Followup fixes for sync step function support

.changeset/sync-step-followup.md

@@ -0,0 +1,5 @@
+---
+"@workflow/swc-plugin": patch
+---
+
+Restore export validation for file-level `"use step"` files: only function exports (sync or async) are allowed; non-function exports (constants, classes, re-exports) emit an error

packages/swc-plugin-workflow/spec.md

@@ -1060,26 +1060,33 @@ The plugin emits errors for invalid usage:
 
 | Error | Description |
 |-------|-------------|
-| Non-async function | Functions with `"use step"` or `"use workflow"` must be async |
+| Non-async workflow function | Functions with `"use workflow"` must be async (step functions may be sync) |
 | Instance methods with `"use workflow"` | Only static methods can have `"use workflow"` (not instance methods) |
 | Getters with `"use workflow"` | Getters cannot be marked with `"use workflow"` |
 | Misplaced directive | Directive must be at top of file or start of function body |
 | Conflicting directives | Cannot have both `"use step"` and `"use workflow"` at module level |
-| Invalid exports | Module-level directive files can only export async functions |
+| Invalid exports (`"use workflow"`) | Module-level `"use workflow"` files can only export async functions |
+| Invalid exports (`"use step"`) | Module-level `"use step"` files can only export functions (sync or async) |
 | Misspelled directive | Detects typos like `"use steps"` or `"use workflows"` |
 
 ---
 
 ## Supported Function Forms
 
-The plugin supports various function declaration styles:
-
-- `async function name() { "use step"; }` - Function declaration
-- `const name = async () => { "use step"; }` - Arrow function with const
-- `let name = async () => { "use step"; }` - Arrow function with let
-- `var name = async () => { "use step"; }` - Arrow function with var
-- `const name = async function() { "use step"; }` - Function expression
-- `{ async method() { "use step"; } }` - Object method
+The plugin supports various function declaration styles. Step functions may be synchronous or asynchronous. Workflow functions must be async.
+
+- `async function name() { "use step"; }` - Async function declaration
+- `function name() { "use step"; }` - Sync function declaration
+- `const name = async () => { "use step"; }` - Async arrow function
+- `const name = () => { "use step"; }` - Sync arrow function
+- `let name = async () => { "use step"; }` - Async arrow function with let
+- `let name = () => { "use step"; }` - Sync arrow function with let
+- `var name = async () => { "use step"; }` - Async arrow function with var
+- `var name = () => { "use step"; }` - Sync arrow function with var
+- `const name = async function() { "use step"; }` - Async function expression
+- `const name = function() { "use step"; }` - Sync function expression
+- `{ async method() { "use step"; } }` - Async object method
+- `{ method() { "use step"; } }` - Sync object method
 - `{ nested: { execute: async () => { "use step"; } } }` - Nested object property
 - `static async method() { "use step"; }` - Static class method
 - `async method() { "use step"; }` - Instance class method (requires custom serialization)

packages/swc-plugin-workflow/transform/src/lib.rs

@@ -102,10 +102,17 @@ fn emit_error(error: WorkflowErrorKind) {
         ),
         WorkflowErrorKind::InvalidExport { span, directive } => (
             span,
-            format!(
-                "Only async functions can be exported from a \"{}\" file",
-                directive
-            ),
+            if directive == "use step" {
+                format!(
+                    "Only functions can be exported from a \"{}\" file",
+                    directive
+                )
+            } else {
+                format!(
+                    "Only async functions can be exported from a \"{}\" file",
+                    directive
+                )
+            },
         ),
     };
 
@@ -4054,8 +4061,6 @@ impl StepTransform {
         }
     }
 
-    // Previously validated that step functions must be async. The restriction
-
     // Check if a function should be treated as a step function
     fn should_transform_function(&self, function: &Function, is_exported: bool) -> bool {
         let has_directive = self.has_use_step_directive(&function.body);
@@ -6051,9 +6056,107 @@ impl VisitMut for StepTransform {
         let mut items_to_insert = Vec::new();
 
         for (i, item) in items.iter_mut().enumerate() {
-            // Validate exports if we have a file-level workflow directive.
-            // Step files allow any exports (sync or async), but workflow files
-            // require exported functions to be async.
+            // Validate exports for file-level step directives.
+            // Step files allow sync or async function exports but reject
+            // non-function exports (constants, classes, re-exports) which
+            // can pull Node-only code into the workflow/client bundles.
+            if self.has_file_step_directive {
+                match item {
+                    ModuleItem::ModuleDecl(ModuleDecl::ExportDecl(export)) => {
+                        match &export.decl {
+                            Decl::Fn(_) => {
+                                // Sync or async function declarations are allowed
+                            }
+                            Decl::Var(var_decl) => {
+                                for decl in &var_decl.decls {
+                                    match &decl.init {
+                                        Some(init) => match &**init {
+                                            Expr::Fn(_) | Expr::Arrow(_) => {
+                                                // Function/arrow expressions are allowed
+                                            }
+                                            _ => {
+                                                emit_error(WorkflowErrorKind::InvalidExport {
+                                                    span: export.span,
+                                                    directive: "use step",
+                                                });
+                                            }
+                                        },
+                                        None => {
+                                            // Uninitialized exports are not functions
+                                            emit_error(WorkflowErrorKind::InvalidExport {
+                                                span: export.span,
+                                                directive: "use step",
+                                            });
+                                        }
+                                    }
+                                }
+                            }
+                            Decl::TsInterface(_)
+                            | Decl::TsTypeAlias(_)
+                            | Decl::TsEnum(_)
+                            | Decl::TsModule(_) => {
+                                // TypeScript declarations are okay
+                            }
+                            _ => {
+                                emit_error(WorkflowErrorKind::InvalidExport {
+                                    span: export.span,
+                                    directive: "use step",
+                                });
+                            }
+                        }
+                    }
+                    ModuleItem::ModuleDecl(ModuleDecl::ExportNamed(named)) => {
+                        // Re-exports (`export { x } from '...'`) are not allowed.
+                        // Local named exports (`export { x }`) are also rejected
+                        // because we cannot statically verify the binding is a function.
+                        if named.src.is_some() || !named.specifiers.is_empty() {
+                            emit_error(WorkflowErrorKind::InvalidExport {
+                                span: named.span,
+                                directive: "use step",
+                            });
+                        }
+                    }
+                    ModuleItem::ModuleDecl(ModuleDecl::ExportDefaultDecl(default)) => {
+                        match &default.decl {
+                            DefaultDecl::Fn(_) => {
+                                // Sync or async function declarations are allowed
+                            }
+                            DefaultDecl::TsInterfaceDecl(_) => {
+                                // TypeScript interface is okay
+                            }
+                            _ => {
+                                emit_error(WorkflowErrorKind::InvalidExport {
+                                    span: default.span,
+                                    directive: "use step",
+                                });
+                            }
+                        }
+                    }
+                    ModuleItem::ModuleDecl(ModuleDecl::ExportDefaultExpr(expr)) => {
+                        match &*expr.expr {
+                            Expr::Fn(_) | Expr::Arrow(_) => {
+                                // Function/arrow expressions are allowed
+                            }
+                            _ => {
+                                emit_error(WorkflowErrorKind::InvalidExport {
+                                    span: expr.span,
+                                    directive: "use step",
+                                });
+                            }
+                        }
+                    }
+                    ModuleItem::ModuleDecl(ModuleDecl::ExportAll(export_all)) => {
+                        emit_error(WorkflowErrorKind::InvalidExport {
+                            span: export_all.span,
+                            directive: "use step",
+                        });
+                    }
+                    _ => {}
+                }
+            }
+
+            // Validate exports for file-level workflow directives.
+            // Workflow files require exported functions to be async.
             if self.has_file_workflow_directive {
                 match item {
                     ModuleItem::ModuleDecl(ModuleDecl::ExportDecl(export)) => {

packages/swc-plugin-workflow/transform/tests/errors/invalid-default-exports-step/input.js

@@ -0,0 +1,6 @@
+'use step';
+
+// Error: default class export
+export default class MyClass {
+  method() {}
+}

packages/swc-plugin-workflow/transform/tests/errors/invalid-default-exports-step/output-client.js

@@ -0,0 +1,4 @@
+// Error: default class export
+export default class MyClass {
+    method() {}
+}

packages/swc-plugin-workflow/transform/tests/errors/invalid-default-exports-step/output-client.stderr

@@ -0,0 +1,7 @@
+  x Only functions can be exported from a "use step" file
+   ,-[input.js:4:1]
+ 3 |     // Error: default class export
+ 4 | ,-> export default class MyClass {
+ 5 | |     method() {}
+ 6 | `-> }
+   `----

packages/swc-plugin-workflow/transform/tests/errors/invalid-default-exports-step/output-step.js

@@ -0,0 +1,4 @@
+// Error: default class export
+export default class MyClass {
+    method() {}
+}

packages/swc-plugin-workflow/transform/tests/errors/invalid-default-exports-step/output-step.stderr

@@ -0,0 +1,7 @@
+  x Only functions can be exported from a "use step" file
+   ,-[input.js:4:1]
+ 3 |     // Error: default class export
+ 4 | ,-> export default class MyClass {
+ 5 | |     method() {}
+ 6 | `-> }
+   `----

packages/swc-plugin-workflow/transform/tests/errors/invalid-default-exports-step/output-workflow.js

@@ -0,0 +1,5 @@
+'use step';
+// Error: default class export
+export default class MyClass {
+    method() {}
+}

packages/swc-plugin-workflow/transform/tests/errors/invalid-default-exports-step/output-workflow.stderr

@@ -0,0 +1,7 @@
+  x Only functions can be exported from a "use step" file
+   ,-[input.js:4:1]
+ 3 |     // Error: default class export
+ 4 | ,-> export default class MyClass {
+ 5 | |     method() {}
+ 6 | `-> }
+   `----

packages/swc-plugin-workflow/transform/tests/errors/invalid-exports/input.js

@@ -6,13 +6,23 @@ export class MyClass {
   method() {}
 }
 export * from './other';
+export let uninitVar;
 
-// This is ok - sync functions are allowed in "use step" files
+// Local named exports also error (can't verify binding is a function)
+const helper = 'not a function';
+export { helper };
+
+// Re-export with specifiers also errors
+export { something } from './re-export';
+
+// These are ok - sync and async functions are allowed in "use step" files
 export function syncFunc() {
   return 'allowed';
 }
 
-// This is ok
 export async function validStep() {
   return 'allowed';
 }
+
+export const arrowStep = () => 'allowed';
+export const asyncArrow = async () => 'allowed';

packages/swc-plugin-workflow/transform/tests/errors/invalid-exports/output-client.js

@@ -1,17 +1,26 @@
-/**__internal_workflows{"steps":{"input.js":{"syncFunc":{"stepId":"step//./input//syncFunc"},"validStep":{"stepId":"step//./input//validStep"}}}}*/;
+/**__internal_workflows{"steps":{"input.js":{"arrowStep":{"stepId":"step//./input//arrowStep"},"asyncArrow":{"stepId":"step//./input//asyncArrow"},"syncFunc":{"stepId":"step//./input//syncFunc"},"validStep":{"stepId":"step//./input//validStep"}}}}*/;
 // These should all error - not functions
 export const value = 42;
 export class MyClass {
     method() {}
 }
 export * from './other';
-// This is ok - sync functions are allowed in "use step" files
+export let uninitVar;
+// Local named exports also error (can't verify binding is a function)
+const helper = 'not a function';
+export { helper };
+// Re-export with specifiers also errors
+export { something } from './re-export';
+// These are ok - sync and async functions are allowed in "use step" files
 export function syncFunc() {
     return 'allowed';
 }
 syncFunc.stepId = "step//./input//syncFunc";
-// This is ok
 export async function validStep() {
     return 'allowed';
 }
 validStep.stepId = "step//./input//validStep";
+export const arrowStep = ()=>'allowed';
+arrowStep.stepId = "step//./input//arrowStep";
+export const asyncArrow = async ()=>'allowed';
+asyncArrow.stepId = "step//./input//asyncArrow";

packages/swc-plugin-workflow/transform/tests/errors/invalid-exports/output-client.stderr

@@ -0,0 +1,40 @@
+  x Only functions can be exported from a "use step" file
+   ,-[input.js:4:1]
+ 3 | // These should all error - not functions
+ 4 | export const value = 42;
+   : ^^^^^^^^^^^^^^^^^^^^^^^^
+ 5 | export class MyClass {
+   `----
+  x Only functions can be exported from a "use step" file
+   ,-[input.js:5:1]
+ 4 |     export const value = 42;
+ 5 | ,-> export class MyClass {
+ 6 | |     method() {}
+ 7 | `-> }
+ 8 |     export * from './other';
+   `----
+  x Only functions can be exported from a "use step" file
+   ,-[input.js:8:1]
+ 7 | }
+ 8 | export * from './other';
+   : ^^^^^^^^^^^^^^^^^^^^^^^^
+ 9 | export let uninitVar;
+   `----
+  x Only functions can be exported from a "use step" file
+   ,-[input.js:9:1]
+ 8 | export * from './other';
+ 9 | export let uninitVar;
+   : ^^^^^^^^^^^^^^^^^^^^^
+   `----
+  x Only functions can be exported from a "use step" file
+    ,-[input.js:13:1]
+ 12 | const helper = 'not a function';
+ 13 | export { helper };
+    : ^^^^^^^^^^^^^^^^^^
+    `----
+  x Only functions can be exported from a "use step" file
+    ,-[input.js:16:1]
+ 15 | // Re-export with specifiers also errors
+ 16 | export { something } from './re-export';
+    : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+    `----

packages/swc-plugin-workflow/transform/tests/errors/invalid-exports/output-step.js

@@ -1,11 +1,17 @@
-/**__internal_workflows{"steps":{"input.js":{"syncFunc":{"stepId":"step//./input//syncFunc"},"validStep":{"stepId":"step//./input//validStep"}}}}*/;
+/**__internal_workflows{"steps":{"input.js":{"arrowStep":{"stepId":"step//./input//arrowStep"},"asyncArrow":{"stepId":"step//./input//asyncArrow"},"syncFunc":{"stepId":"step//./input//syncFunc"},"validStep":{"stepId":"step//./input//validStep"}}}}*/;
 // These should all error - not functions
 export const value = 42;
 export class MyClass {
     method() {}
 }
 export * from './other';
-// This is ok - sync functions are allowed in "use step" files
+export let uninitVar;
+// Local named exports also error (can't verify binding is a function)
+const helper = 'not a function';
+export { helper };
+// Re-export with specifiers also errors
+export { something } from './re-export';
+// These are ok - sync and async functions are allowed in "use step" files
 export function syncFunc() {
     return 'allowed';
 }
@@ -14,7 +20,6 @@ export function syncFunc() {
     __wf_reg.set(__wf_id, __wf_fn);
     __wf_fn.stepId = __wf_id;
 })(syncFunc, "step//./input//syncFunc");
-// This is ok
 export async function validStep() {
     return 'allowed';
 }
@@ -23,3 +28,15 @@ export async function validStep() {
     __wf_reg.set(__wf_id, __wf_fn);
     __wf_fn.stepId = __wf_id;
 })(validStep, "step//./input//validStep");
+export const arrowStep = ()=>'allowed';
+(function(__wf_fn, __wf_id) {
+    var __wf_sym = Symbol.for("@workflow/core//registeredSteps"), __wf_reg = globalThis[__wf_sym] || (globalThis[__wf_sym] = new Map());
+    __wf_reg.set(__wf_id, __wf_fn);
+    __wf_fn.stepId = __wf_id;
+})(arrowStep, "step//./input//arrowStep");
+export const asyncArrow = async ()=>'allowed';
+(function(__wf_fn, __wf_id) {
+    var __wf_sym = Symbol.for("@workflow/core//registeredSteps"), __wf_reg = globalThis[__wf_sym] || (globalThis[__wf_sym] = new Map());
+    __wf_reg.set(__wf_id, __wf_fn);
+    __wf_fn.stepId = __wf_id;
+})(asyncArrow, "step//./input//asyncArrow");