recursive use of an object detected which would lead to unsafe aliasing in rust

[dtysky]

Summary

Hi, I am porting the "gl-matrix" to webassembly with wasm-bindgen. In "gl-matrix", we can find lots of those methods:

// module vec3

function add(out, a, b) {
xxx
}

In most time, arguments "out" and "a" are same variable.

I implement it in rust and have built it to wasm:

impl Vector3 {
    pub fn add(out: &mut Vector3, a: &Vector3, b: &Vector3) {
xxx
}
}

It seems lt work fine, but after running these code:

const a = Vector3.create();
const b = Vector3.create();
Vector3.add(a, a, b);

Rust thrown an error:

debug.js:21 Error: recursive use of an object detected which would lead to unsafe aliasing in rust

Do you have any good idea to solve this problem? Thanks!

[alexcrichton]

Thanks for the report! This is indeed a dynamic error detected by the runtime, and the issue here is that you can't have both & and &mut pointers to the same object in Rust. In this case for the add function both out and a have the same pointer, so that's disallowed by rustc at compile time but has to be detected at runtime in JS.

You'll either want to return a Vector3 or otherwise have a third object here, and that should do the trick!

[dtysky]

Thanks for the report! This is indeed a dynamic error detected by the runtime, and the issue here is that you can't have both & and &mut pointers to the same object in Rust. In this case for the add function both out and a have the same pointer, so that's disallowed by rustc at compile time but has to be detected at runtime in JS.

You'll either want to return a Vector3 or otherwise have a third object here, and that should do the trick!

@alexcrichton

Thanks for your quick reply! I have guessed this issue is about "borrow", and have tried to change the function to these:

pub fn add(out: &mut Vector3, a: &mut Vector3, b: &Vector3) {
}

But still get the same error...I cannot understand it...

Return a Vector3 or a third object will make a copy, in my application I cannot afford the overhead...

Any ideas

[alexcrichton]

I'd recomment exploring this example in Rust with the assistance of the Rust compiler. The idioms that work in Rust are what you'll want to transfer to JS.

[dtysky]

@alexcrichton

It seems no better way to solve this problem, I had tried some unsafe code like these:

pub fn add(out: *mut Vector4, a: *mut Vector4, b: *mut Vector4) {
        unsafe {
            let out = &mut *out;
            let a = & *a;
            let b = & *b;
            out.0 = a.0 + b.0;
            out.1 = a.1 + b.1;
            out.2 = a.2 + b.2;
            out.3 = a.3 + b.3;
        }
    }

It works, but wasm-bindgen will generate js code that lose the type information and I had to pass ptr to rust myself:

math.Vector4.add((v1 as any).ptr + 4, (v1 as any).ptr + 4, (v2 as any).ptr + 4);

It's painful...If we cannot reuse out and a, the goals of performance will be lost. Rust is a great language and wasm-bindgen is a great toolchain, but in this application I cannot find any viable solutions.

Or are there any options to turn off the runtime checking?

Thanks.

[Pauan]

I'm not 100% sure, but I'm pretty sure your code isn't sound.

Why not create a new method which treats out and a as the same?

    pub fn increment(a: &mut Vector4, b: &Vector4) {
        a.0 = a.0 + b.0;
        a.1 = a.1 + b.1;
        a.2 = a.2 + b.2;
        a.3 = a.3 + b.3;
    }

It's quite common in Rust to have different methods for different uses (e.g. new and new_with_extra_stuff), so this is quite idiomatic.

[dtysky]

@Pauan

Yes your way is natural, but in my case, I must make sure my implements is as same as gl-matrix library.

We use gl-matrix in our web game engine, we want to replace the math library and ensure it could fallback to js version.

[dtysky]

Emm...After all I solve this problem by some real tricks:

let fp = path.resolve(__dirname, './pkg/xxx.wasm');
const originBuffer = fs.readFileSync(fp);

const wasm = binaryen.readBinary(originBuffer);
const wast = wasm.emitText()
  .replace(/\(br_if \$label\$1[\s\n]+?\(i32.eq\n[\s\S\n]+?i32.const -1\)[\s\n]+\)[\s\n]+\)/g, '');
const distBuffer = binaryen.parseText(wast).emitBinary();

fs.writeFileSync(fp, distBuffer);

I convert the wasm file to wast, them remove borrow checking from it, at last convert it back to wasm, it works(in this case we do not need borrow checking).

[smolck]

Is there a recommended way of debugging this issue and fixing it? Or documentation with examples of it happening that explain why it occurs in those examples? I'm running into this myself in this PR for a text editor I maintain, but I have no real idea why it's happening/how to fix it (I only ever call functions on a struct from a single reference to said struct, so no obvious violations of the borrow checker from the JS side), and this thread is the only real information on the topic I could find after searching around a bit.

It seems like the underlying problem here is simply that we don't want references, we want pointers. @dtysky Would having Wasm-bindgen somehow support allowing pointers to/from the ABI fit your original use case?

[dtysky]

It seems like the underlying problem here is simply that we don't want referencesz we want pointers. @dtysky Would having Wasm-bindgen somehow support allowing pointers to/from the ABI fit your original use case?

Yes, in my case i just expect the member functions could receive a pointer as self, and return a pointer as result: https://github.com/dtysky/gl-matrix-wasm