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* [swc-plugin] Capture lexical `this` for nested arrow step functions When a nested arrow `"use step"` references the enclosing function/method's `this`, plumb that `this` through the workflow runtime so the step body sees the correct receiver. - Workflow mode wraps the step proxy with `.bind(this)`, so invoking the proxy captures the caller's `this` as `thisVal` on the queue item. - Step mode hoists the body as a regular `function` (not an arrow) so the runtime's `stepFn.apply(thisVal, args)` rebinds `this` inside the hoisted body. Detection only fires for arrows, since arrows inherit `this` lexically. Nested non-arrow functions/methods/getters/setters introduce their own `this`, so the detector stops at those boundaries. The runtime already supported `thisVal` for instance-method steps; this PR is purely a compiler change to feed the existing pipeline. Caveat: capture works at runtime only when the captured value is serializable across the workflow->step boundary (i.e. the enclosing class implements `WORKFLOW_SERIALIZE`/`WORKFLOW_DESERIALIZE`). Refs vercel/workflow#1865 * Address PR review: preserve step proxy metadata + tighter `this` detection - core: Override `.bind` on step proxies so the bound function retains `stepId` and `__closureVarsFn`. Without this, a bound proxy that flows through workflow serialization (e.g. as a step argument) would be treated as a non-serializable plain function by `getStepFunctionReducer`. - swc-plugin: Detector now also walks `arrow.params` so `this` references in default values / destructuring initializers (e.g. `(x = this.foo) => ...`) trigger the `.bind(this)` path. - swc-plugin: Class bodies inside the arrow body are now treated as `this`-binding boundaries — `this` inside class field initializers, methods, etc. is bound to the class instance, not the outer arrow. The detector still walks `extends` clauses and computed property keys because those are evaluated in the surrounding scope. - spec.md: Sharpen the note about `this` in step bodies — it's syntactically allowed but only meaningful for instance-method steps and lexical-`this` arrow steps; other shapes compile but `this` will be whatever the caller of the step proxy passes. - Add `lexical-this-detector-edge-cases` fixture covering both the default-param positive case and the inner-class false-positive guard. - Strengthen the runtime test to assert `stepId` / `__closureVarsFn` survive `.bind(...)`. * [swc-plugin] Fix `arguments` closure-var capture; drop dead `this`/`arguments` checks - Add `arguments` to `is_global_identifier` so it's not captured as a closure variable. Previously a nested `function`-form step like function step() { 'use step'; return arguments[0]; } was hoisted with `const { arguments } = ...` (a strict-mode syntax error) and the body's `arguments[0]` resolved against the destructured binding instead of the function's intrinsic `arguments` object. - Remove dead `ForbiddenExpression` checks for `this` and `arguments` in `visit_mut_this_expr` / `visit_mut_ident`. The `'use step'` / `'use workflow'` directives are stripped during the module-level traversal before children are visited, so `in_step_function` / `in_workflow_function` are never observed as true here in practice. The existing `step-with-this-arguments-super` fixture explicitly documents that all three identifiers are allowed in step bodies. - Tighten the spec note about `arguments` accordingly: it works in `function`-form steps (reflecting positional args) but is not captured for arrow-form steps; use `...args` for that case. - Add `nested-step-arguments` fixture pinning down the new behavior. Signed-off-by: Nathan Rajlich <n@n8.io> Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com> Co-authored-by: Nathan Rajlich <n@n8.io>