feat(es/minifier): Consider arguments while dropping unused assignments (#3775)

swc_ecma_minifier:
 - `analyze`: Track usage of `arguments`.

.github/workflows/bot-pr.yml

@@ -1,6 +1,7 @@
 name: Maintenance
 
 on:
+  pull_request:
   pull_request_review:
 
 jobs:

crates/swc_ecma_minifier/src/analyzer/mod.rs

@@ -48,7 +48,9 @@ where
     };
     n.visit_with(&mut v);
     let top_scope = v.scope;
-    v.data.top_scope().merge(top_scope, false);
+    v.data.top_scope().merge(top_scope.clone(), false);
+
+    v.data.scope(SyntaxContext::empty()).merge(top_scope, false);
 
     v.data
 }
@@ -138,6 +140,7 @@ pub(crate) enum ScopeKind {
 pub(crate) struct ScopeData {
     pub has_with_stmt: bool,
     pub has_eval_call: bool,
+    pub used_arguments: bool,
 }
 
 /// Analyzed info of a whole program we are working on.
@@ -218,6 +221,10 @@ where
     }
 
     fn report_usage(&mut self, i: &Ident, is_assign: bool) {
+        if i.sym == js_word!("arguments") {
+            self.scope.mark_used_arguments();
+        }
+
         self.data.report_usage(self.ctx, i, is_assign)
     }
 
@@ -253,9 +260,7 @@ where
 
             match &n.body {
                 BlockStmtOrExpr::BlockStmt(body) => {
-                    // We use visit_children_with instead of visit_with to bypass block scope
-                    // handler.
-                    body.visit_children_with(child);
+                    body.visit_with(child);
                 }
                 BlockStmtOrExpr::Expr(body) => {
                     body.visit_with(child);

crates/swc_ecma_minifier/src/analyzer/storage/mod.rs

@@ -30,11 +30,13 @@ pub(crate) trait Storage: Sized + Default {
     ) -> &mut Self::VarData;
 }
 
-pub(crate) trait ScopeDataLike: Sized + Default {
+pub(crate) trait ScopeDataLike: Sized + Default + Clone {
     fn add_declared_symbol(&mut self, id: &Ident);
 
     fn merge(&mut self, other: Self, is_child: bool);
 
+    fn mark_used_arguments(&mut self);
+
     fn mark_eval_called(&mut self);
 
     fn mark_with_stmt(&mut self);

crates/swc_ecma_minifier/src/analyzer/storage/normal.rs

@@ -164,6 +164,11 @@ impl ScopeDataLike for ScopeData {
     fn merge(&mut self, other: Self, _: bool) {
         self.has_with_stmt |= other.has_with_stmt;
         self.has_eval_call |= other.has_eval_call;
+        self.used_arguments |= other.used_arguments;
+    }
+
+    fn mark_used_arguments(&mut self) {
+        self.used_arguments = true;
     }
 
     fn mark_eval_called(&mut self) {

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

@@ -440,6 +440,20 @@ where
             return;
         }
 
+        let used_arguments = self
+            .data
+            .as_ref()
+            .map(|data| {
+                data.scopes.get(&self.ctx.scope).unwrap_or_else(|| {
+                    unreachable!(
+                        "scope should exist\nScopes: {:?};\nCtxt: {:?}",
+                        data.scopes, self.ctx.scope
+                    )
+                })
+            })
+            .map(|scope| scope.used_arguments)
+            .unwrap_or(false);
+
         if cfg!(feature = "debug") {
             tracing::trace!(
                 "unused: drop_unused_assignments: Target: `{}`",
@@ -480,7 +494,11 @@ where
                         .as_ref()
                         .and_then(|data| data.vars.get(&i.to_id()))
                     {
-                        if var.is_fn_local && var.usage_count == 0 && var.declared {
+                        if var.is_fn_local
+                            && var.usage_count == 0
+                            && var.declared
+                            && (!var.declared_as_fn_param || !used_arguments)
+                        {
                             tracing::debug!(
                                 "unused: Dropping assignment to var '{}{:?}', which is never used",
                                 i.id.sym,

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

@@ -83,6 +83,15 @@ where
     /// RAII guard to change context temporarically
     #[inline]
     pub(super) fn with_ctx(&mut self, ctx: Ctx) -> WithCtx<'_, 'b, M> {
+        if cfg!(debug_assertions) {
+            let scope_ctxt = ctx.scope;
+            if self.ctx.scope != scope_ctxt {
+                if let Some(data) = &self.data {
+                    data.scopes.get(&scope_ctxt).expect("scope not found");
+                }
+            }
+        }
+
         let orig_ctx = self.ctx;
         self.ctx = ctx;
         WithCtx {

crates/swc_ecma_minifier/tests/golden.txt

@@ -1,4 +1,6 @@
 arguments/arguments_and_destructuring_1/input.js
+arguments/arguments_and_destructuring_2/input.js
+arguments/arguments_and_destructuring_3/input.js
 arguments/arguments_in_arrow_func_1/input.js
 arguments/arguments_in_arrow_func_2/input.js
 arguments/modified_strict/input.js

crates/swc_ecma_minifier/tests/postponed.txt

@@ -1,5 +1,3 @@
-arguments/arguments_and_destructuring_2/input.js
-arguments/arguments_and_destructuring_3/input.js
 arguments/duplicate_parameter_with_arguments/input.js
 arguments/issue_687/input.js
 arguments/modified/input.js