#[wasm_bindgen] attribute not work in macro_rules

[sshwy]

Describe the Bug

When putting struct attributed #[wasm_bindgen] in a custom macro_rules, it throws error cannot find value 'js' in this scope.

Steps to Reproduce

wasm_bindgen version: 0.2.88.

code snippet:

// a macro that output exactly the same input struct
macro_rules! make_struct {
    {
        $( #[$attr:meta] )*
        $svis:vis struct $record_name:ident {
            $(
                $fvis:vis $fname:ident : $ftype:ty
            ),* $(,)?
        }
    } => {
        $( #[$attr] )*
        $svis struct $record_name {
            $(
                $fvis $fname : $ftype
            ),*
        }
    };
}

// testing
make_struct!{
    #[wasm_bindgen]
    #[derive(Debug, Clone, Copy)]
    pub struct Test {
        pub a: i32 // error: cannot find value `js` in this scope
    }
}

Expected Behavior

In my imagination, macro rules should be expanded before the proc macros, thus the above code should work as normal.

Correct me if I'm wrong.

[daxpedda]

This is very strange. When I used cargo expand it seemed to output code that worked just fine, So I'm not sure what the problem here is.

[ChiaMineJP]

Hello, I'm also experiencing this issue. I've created a simple reproducible repository here: https://github.com/ChiaMineJP/error-report/tree/main/wasm-bindgen-with-macro

Summary

I defined a simple macro rule as below, which just adds a line #[wasm-bindgen] to a struct definition.

macro_rules! with_wasm {
    ($name:ident {$field:ident: $t:ty}) => {
        #[wasm_bindgen::prelude::wasm_bindgen]
        pub struct $name {
            pub $field: $t
        }
    }
}

This macro simply converts

with_wasm!(AStruct {
    a_field: u8
});

to

#[wasm_bindgen::prelude::wasm_bindgen]
pub struct AStruct {
    pub a_field: u8,
}

While the expanded code (the below) passes wasm-pack build without error, the original code with macro (the above) fails with error:

error[E0425]: cannot find value `js` in this scope

Possible hint to solve this issue

When I replace pub $field: $t in the macro with pub a_field: $t like below, then wasm-pack build runs without error.

macro_rules! with_wasm {
    ($name:ident {$field:ident: $t:ty}) => {
        #[wasm_bindgen::prelude::wasm_bindgen]
        pub struct $name {
            // Previously `pub $field: $t`
            pub a_field: $t
        }
    }
}

This suggests something weird happenning on macro expansion. especially field name evaluation.

Possible hint to solve this issue 2

When I remove pub in the macro, wasm-pack build succeeded.

macro_rules! with_wasm {
    ($name:ident {$field:ident: $t:ty}) => {
        #[wasm_bindgen::prelude::wasm_bindgen]
        pub struct $name {
            // Previously `pub $field: $t`
            $field: $t
        }
    }
}

[sshwy]

@ChiaMineJP thank you for your solutions, however they won't work for me :). I expect the field of the struct to be customizable, and exported to the js side, thus $field and pub are both required.

[ChiaMineJP]

Hi @sshwy Sorry my comment was a bit confusing. Possible hint to solve this issue was not a direct fix suggestion but just a bunch of helpful findings to locate the root cause of this issue.

For example, the fact that replacing $field with a fixed field name does not emit error suggests that #[wasm_bindgen] custom attribute does not recognize the field name which is output from the macro.

[ChiaMineJP]

Update:

I tried to compare token output of #[wasm_bindgen] by outputing it to a file as below. https://github.com/rustwasm/wasm-bindgen/blob/main/crates/macro/src/lib.rs

(I cloned this wasm_bindgen repository and use it as a path dependency like this)

As I wrote in my comment above, replacing $field with a_field (fixed field name) does not raise an error. I wanted to know whether token output by #[wasm_bindgen] is different with $field or a_field. The result was the same and even when the formar complained error(cannot find value js in this scope), wasm_bindgen custom attribute itself returns Ok(tokens).

This suggests that there might be a bug in rust compiler and not in wasm_bindgen.

[ChiaMineJP]

@sshwy I've come up with a violent solution.

The idea is to let some custom attribute add #[wasm_bindgen] to target struct. In my environment this strangely works. Hope it works well for you.

Please clone my repository to try my solution. https://github.com/ChiaMineJP/error-report/tree/main/wasm-bindgen-with-macro#temporal-solution