fix(es/compat): Preserve spread in the generator pass (#8401)

**Related issue:**
 - Closes #8397

crates/swc/tests/fixture/issues-8xxx/8397/input/.swcrc

@@ -0,0 +1,21 @@
+{
+    "jsc": {
+        "parser": {
+            "syntax": "typescript",
+            "tsx": false
+        },
+        "loose": false,
+        "minify": {
+            "compress": false,
+            "mangle": false
+        }
+    },
+    "module": {
+        "type": "es6"
+    },
+    "minify": false,
+    "isModule": true,
+    "env": {
+        "targets": "Chrome >= 46"
+    }
+}

crates/swc/tests/fixture/issues-8xxx/8397/input/index.js

@@ -0,0 +1,9 @@
+function f() {
+    const a = [1, 2, 3];
+    const b = [...a, 4, 5];
+}
+
+async function af() {
+    const a = [1, 2, 3];
+    const b = [...a, 4, 5];
+}

crates/swc/tests/fixture/issues-8xxx/8397/output/index.js

@@ -0,0 +1,38 @@
+import { _ as _async_to_generator } from "@swc/helpers/_/_async_to_generator";
+import { _ as _ts_generator } from "@swc/helpers/_/_ts_generator";
+function f() {
+    var a = [
+        1,
+        2,
+        3
+    ];
+    var b = [
+        ...a,
+        4,
+        5
+    ];
+}
+function af() {
+    return _af.apply(this, arguments);
+}
+function _af() {
+    _af = _async_to_generator(function() {
+        var a, b;
+        return _ts_generator(this, function(_state) {
+            a = [
+                1,
+                2,
+                3
+            ];
+            b = [
+                ...a,
+                4,
+                5
+            ];
+            return [
+                2
+            ];
+        });
+    });
+    return _af.apply(this, arguments);
+}

crates/swc_ecma_compat_es2015/src/generator.rs

@@ -1098,7 +1098,7 @@ impl Generator {
             );
         }
 
-        let mut expressions = elements
+        let expressions = elements
             .iter_mut()
             .skip(num_initial_elements)
             .map(|v| v.take())
@@ -1114,11 +1114,7 @@ impl Generator {
                     spread: None,
                     expr: Box::new(Expr::Array(ArrayLit {
                         span: DUMMY_SP,
-                        elems: expressions
-                            .take()
-                            .into_iter()
-                            .map(|expr| Some(ExprOrSpread { spread: None, expr }))
-                            .collect(),
+                        elems: expressions,
                     })),
                 }],
                 type_args: Default::default(),
@@ -1130,24 +1126,19 @@ impl Generator {
                     .take()
                     .into_iter()
                     .map(Some)
-                    .chain(
-                        expressions
-                            .take()
-                            .into_iter()
-                            .map(|expr| Some(ExprOrSpread { spread: None, expr })),
-                    )
+                    .chain(expressions)
                     .collect(),
             })
         }
     }
 
     fn reduce_element(
         &mut self,
-        mut expressions: Vec<Box<Expr>>,
+        mut expressions: Vec<Option<ExprOrSpread>>,
         mut element: Option<ExprOrSpread>,
         leading_element: &mut Option<ExprOrSpread>,
         temp: &mut Option<Ident>,
-    ) -> Vec<Box<Expr>> {
+    ) -> Vec<Option<ExprOrSpread>> {
         if contains_yield(&element) && !expressions.is_empty() {
             let has_assigned_temp = temp.is_some();
             if temp.is_none() {
@@ -1166,11 +1157,7 @@ impl Generator {
                             .as_callee(),
                         args: vec![Box::new(Expr::Array(ArrayLit {
                             span: DUMMY_SP,
-                            elems: expressions
-                                .take()
-                                .into_iter()
-                                .map(|expr| Some(ExprOrSpread { spread: None, expr }))
-                                .collect(),
+                            elems: expressions.take(),
                         }))
                         .as_arg()],
                         type_args: Default::default(),
@@ -1183,12 +1170,7 @@ impl Generator {
                                 .take()
                                 .into_iter()
                                 .map(Some)
-                                .chain(
-                                    expressions
-                                        .take()
-                                        .into_iter()
-                                        .map(|expr| Some(ExprOrSpread { spread: None, expr })),
-                                )
+                                .chain(expressions.take())
                                 .collect(),
                         }
                         .into(),
@@ -1200,7 +1182,9 @@ impl Generator {
         }
 
         element.visit_mut_with(self);
-        expressions.extend(element.map(|v| v.expr));
+        if element.is_some() {
+            expressions.push(element);
+        }
         expressions
     }