Summary

Currently, Scoping::no_side_effects only tracks symbols annotated with @__NO_SIDE_EFFECTS__ (the .pure flag).

Propose also marking functions with empty bodies and simple parameters (no destructuring, no defaults).

This would allow Rolldown to use scoping.no_side_effects() as the single source of truth for "calling this function has no side effects", eliminating the need for a separate SymbolRefFlags::SideEffectsFreeFunction flag that currently duplicates this logic in the bundler.

Current Rolldown duplication

Rolldown maintains its own empty-function detection in rolldown_ecmascript_utils/src/extensions/ast_ext/function.rs and uses it alongside .pure in the scanner (ast_scanner/mod.rs#L738-L748):

let is_side_effect_free_function = decl
  .init
  .as_ref()
  .map(|expr| match expr {
    Expression::FunctionExpression(func) => func.is_side_effect_free() || func.pure,
    Expression::ArrowFunctionExpression(func) => {
      func.is_side_effect_free() || func.pure
    }
    _ => false,
  })
  .unwrap_or(false);

This sets SymbolRefFlags::SideEffectsFreeFunction, which is then consumed during cross-module optimization (cross_module_optimization.rs#L62-L85) to identify pure function calls across module boundaries.

Proposed change

In crates/oxc_semantic/src/binder.rs:

Function::bind (line ~155):

// Current
if self.pure {
    builder.scoping.no_side_effects.insert(symbol_id);
}

// Proposed
if self.pure || is_empty_and_simple_params(self) {
    builder.scoping.no_side_effects.insert(symbol_id);
}

VariableDeclarator::bind (line ~101): same extension for function expression / arrow function initializers.

Where "empty and simple params" means:

• Body has no statements (or None)
• All params are BindingIdentifier with no initializer

Industry precedent

3 out of 4 major tools treat empty function calls as side-effect-free. Verified with actual test runs against Rollup 4.59 and esbuild 0.27.