feat(es/minifier): Drop unused rest paramters (#10857)

kdy1 · web-flow · commit 5305486d58d9 · 2025-07-15T14:03:10.000+09:00
diff --git a/.changeset/minifier-foo.md b/.changeset/minifier-foo.md
@@ -0,0 +1,6 @@
+---
+swc_ecma_minifier: patch
+swc_core: patch
+---
+
+init
diff --git a/crates/swc/tests/tsc-references/destructuringParameterDeclaration3ES5.2.minified.js b/crates/swc/tests/tsc-references/destructuringParameterDeclaration3ES5.2.minified.js
@@ -1,12 +1,5 @@
 //// [destructuringParameterDeclaration3ES5.ts]
-var E, array = [
-    1,
-    2,
-    3
-];
-!function(...a) {}([
-    ...array
-]), function(...x) {}(...array), function([a, b, [[c]]]) {}([
+!function([a, b, [[c]]]) {}([
     1,
     2,
     [
@@ -20,4 +13,4 @@ var E, array = [
     1,
     2
 ]);
-var E1 = ((E = E1 || {})[E.a = 0] = "a", E[E.b = 1] = "b", E);
+var E, E1 = ((E = E1 || {})[E.a = 0] = "a", E[E.b = 1] = "b", E);
diff --git a/crates/swc/tests/tsc-references/destructuringParameterDeclaration3ES6.2.minified.js b/crates/swc/tests/tsc-references/destructuringParameterDeclaration3ES6.2.minified.js
@@ -1,12 +1,5 @@
 //// [destructuringParameterDeclaration3ES6.ts]
-var E, array = [
-    1,
-    2,
-    3
-];
-!function(...a) {}([
-    ...array
-]), function(...x) {}(...array), function([a, b, [[c]]]) {}([
+!function([a, b, [[c]]]) {}([
     1,
     2,
     [
@@ -20,4 +13,4 @@ var E, array = [
     1,
     2
 ]);
-var E1 = ((E = E1 || {})[E.a = 0] = "a", E[E.b = 1] = "b", E);
+var E, E1 = ((E = E1 || {})[E.a = 0] = "a", E[E.b = 1] = "b", E);
diff --git a/crates/swc/tests/tsc-references/emitDefaultParametersFunctionExpressionES6.2.minified.js b/crates/swc/tests/tsc-references/emitDefaultParametersFunctionExpressionES6.2.minified.js
@@ -1,2 +1,2 @@
 //// [emitDefaultParametersFunctionExpressionES6.ts]
-!function(num, boo = !1, ...rest) {}(0);
+!function(num, boo = !1) {}(0);
diff --git a/crates/swc/tests/tsc-references/emitRestParametersFunctionExpressionES6.2.minified.js b/crates/swc/tests/tsc-references/emitRestParametersFunctionExpressionES6.2.minified.js
@@ -1,2 +1 @@
 //// [emitRestParametersFunctionExpressionES6.ts]
-!function(...rest) {}();
diff --git a/crates/swc/tests/tsc-references/iteratorSpreadInCall3.2.minified.js b/crates/swc/tests/tsc-references/iteratorSpreadInCall3.2.minified.js
@@ -10,4 +10,4 @@ class SymbolIterator {
         return this;
     }
 }
-!function(...s) {}(...new SymbolIterator);
+new SymbolIterator;
diff --git a/crates/swc/tests/tsc-references/iteratorSpreadInCall4.2.minified.js b/crates/swc/tests/tsc-references/iteratorSpreadInCall4.2.minified.js
@@ -10,4 +10,4 @@ class SymbolIterator {
         return this;
     }
 }
-!function(s1, ...s) {}(...new SymbolIterator);
+new SymbolIterator;
diff --git a/crates/swc/tests/tsc-references/iteratorSpreadInCall5.2.minified.js b/crates/swc/tests/tsc-references/iteratorSpreadInCall5.2.minified.js
@@ -21,4 +21,4 @@ class _StringIterator {
         return this;
     }
 }
-!function(...s) {}(...new SymbolIterator, ...new _StringIterator);
+new SymbolIterator, new _StringIterator;
diff --git a/crates/swc/tests/tsc-references/iteratorSpreadInCall6.2.minified.js b/crates/swc/tests/tsc-references/iteratorSpreadInCall6.2.minified.js
@@ -21,4 +21,4 @@ class _StringIterator {
         return this;
     }
 }
-!function(...s) {}(...new SymbolIterator, ...new _StringIterator);
+new SymbolIterator, new _StringIterator;
diff --git a/crates/swc/tests/tsc-references/jsDeclarationsGetterSetter.2.minified.js b/crates/swc/tests/tsc-references/jsDeclarationsGetterSetter.2.minified.js
@@ -32,7 +32,7 @@ Object.defineProperty(F.prototype, "x", {
 export class G {
 }
 Object.defineProperty(G.prototype, "x", {
-    set (...args) {}
+    set () {}
 });
 export class H {
 }
diff --git a/crates/swc/tests/tsc-references/newWithSpreadES6.2.minified.js b/crates/swc/tests/tsc-references/newWithSpreadES6.2.minified.js
diff --git a/crates/swc/tests/tsc-references/privateNameStaticAccessorsCallExpression.2.minified.js b/crates/swc/tests/tsc-references/privateNameStaticAccessorsCallExpression.2.minified.js
@@ -23,7 +23,7 @@ var _fieldFunc = {
     set: void 0
 }, _fieldFunc2 = {
     get: function() {
-        return function(a, ...b) {};
+        return function(a) {};
     },
     set: void 0
 }, _x = {
diff --git a/crates/swc/tests/tsc-references/privateNameStaticMethodCallExpression.2.minified.js b/crates/swc/tests/tsc-references/privateNameStaticMethodCallExpression.2.minified.js
@@ -16,5 +16,5 @@ class AA {
 function method() {
     this.x = 10;
 }
-function method2(a, ...b) {}
+function method2(a) {}
 AA.x = 1;
diff --git a/crates/swc/tests/tsc-references/taggedTemplateContextualTyping1.2.minified.js b/crates/swc/tests/tsc-references/taggedTemplateContextualTyping1.2.minified.js
@@ -1,3 +1,3 @@
 //// [taggedTemplateContextualTyping1.ts]
-function tempTag1(...rest) {}
+function tempTag1() {}
 tempTag1`${(x)=>(x(void 0), x)}${10}`, tempTag1`${(x)=>(x(void 0), x)}${(y)=>(y(void 0), y)}${10}`, tempTag1`${(x)=>(x(void 0), x)}${(y)=>(y(void 0), y)}${void 0}`, tempTag1`${(x)=>(x(void 0), x)}${(y)=>(y(void 0), y)}${void 0}`;
diff --git a/crates/swc/tests/tsc-references/taggedTemplateContextualTyping2.2.minified.js b/crates/swc/tests/tsc-references/taggedTemplateContextualTyping2.2.minified.js
@@ -1,3 +1,3 @@
 //// [taggedTemplateContextualTyping2.ts]
-function tempTag2(...rest) {}
+function tempTag2() {}
 tempTag2`${(x)=>(x(void 0), x)}${0}`, tempTag2`${(x)=>(x(void 0), x)}${(y)=>(y(null), y)}${"hello"}`, tempTag2`${(x)=>(x(void 0), x)}${void 0}${"hello"}`;
diff --git a/crates/swc/tests/tsc-references/taggedTemplateStringsPlainCharactersThatArePartsOfEscapes01_ES6.2.minified.js b/crates/swc/tests/tsc-references/taggedTemplateStringsPlainCharactersThatArePartsOfEscapes01_ES6.2.minified.js
@@ -1,2 +1 @@
 //// [taggedTemplateStringsPlainCharactersThatArePartsOfEscapes01_ES6.ts]
-(function(...x) {})`0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 2028 2029 0085 t v f b r n`;
diff --git a/crates/swc/tests/tsc-references/taggedTemplateStringsPlainCharactersThatArePartsOfEscapes02_ES6.2.minified.js b/crates/swc/tests/tsc-references/taggedTemplateStringsPlainCharactersThatArePartsOfEscapes02_ES6.2.minified.js
@@ -1,2 +1 @@
 //// [taggedTemplateStringsPlainCharactersThatArePartsOfEscapes02_ES6.ts]
-(function(...x) {})`0${" "}1${" "}2${" "}3${" "}4${" "}5${" "}6${" "}7${" "}8${" "}9${" "}10${" "}11${" "}12${" "}13${" "}14${" "}15${" "}16${" "}17${" "}18${" "}19${" "}20${" "}2028${" "}2029${" "}0085${" "}t${" "}v${" "}f${" "}b${" "}r${" "}n`;
diff --git a/crates/swc/tests/tsc-references/taggedTemplateStringsWithOverloadResolution1_ES6.2.minified.js b/crates/swc/tests/tsc-references/taggedTemplateStringsWithOverloadResolution1_ES6.2.minified.js
@@ -1,3 +1,3 @@
 //// [taggedTemplateStringsWithOverloadResolution1_ES6.ts]
-function foo(...stuff) {}
+function foo() {}
 foo``, foo`${1}`, foo`${1}${2}`, foo`${1}${!0}`, foo`${1}${"2"}`, foo`${1}${2}${3}`;
diff --git a/crates/swc/tests/tsc-references/taggedTemplateStringsWithOverloadResolution2_ES6.2.minified.js b/crates/swc/tests/tsc-references/taggedTemplateStringsWithOverloadResolution2_ES6.2.minified.js
@@ -1,2 +1 @@
 //// [taggedTemplateStringsWithOverloadResolution2_ES6.ts]
-(function(...stuff) {})`${1}`, (function(...stuff) {})`${1}`;
diff --git a/crates/swc/tests/tsc-references/taggedTemplateStringsWithTagNamedDeclareES6.2.minified.js b/crates/swc/tests/tsc-references/taggedTemplateStringsWithTagNamedDeclareES6.2.minified.js
@@ -1,2 +1 @@
 //// [taggedTemplateStringsWithTagNamedDeclareES6.ts]
-(function(x, ...ys) {})`Hello ${0} world!`;
diff --git a/crates/swc/tests/tsc-references/taggedTemplateStringsWithTypeErrorInFunctionExpressionsInSubstitutionExpressionES6.2.minified.js b/crates/swc/tests/tsc-references/taggedTemplateStringsWithTypeErrorInFunctionExpressionsInSubstitutionExpressionES6.2.minified.js
@@ -1,2 +1 @@
 //// [taggedTemplateStringsWithTypeErrorInFunctionExpressionsInSubstitutionExpressionES6.ts]
-(function(...rest) {})`${function(x) {}}`;
diff --git a/crates/swc_ecma_minifier/src/compress/optimize/mod.rs b/crates/swc_ecma_minifier/src/compress/optimize/mod.rs
@@ -47,6 +47,7 @@ mod inline;
 mod loops;
 mod ops;
 mod props;
+mod rest_params;
 mod sequences;
 mod strings;
 mod unused;
@@ -2137,6 +2138,10 @@ impl VisitMut for Optimizer<'_> {
                 .optimize_usage_of_arguments(n);
         }
 
+        {
+            self.with_ctx(self.ctx.clone()).drop_unused_rest_params(n);
+        }
+
         self.ctx.bit_ctx.set(BitCtx::InAsm, old_in_asm);
 
         if let Some(body) = &mut n.body {
diff --git a/crates/swc_ecma_minifier/src/compress/optimize/rest_params.rs b/crates/swc_ecma_minifier/src/compress/optimize/rest_params.rs
@@ -0,0 +1,58 @@
+use swc_ecma_ast::*;
+
+use super::Optimizer;
+
+/// Methods related to rest parameter optimization.
+impl Optimizer<'_> {
+    /// Removes unused rest parameters from functions.
+    ///
+    /// Example:
+    /// ```js
+    /// function f(a, ...b) {
+    ///     console.log(a);
+    /// }
+    /// ```
+    /// =>
+    /// ```js
+    /// function f(a) {
+    ///     console.log(a);
+    /// }
+    /// ```
+    pub(super) fn drop_unused_rest_params(&mut self, f: &mut Function) {
+        if !self.options.arguments && !self.options.unused {
+            return;
+        }
+
+        // Don't optimize if there's no rest parameter
+        if f.params.is_empty() {
+            return;
+        }
+
+        let last_param = match f.params.last() {
+            Some(p) => p,
+            None => return,
+        };
+
+        // Check if the last parameter is a rest parameter
+        let rest_pat = match &last_param.pat {
+            Pat::Rest(rest) => rest,
+            _ => return,
+        };
+
+        // Get the identifier of the rest parameter
+        let rest_id = match &*rest_pat.arg {
+            Pat::Ident(BindingIdent { id, .. }) => id.to_id(),
+            _ => return,
+        };
+
+        // Check if the rest parameter is used using ProgramData
+        if let Some(usage) = self.data.vars.get(&rest_id) {
+            // If the parameter is not referenced, we can remove it
+            if usage.ref_count == 0 {
+                self.changed = true;
+                report_change!("rest_params: Removing unused rest parameter");
+                f.params.pop();
+            }
+        }
+    }
+}
diff --git a/crates/swc_ecma_minifier/tests/TODO.txt b/crates/swc_ecma_minifier/tests/TODO.txt
@@ -163,7 +163,6 @@ harmony/array_literal_with_spread_4b/input.js
 harmony/class_name_can_be_preserved_with_reserved/input.js
 harmony/classes_extending_classes_out_of_pure_iifes/input.js
 harmony/default_assign/input.js
-harmony/expansion/input.js
 harmony/issue_2349b/input.js
 harmony/issue_2794_1/input.js
 harmony/issue_2794_2/input.js

-Original file line number
+Diff line change
@@ @@ -0,0 +1,6 @@ @@
 +---
 +swc_ecma_minifier: patch
 +swc_core: patch
 +---
++
 +init
Original file line number	Diff line number	Diff line change
`@@ -1,2 +1,2 @@`
`1`	`1`	`//// [emitDefaultParametersFunctionExpressionES6.ts]`
`2`		`-!function(num, boo = !1, ...rest) {}(0);`
	`2`	`+!function(num, boo = !1) {}(0);`
Original file line number	Diff line number	Diff line change
`@@ -1,2 +1 @@`
`1`	`1`	`//// [emitRestParametersFunctionExpressionES6.ts]`
`2`		`-!function(...rest) {}();`
Original file line number	Diff line number	Diff line change
`@@ -10,4 +10,4 @@ class SymbolIterator {`
`10`	`10`	`return this;`
`11`	`11`	`}`
`12`	`12`	`}`
`13`		`-!function(...s) {}(...new SymbolIterator);`
	`13`	`+new SymbolIterator;`
Original file line number	Diff line number	Diff line change
`@@ -21,4 +21,4 @@ class _StringIterator {`
`21`	`21`	`return this;`
`22`	`22`	`}`
`23`	`23`	`}`
`24`		`-!function(...s) {}(...new SymbolIterator, ...new _StringIterator);`
	`24`	`+new SymbolIterator, new _StringIterator;`
Original file line number	Diff line number	Diff line change
`@@ -32,7 +32,7 @@ Object.defineProperty(F.prototype, "x", {`
`32`	`32`	`export class G {`
`33`	`33`	`}`
`34`	`34`	`Object.defineProperty(G.prototype, "x", {`
`35`		`- set (...args) {}`
	`35`	`+ set () {}`
`36`	`36`	`});`
`37`	`37`	`export class H {`
`38`	`38`	`}`