so it should be easy to do from Rust.

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