Add 'data' getter for the jvalue

[stanislav-tkach]

Compatibility has been broken in the f075705a9b092bfa5fccc01763ad89d352caeb8d commit, because public field becomes private.

Unfortunately data field is used in the jni-rs crate. I suppose that trace! can be useful, so I propose to add getter for the data field.

[sfackler]

Oops, I was not aware that anyone was using the internal representation. We should just make the field public again then.

FYI, this type is gong to be turned into a "real" union after Rust 1.20 releases, and it'll be undefined behavior to do what that line is doing.

[stanislav-tkach]

I have updated pull request according to your comment.

this type is gong to be turned into a "real" union after Rust 1.20 releases

Untagged unions will be stabilized? Nice, but could you please clarify what exactly will cause undefined behavior? If unions don't implement Debug trait, then it just breaks compilation. Am I missing something?

[sfackler]

It's undefined behavior to read the union on a different type than it was written. Something like this:

value.z = true;
println!("{}", value.j);

You'll need to configure systest to skip that field since it doesn't exist in C: https://github.com/sfackler/rust-jni-sys/blob/master/systest/build.rs

Should just be

cfg.skip_field(|struct, field| {
    struct == "jvalue" && field == "_data"
});

[stanislav-tkach]

Fixed. :sweat_smile:

[sfackler]

Thanks! Sorry about the breakage. Will publish now.