feat(es/minifier): Remove unused parameters of arrow functions (#8636)

**Related issue:**

 - Closes #8626

Author: kdy1

crates/swc_ecma_minifier/src/compress/optimize/mod.rs

@@ -1454,6 +1454,8 @@ impl VisitMut for Optimizer<'_> {
 
     #[cfg_attr(feature = "debug", tracing::instrument(skip_all))]
     fn visit_mut_arrow_expr(&mut self, n: &mut ArrowExpr) {
+        self.drop_unused_arrow_params(&mut n.params);
+
         let prepend = self.prepend_stmts.take();
 
         let ctx = self.ctx;
@@ -1685,9 +1687,7 @@ impl VisitMut for Optimizer<'_> {
         match n {
             DefaultDecl::Class(_) => {}
             DefaultDecl::Fn(f) => {
-                if !self.options.keep_fargs && self.options.unused {
-                    self.drop_unused_params(&mut f.function.params);
-                }
+                self.drop_unused_params(&mut f.function.params);
             }
             DefaultDecl::TsInterfaceDecl(_) => {}
         }
@@ -1708,9 +1708,7 @@ impl VisitMut for Optimizer<'_> {
     #[cfg_attr(feature = "debug", tracing::instrument(skip_all))]
     fn visit_mut_export_decl(&mut self, n: &mut ExportDecl) {
         if let Decl::Fn(f) = &mut n.decl {
-            if !self.options.keep_fargs && self.options.unused {
-                self.drop_unused_params(&mut f.function.params);
-            }
+            self.drop_unused_params(&mut f.function.params);
         }
 
         let ctx = Ctx {
@@ -2027,9 +2025,7 @@ impl VisitMut for Optimizer<'_> {
             .entry(f.ident.to_id())
             .or_insert_with(|| FnMetadata::from(&*f.function));
 
-        if !self.options.keep_fargs && self.options.unused {
-            self.drop_unused_params(&mut f.function.params);
-        }
+        self.drop_unused_params(&mut f.function.params);
 
         let ctx = Ctx {
             top_level: false,

crates/swc_ecma_minifier/src/compress/optimize/unused.rs

@@ -132,13 +132,36 @@ impl Optimizer<'_> {
 
     #[cfg_attr(feature = "debug", tracing::instrument(skip_all))]
     pub(super) fn drop_unused_params(&mut self, params: &mut Vec<Param>) {
+        if self.options.keep_fargs || !self.options.unused {
+            return;
+        }
+
         for param in params.iter_mut().rev() {
             self.take_pat_if_unused(&mut param.pat, None, false);
 
             if !param.pat.is_invalid() {
-                return;
+                break;
+            }
+        }
+
+        params.retain(|p| !p.pat.is_invalid());
+    }
+
+    #[cfg_attr(feature = "debug", tracing::instrument(skip_all))]
+    pub(super) fn drop_unused_arrow_params(&mut self, params: &mut Vec<Pat>) {
+        if self.options.keep_fargs || !self.options.unused {
+            return;
+        }
+
+        for param in params.iter_mut().rev() {
+            self.take_pat_if_unused(param, None, false);
+
+            if !param.is_invalid() {
+                break;
             }
         }
+
+        params.retain(|p| !p.is_invalid());
     }
 
     #[cfg_attr(feature = "debug", tracing::instrument(skip_all))]

crates/swc_ecma_minifier/tests/fixture/issues/3126/1/output.js

@@ -1,12 +1,12 @@
 (()=>{
     var __webpack_modules__ = {
-        746: (__unused_webpack_module, __unused_webpack___webpack_exports__, __webpack_require__1)=>{
+        746: ()=>{
             Object.prototype.hasOwnProperty;
         }
     };
     __webpack_require__.m = __webpack_modules__;
     (()=>{
-        __webpack_require__.O = (result, chunkIds, fn, priority)=>{
+        __webpack_require__.O = (result, chunkIds)=>{
             for(var j = 0; j < chunkIds.length; j++)Object.keys(__webpack_require__.O).every((key)=>__webpack_require__.O[key](chunkIds[j]));
         };
     })();

crates/swc_ecma_minifier/tests/fixture/issues/8626/config.json

@@ -0,0 +1,4 @@
+{
+    "defaults": true,
+    "keep_fargs": false
+}

crates/swc_ecma_minifier/tests/fixture/issues/8626/input.js

@@ -0,0 +1,5 @@
+export function foo(cb) {
+    cb();
+}
+
+foo((a, b) => true);

crates/swc_ecma_minifier/tests/fixture/issues/8626/output.js

@@ -0,0 +1,4 @@
+export function foo(cb) {
+    cb();
+}
+foo(()=>!0);