fix(es/compat): Fix loose mode of the spread pass (#7760)

Author: isker

crates/swc/tests/fixture/issues-7xxx/7736/case1/input/.swcrc

@@ -0,0 +1,11 @@
+{
+    "jsc": {
+        "parser": {
+            "syntax": "ecmascript",
+            "jsx": false
+        },
+        "target": "es5",
+        "loose": true
+    },
+    "isModule": false
+}

crates/swc/tests/fixture/issues-7xxx/7736/case1/input/index.js

@@ -0,0 +1,11 @@
+[
+  1,
+  2,
+  ...['foo', 3],
+  4,
+  ...['bar', 5],
+  ['baz', 6],
+  7,
+  8,
+  9,
+]

crates/swc/tests/fixture/issues-7xxx/7736/case1/output/index.js

@@ -0,0 +1,20 @@
+[].concat([
+    1,
+    2
+], [
+    "foo",
+    3
+], [
+    4
+], [
+    "bar",
+    5
+], [
+    [
+        "baz",
+        6
+    ],
+    7,
+    8,
+    9
+]);

crates/swc_ecma_transforms_compat/src/es2015/spread.rs

@@ -251,12 +251,45 @@ impl Spread {
             };
         }
 
-        // shorthand [].concat(arr1, arr2) shoule be used under loose mode
+        // Shorthand [].concat(arr1, arr2) should be used under loose mode.
+        // Array.prototype.concat has the lovely feature by which arrays passed
+        // to it are depth-1 flattened. This effectively implements the loose
+        // spread. But if any arrays that are not being spread are present, they
+        // will be incorrectly flattened. The solution is to wrap every
+        // contiguous slice of non-spread args in an array, which will protect
+        // array args from being flattened.
         if self.c.loose {
             let mut arg_list = vec![];
+            let mut current_elems = vec![];
             for arg in args.flatten() {
                 let expr = arg.expr;
-                arg_list.push(expr.as_arg());
+                match arg.spread {
+                    Some(_) => {
+                        if !current_elems.is_empty() {
+                            arg_list.push(
+                                ArrayLit {
+                                    span: DUMMY_SP,
+                                    elems: current_elems,
+                                }
+                                .as_arg(),
+                            );
+                            current_elems = vec![];
+                        }
+                        arg_list.push(expr.as_arg());
+                    }
+                    None => {
+                        current_elems.push(Some(expr.as_arg()));
+                    }
+                }
+            }
+            if !current_elems.is_empty() {
+                arg_list.push(
+                    ArrayLit {
+                        span: DUMMY_SP,
+                        elems: current_elems,
+                    }
+                    .as_arg(),
+                );
             }
 
             return Expr::Call(CallExpr {