Confusing E0308 Diagnostic Note for `web_sys::RequestInit::set_body` on WASM Target

[SauersML]

Environment:

• rustc: 1.78
• web-sys version: 0.3.77
• Build tool: trunk 0.21.13
• Target: wasm32-unknown-unknown
• OS: Fedora Linux

Problem Description:

When compiling Rust code targeting wasm32-unknown-unknown using trunk build, I encountered a contradictory E0308: mismatched types error when calling web_sys::RequestInit::set_body.

The code was preparing an optional request body as an Option<JsValue> and then attempting to pass it to opts.set_body(...) where opts is a web_sys::RequestInit.

1. Error Message: Stated the method expected a direct reference:

   error[E0308]: mismatched types
    --> src/client/api.rs:XX:YY
     |
   XX|         opts.set_body(some_variable);
     |              -------- ^^^^^^^^^^^^^ expected `&JsValue`, found `Option<&JsValue>`
     |              |
     |              arguments to this method are incorrect
     |
     = note: expected reference `&JsValue`
                     found enum `std::option::Option<&JsValue>`
   

2. ** note::** Simultaneously, the note pointing to the method definition suggested a different signature requiring an Option:

   note: method defined here
    --> /home/user/.cargo/registry/src/index.crates.io-.../web-sys-0.3.77/src/features/gen_RequestInit.rs:22:12
     |
   22 |     pub fn set_body(this: &RequestInit, val: Option<&::wasm_bindgen::JsValue>); // <-- Signature in note differs from primary error
     |            ^^^^^^^^
   

Investigation & Resolution:

• Attempts to satisfy the signature shown in the note: by passing Some(&jsvalue) or using option.as_ref() to create an Option<&JsValue> still failed compilation, resulting in the primary error demanding &JsValue.
• Consulting the web-sys 0.3.77 documentation and source code confirmed that the actual current signature for the method is:

  pub fn set_body(&self, val: &JsValue)

• This confirms that the rimary error message (expected &JsValue) was correct, and the diagnostic note: pointing to the definition was misleading.
• The working code that successfully compiles involves handling the Some and None cases explicitly to always pass a &JsValue, matching the actual signature:

  // body_for_request is Option<JsValue>
  if let Some(ref body_value) = body_for_request {
      opts.set_body(body_value); // Pass &JsValue directly when Some
  } else {
      opts.set_body(&JsValue::NULL); // Pass reference to JsValue::NULL when None
  }

Summary:

I am are sharing this information in case others encounter a similar contradictory diagnostic message for web_sys::RequestInit::set_body. The key finding is that the actual method signature expects &JsValue, and the diagnostic note: suggesting Option<&JsValue> appears to be inaccurate for web-sys version 0.3.77. The correct approach is to pass &JsValue directly or &JsValue::NULL.