neon-bindings/neon · error

Expected `&mut Cx` instead of `Channel`.

Error message

Expected `&mut Cx` instead of `Channel`.

What it means

Compile-time error from Neon's context check: a parameter in the context position was an owned `Channel` (by value, not a reference) where a mutable context reference is required. Neon explicitly hints that `Channel` should be swapped for `&mut Cx`.

Solutions

1. Replace the `Channel` parameter with `mut cx: &mut Cx` / `&mut FunctionContext`.
2. If a Channel is needed for background work, obtain it inside the function via `cx.channel()`.
3. If you need a Channel as data, accept it through a different mechanism (e.g. stored in a struct passed as an argument), not in the context position.

Example fix

// before
fn my_fn(channel: Channel) -> JsResult<JsUndefined> { ... }
// after
fn my_fn(mut cx: &mut FunctionContext) -> JsResult<JsUndefined> {
    let channel = cx.channel();
    ...
}

// Ensure no parameter in the context position is an owned Channel:
// bad: fn f(ch: Channel)
// good: fn f(mut cx: &mut FunctionContext)
fn valid_context(cx: &mut neon::context::FunctionContext) {}

// Compile-time error; pattern for CI gating:
// if `cargo check` output contains "Expected `&mut Cx` instead of `Channel`."
// then replace the Channel parameter with a context reference.

When it happens

Trigger: Declaring `fn my_fn(channel: Channel)` — an un-borrowed `Channel` type in the parameter list that `is_channel_type()` matches, in the context slot of an exported function.

Common situations: Developers copying `Channel` from async/threading examples into the context parameter position, or trying to receive a Channel as the first argument of an exported function instead of the context.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• Context must be a `&mut` reference.
• Expected `&mut Cx` instead of a `Channel` reference.
• Context must be a `&mut` reference.
• Expected an owned `Channel` instead of a context reference.
• Context is not available in async functions. Try a…

AI-assisted analysis of neon-bindings/neon@38960e4381 (2026-09-13). Data as JSON: /api/errors/660c64c721a4a1a4. Report an issue: GitHub.

Appendix: source

Thrown at crates/neon-macros/src/export/function/mod.rs:224

                ty.elem.span(),
                "Expected `&mut Cx` instead of a `Channel` reference.",
            ))
        }

        syn::Type::Reference(ty) => ty,

        // Context needs to be a reference
        _ if opts.context || is_context_type(&ty.ty) => {
            return Err(syn::Error::new(
                ty.ty.span(),
                "Context must be a `&mut` reference.",
            ))
        }

        // Hint that `Channel` should be swapped for `&mut Cx`
        _ if is_channel_type(&ty.ty) => {
            return Err(syn::Error::new(
                ty.ty.span(),
                "Expected `&mut Cx` instead of `Channel`.",
            ))
        }

        _ => return Ok(false),
    };

    // Not a forced or inferred context
    if !opts.context && !is_context_type(&ty.elem) {
        return Ok(false);
    }

    // Context argument must be mutable
    if ty.mutability.is_none() {
        return Err(syn::Error::new(ty.span(), "Must be a `&mut` reference."));
    }

    // All tests passed!

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