Unclear compiler error missing `#[wasm_bindgen]` on `impl`

[ThomasdenH]

Updated Report

use wasm_bindgen::prelude::*;

impl TestStruct {
    #[wasm_bindgen(constructor)]
    pub fn new() -> Self {
        unimplemented!();
    }
}

yields...

error: expected one of `(`, `async`, `const`, `default`, `existential`, `extern`, `fn`, `type`, or `unsafe`, found `static`
 --> examples/add/src/lib.rs:4:5
  |
4 |     #[wasm_bindgen(constructor)]
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0412]: cannot find type `TestStruct` in this scope
 --> examples/add/src/lib.rs:3:6
  |
3 | impl TestStruct {
  |      ^^^^^^^^^^ not found in this scope

error: aborting due to 2 previous errors

which is a pretty bad error!

Original Report

Note: I edited the issue to suggest a better error message since I found the cause.

Describe the Bug

When building the provided code, I get an unclear compiler error.

Steps to Reproduce

1. Compile the following code with wasm-pack build:

#[cfg(target_arch = "wasm32")]
use wasm_bindgen::prelude::*;

#[cfg_attr(target_arch = "wasm32", wasm_bindgen)]
pub struct TestStruct {}

impl TestStruct {
    #[cfg(target_arch = "wasm32")]
    #[wasm_bindgen(constructor)]
    pub fn new() -> Self {
        unimplemented!();
    }
}

Expected Behavior

I expect a compiler error that suggests the impl block have a #[wasm-bindgen] above it.

Actual Behavior

I get the following compiler error:

error: expected one of `(`, `async`, `const`, `default`, `existential`, `extern`, `fn`, `type`, or `unsafe`, found `static`
 --> src\core\crypto.rs:9:5
  |
9 |     #[wasm_bindgen(constructor)]
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[c410-f3r]

I guess it is expected, you are not guarding the wasm_bindgen(constructor) macro for non-wasm32 targets.

#[cfg(target_arch = "wasm32")]
use wasm_bindgen::prelude::*;

#[cfg_attr(target_arch = "wasm32", wasm_bindgen)]
pub struct TestStruct {}

impl TestStruct {
    #[cfg_attr(target_arch = "wasm32", wasm_bindgen(constructor))]
    pub fn new() -> Self {
        unimplemented!();
    }
}

[ThomasdenH]

Doesn't the attribute above it make the entire function conditional on target_arch = "wasm32"?

[c410-f3r]

@ThomasdenH I am sorry, it was nothing to do with macro resolution or cfg_attr. To make your code work, you just have to add #[wasm_bindgen] in the implementation block.

Edit: Not sure if it is possible on the wasm-bindgen side because each macro is parsed individually.

[alexcrichton]

Thanks for the reprot! I think this isn't related to #[cfg] but rather than #[wasm_bindgen] is missing on the impl block. Definitely something we should fix!

[c410-f3r]

@alexcrichton Same as #1500, I don't think it is possible on parsing. The #[wasm_bindgen] fn block is self-contained and doesn't know the existence of other #[wasm_bindgen] blocks/structs.

[alexcrichton]

Yeah this is gonna be a tricky one to fix, I don't think it'll be impossible but we should try at some point!