oven-sh/bun · error · Error

$$${name} takes three arguments, but got '$$${name}${inner.r

Error message

$$${name} takes three arguments, but got '$$${name}${inner.result}'

What it means

$newRustFunction/$newCppFunction create native function objects at bundle time and require exactly three literal arguments: (structName: string, functionName: string, length: number). The arity/type check runs after JSON parsing succeeds, so this error means the call parsed but did not match that shape.

Source

Thrown at src/codegen/replacements.ts:258

        const str =
          "[" +
          inner.result
            .slice(1, -1)
            .replaceAll("'", '"')
            .replace(/,[\s\n]*$/s, "") +
          "]";
        args = JSON.parse(str);
      } catch {
        throw new Error(`Call is not known at bundle-time: '$${name}${inner.result}'`);
      }
      if (
        args.length != (is_create_fn ? 3 : 2) ||
        typeof args[0] !== "string" ||
        typeof args[1] !== "string" ||
        (is_create_fn && typeof args[2] !== "number")
      ) {
        if (is_create_fn) {
          throw new Error(`$${name} takes three arguments, but got '$${name}${inner.result}'`);
        } else {
          throw new Error(`$${name} takes two string arguments, but got '$${name}${inner.result}'`);
        }
      }

      const id = registerNativeCall(kind, args[0], args[1], is_create_fn ? args[2] : null);

      return [slice.slice(0, match.index) + "__intrinsic__lazy(" + id + ")", inner.rest, true];
    } else if (name === "isPromiseFulfilled" || name === "isPromiseRejected" || name === "isPromisePending") {
      const inner = sliceSourceCode(rest, true);
      // JSC::JSPromise::Status: Pending = 0, Fulfilled = 1, Rejected = 2.
      const status = name === "isPromisePending" ? 0 : name === "isPromiseFulfilled" ? 1 : 2;
      let args;
      if (debug) {
        // use a property on @lazy as a temporary holder for the expression. only in debug!
        args = `($assert(__intrinsic__isPromise(__intrinsic__lazy.temp=${inner.result.slice(0, -1)}))),__intrinsic__peekPromiseStatus(__intrinsic__lazy.temp) === (__intrinsic__lazy.temp = undefined, ${status}))`;
      } else {
        args = `(__intrinsic__peekPromiseStatus${inner.result} === ${status})`;

View on GitHub (pinned to 8c5296ac45)

Solutions

1. Supply all three literals: $newRustFunction("MyStruct", "myFn", 2).
2. Make the third argument a plain number literal (not "2" and not a variable).
3. If you only need an existing function, use $rust/$cpp (two string args) instead of the new* form.

Example fix

// before
const fn = $newRustFunction("MyStruct", "myFn");

// after
const fn = $newRustFunction("MyStruct", "myFn", 2);

const newFnRe = /\$new(Rust|Cpp)Function\((.*?)\)/g;
for (const m of src.matchAll(newFnRe)) {
  const args = m[2].split(",").map(s => s.trim());
  const ok = args.length === 3 && /^(['"]).*\1$/.test(args[0]) && /^(['"]).*\1$/.test(args[1]) && /^\d+$/.test(args[2]);
  if (!ok) throw new Error(`bad $new${m[1]}Function call: ${m[0]}`);
}

When it happens

Trigger: Calling $newRustFunction with two arguments (copying a $rust call site), passing the argument count as a string, or adding a trailing fourth argument.

Common situations: Copy-pasting a $rust(...) binding and just renaming the macro; updating a binding signature and forgetting the length parameter.

Related errors

• $$${name} takes two string arguments, but got '$$${name}${in
• Call is not known at bundle-time: '$$${name}${inner.result}'
• slice/str param `${ty}` is not FFI-safe; pass (ptr, len)
• ${where}: cannot parse param `${raw}`
• ${loc}: HOST_EXPORT(${e.symbol}, rust) param `${p.raw}` is a

AI-assisted analysis of oven-sh/bun@8c5296ac45 (2026-08-16). Data as JSON: /api/errors/d3543bcdafc92f16. Report an issue: GitHub.