fix(es/minifier): Don't drop assignments to unused top-level variables (swc-project#7581)

**Related issue:**

 - Closes swc-project#7568

Author: kdy1

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

@@ -711,6 +711,8 @@ impl Optimizer<'_> {
                             && var.usage_count == 0
                             && var.declared
                             && (!var.declared_as_fn_param || !used_arguments || self.ctx.in_strict)
+                            && (self.options.top_level()
+                                || i.span.ctxt != self.marks.top_level_ctxt)
                         {
                             report_change!(
                                 "unused: Dropping assignment to var '{}{:?}', which is never used",

crates/swc_ecma_minifier/src/lib.rs

@@ -100,6 +100,7 @@ pub fn optimize(
     let _timer = timer!("minify");
 
     let mut marks = Marks::new();
+    marks.top_level_ctxt = SyntaxContext::empty().apply_mark(extra.top_level_mark);
     marks.unresolved_mark = extra.unresolved_mark;
 
     debug_assert_valid(&n);

crates/swc_ecma_minifier/tests/fixture/issues/7568/1/input.js

@@ -0,0 +1,28 @@
+var specific_microfront;
+(function () { // webpackBootstrap
+    "use strict";
+    var __webpack_modules__ = ({
+
+
+
+    });
+    function __webpack_require__(moduleId) {
+        var module = __webpack_module_cache__[moduleId] = {
+            id: moduleId,
+            loaded: false,
+            exports: {}
+
+        };
+
+        __webpack_modules__[moduleId].call(module.exports, module, module.exports, __webpack_require__);
+        module.loaded = true;
+        return module.exports;
+
+    }
+
+    var __webpack_exports__ = __webpack_require__("webpack/container/entry/specific_page");
+    specific_microfront = __webpack_exports__;
+
+
+})()
+    ;

crates/swc_ecma_minifier/tests/fixture/issues/7568/1/output.js

@@ -0,0 +1,13 @@
+var specific_microfront;
+!function() {
+    "use strict";
+    var __webpack_modules__ = {};
+    specific_microfront = function __webpack_require__(moduleId) {
+        var module = __webpack_module_cache__[moduleId] = {
+            id: moduleId,
+            loaded: !1,
+            exports: {}
+        };
+        return __webpack_modules__[moduleId].call(module.exports, module, module.exports, __webpack_require__), module.loaded = !0, module.exports;
+    }("webpack/container/entry/specific_page");
+}();

crates/swc_ecma_minifier/tests/fixture/issues/7568/config.json

@@ -0,0 +1,5 @@
+{
+    "defaults": true,
+    "toplevel": false,
+    "passes": 0
+}

crates/swc_ecma_usage_analyzer/src/marks.rs

@@ -1,6 +1,6 @@
 #![allow(dead_code)]
 
-use swc_common::Mark;
+use swc_common::{Mark, SyntaxContext};
 
 #[derive(Debug, Clone, Copy)]
 pub struct Marks {
@@ -47,6 +47,8 @@ pub struct Marks {
     /// preserve the side effects.
     pub fake_block: Mark,
 
+    pub top_level_ctxt: SyntaxContext,
+
     pub unresolved_mark: Mark,
 }
 
@@ -66,6 +68,7 @@ impl Marks {
             noinline: m(),
             pure: m(),
             fake_block: m(),
+            top_level_ctxt: SyntaxContext::empty().apply_mark(m()),
             unresolved_mark: m(),
         }
     }