feat(minifier): remove pure function calls when the return value is not used

Author: sapphi-red

crates/oxc_minifier/src/peephole/mod.rs

@@ -132,7 +132,7 @@ impl<'a> Traverse<'a, MinifierState<'a>> for PeepholeOptimizations {
 
     fn enter_statement(&mut self, stmt: &mut Statement<'a>, ctx: &mut TraverseCtx<'a>) {
         let ctx = &mut Ctx::new(ctx);
-        Self::keep_track_of_empty_functions(stmt, ctx);
+        Self::keep_track_of_pure_functions(stmt, ctx);
     }
 
     fn exit_statement(&mut self, stmt: &mut Statement<'a>, ctx: &mut TraverseCtx<'a>) {

crates/oxc_minifier/src/peephole/remove_dead_code.rs

@@ -1,7 +1,10 @@
 use oxc_allocator::{TakeIn, Vec};
 use oxc_ast::ast::*;
 use oxc_ast_visit::Visit;
-use oxc_ecmascript::{constant_evaluation::ConstantEvaluation, side_effects::MayHaveSideEffects};
+use oxc_ecmascript::{
+    constant_evaluation::{ConstantEvaluation, ConstantValue},
+    side_effects::MayHaveSideEffects,
+};
 use oxc_span::GetSpan;
 use oxc_traverse::Ancestor;
 
@@ -340,11 +343,11 @@ impl<'a> PeepholeOptimizations {
         }
     }
 
-    pub fn keep_track_of_empty_functions(stmt: &mut Statement<'a>, ctx: &mut Ctx<'a, '_>) {
+    pub fn keep_track_of_pure_functions(stmt: &mut Statement<'a>, ctx: &mut Ctx<'a, '_>) {
         match stmt {
             Statement::FunctionDeclaration(f) => {
                 if let Some(body) = &f.body {
-                    Self::try_save_empty_function(
+                    Self::try_save_pure_function(
                         f.id.as_ref(),
                         &f.params,
                         body,
@@ -359,7 +362,7 @@ impl<'a> PeepholeOptimizations {
                     if let BindingPatternKind::BindingIdentifier(id) = &d.id.kind {
                         match &d.init {
                             Some(Expression::ArrowFunctionExpression(a)) => {
-                                Self::try_save_empty_function(
+                                Self::try_save_pure_function(
                                     Some(id),
                                     &a.params,
                                     &a.body,
@@ -370,7 +373,7 @@ impl<'a> PeepholeOptimizations {
                             }
                             Some(Expression::FunctionExpression(f)) => {
                                 if let Some(body) = &f.body {
-                                    Self::try_save_empty_function(
+                                    Self::try_save_pure_function(
                                         Some(id),
                                         &f.params,
                                         body,
@@ -389,24 +392,30 @@ impl<'a> PeepholeOptimizations {
         }
     }
 
-    fn try_save_empty_function(
+    fn try_save_pure_function(
         id: Option<&BindingIdentifier<'a>>,
         params: &FormalParameters<'a>,
         body: &FunctionBody<'a>,
         r#async: bool,
         generator: bool,
         ctx: &mut Ctx<'a, '_>,
     ) {
-        if !body.is_empty() || r#async || generator {
+        if r#async || generator {
             return;
         }
         // `function foo({}) {} foo(null)` is runtime type error.
         if !params.items.iter().all(|pat| pat.pattern.kind.is_binding_identifier()) {
             return;
         }
+        if body.statements.iter().any(|stmt| stmt.may_have_side_effects(ctx)) {
+            return;
+        }
         let Some(symbol_id) = id.and_then(|id| id.symbol_id.get()) else { return };
         if ctx.scoping().get_resolved_references(symbol_id).all(|r| r.flags().is_read_only()) {
-            ctx.state.empty_functions.insert(symbol_id);
+            ctx.state.pure_functions.insert(
+                symbol_id,
+                if body.is_empty() { Some(ConstantValue::Undefined) } else { None },
+            );
         }
     }
 
@@ -415,7 +424,10 @@ impl<'a> PeepholeOptimizations {
         if let Expression::Identifier(ident) = &e.callee {
             if let Some(reference_id) = ident.reference_id.get() {
                 if let Some(symbol_id) = ctx.scoping().get_reference(reference_id).symbol_id() {
-                    if ctx.state.empty_functions.contains(&symbol_id) {
+                    if matches!(
+                        ctx.state.pure_functions.get(&symbol_id),
+                        Some(Some(ConstantValue::Undefined))
+                    ) {
                         let mut exprs =
                             Self::fold_arguments_into_needed_expressions(&mut e.arguments, ctx);
                         if exprs.is_empty() {

crates/oxc_minifier/src/peephole/remove_unused_expression.rs

@@ -521,7 +521,19 @@ impl<'a> PeepholeOptimizations {
     fn remove_unused_call_expr(e: &mut Expression<'a>, ctx: &mut Ctx<'a, '_>) -> bool {
         let Expression::CallExpression(call_expr) = e else { return false };
 
-        if call_expr.pure && ctx.annotations() {
+        let is_pure = {
+            (call_expr.pure && ctx.annotations())
+                || (if let Expression::Identifier(id) = &call_expr.callee
+                    && let Some(symbol_id) =
+                        ctx.scoping().get_reference(id.reference_id()).symbol_id()
+                {
+                    ctx.state.pure_functions.contains_key(&symbol_id)
+                } else {
+                    false
+                })
+        };
+
+        if is_pure {
             let mut exprs =
                 Self::fold_arguments_into_needed_expressions(&mut call_expr.arguments, ctx);
             if exprs.is_empty() {
@@ -954,6 +966,9 @@ mod test {
         test("/* @__PURE__ */ new Foo(a)", "a");
         test("true && /* @__PURE__ */ noEffect()", "");
         test("false || /* @__PURE__ */ noEffect()", "");
+
+        test("var foo = () => 1; foo(), foo()", "var foo = () => 1");
+        test_same("var foo = () => { bar() }; foo(), foo()");
     }
 
     #[test]

crates/oxc_minifier/src/state.rs

@@ -1,4 +1,5 @@
-use rustc_hash::FxHashSet;
+use oxc_ecmascript::constant_evaluation::ConstantValue;
+use rustc_hash::FxHashMap;
 
 use oxc_span::SourceType;
 use oxc_syntax::symbol::SymbolId;
@@ -10,8 +11,8 @@ pub struct MinifierState<'a> {
 
     pub options: CompressOptions,
 
-    /// Function declarations that are empty
-    pub empty_functions: FxHashSet<SymbolId>,
+    /// The return value of function declarations that are pure
+    pub pure_functions: FxHashMap<SymbolId, Option<ConstantValue<'a>>>,
 
     pub symbol_values: SymbolValues<'a>,
 
@@ -23,7 +24,7 @@ impl MinifierState<'_> {
         Self {
             source_type,
             options,
-            empty_functions: FxHashSet::default(),
+            pure_functions: FxHashMap::default(),
             symbol_values: SymbolValues::default(),
             changed: false,
         }