fix(minifier): Preserve init variable declarations when removing for statements during DCE (#6551)

- Closes: #6547

---------

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

crates/oxc_ast/src/ast_builder_impl.rs

@@ -112,6 +112,20 @@ impl<'a> AstBuilder<'a> {
         mem::replace(decl, empty_decl)
     }
 
+    #[inline]
+    pub fn move_variable_declaration(
+        self,
+        decl: &mut VariableDeclaration<'a>,
+    ) -> VariableDeclaration<'a> {
+        let empty_decl = self.variable_declaration(
+            Span::default(),
+            VariableDeclarationKind::Var,
+            self.vec(),
+            false,
+        );
+        mem::replace(decl, empty_decl)
+    }
+
     #[inline]
     pub fn move_vec<T>(self, vec: &mut Vec<'a, T>) -> Vec<'a, T> {
         mem::replace(vec, self.vec())

crates/oxc_minifier/src/ast_passes/peephole_remove_dead_code.rs

@@ -203,11 +203,26 @@ impl<'a, 'b> PeepholeRemoveDeadCode {
                 // Check vars in statement
                 let mut keep_var = KeepVar::new(ctx.ast);
                 keep_var.visit_statement(&for_stmt.body);
-                Some(
+
-                    keep_var
+                let mut var_decl = keep_var.get_variable_declaration();
-                        .get_variable_declaration_statement()
+
-                        .unwrap_or_else(|| ctx.ast.statement_empty(SPAN)),
+                if let Some(ForStatementInit::VariableDeclaration(var_init)) = &mut for_stmt.init {
-                )
+                    if var_init.kind.is_var() {
+                        if let Some(var_decl) = &mut var_decl {
+                            var_decl
+                                .declarations
+                                .splice(0..0, ctx.ast.move_vec(&mut var_init.declarations));
+                        } else {
+                            var_decl = Some(ctx.ast.move_variable_declaration(var_init));
+                        }
+                    }
+                }
+
+                var_decl
+                    .map(|var_decl| {
+                        ctx.ast.statement_declaration(ctx.ast.declaration_from_variable(var_decl))
+                    })
+                    .or_else(|| Some(ctx.ast.statement_empty(SPAN)))
             }
             Some(true) => {
                 // Remove the test expression.
@@ -350,6 +365,10 @@ mod test {
         // Make sure it plays nice with minimizing
         fold("for(;false;) { foo(); continue }", "");
 
+        fold("for (var { c, x: [d] } = {}; 0;);", "var { c, x: [d] } = {};");
+        fold("for (var se = [1, 2]; false;);", "var se = [1, 2];");
+        fold("for (var se = [1, 2]; false;) { var a = 0; }", "var se = [1, 2], a;");
+
         // fold("l1:for(;false;) {  }", "");
     }
 

crates/oxc_minifier/src/keep_var.rs

@@ -34,19 +34,21 @@ impl<'a> Visit<'a> for KeepVar<'a> {
             // match_module_declaration!(Statement) => {
             // visitor.visit_module_declaration(it.to_module_declaration())
             // }
-            Statement::VariableDeclaration(decl) => {
+            Statement::VariableDeclaration(decl) => self.visit_variable_declaration(decl),
-                if decl.kind.is_var() {
+            _ => {}
-                    decl.bound_names(&mut |ident| {
+        }
+    }
+
+    fn visit_variable_declaration(&mut self, it: &VariableDeclaration<'a>) {
+        if it.kind.is_var() {
+            it.bound_names(&mut |ident| {
                 self.vars.push((ident.name.clone(), ident.span));
             });
-                    if decl.has_init() {
+            if it.has_init() {
                 self.all_hoisted = false;
             }
         }
     }
-            _ => {}
-        }
-    }
 }
 
 impl<'a> KeepVar<'a> {
@@ -58,7 +60,7 @@ impl<'a> KeepVar<'a> {
         self.all_hoisted
     }
 
-    pub fn get_variable_declaration_statement(self) -> Option<Statement<'a>> {
+    pub fn get_variable_declaration(self) -> Option<VariableDeclaration<'a>> {
         if self.vars.is_empty() {
             return None;
         }
@@ -71,7 +73,13 @@ impl<'a> KeepVar<'a> {
         }));
 
         let decl = self.ast.variable_declaration(SPAN, kind, decls, false);
-        let stmt = self.ast.statement_declaration(self.ast.declaration_from_variable(decl));
+        Some(decl)
+    }
+
+    pub fn get_variable_declaration_statement(self) -> Option<Statement<'a>> {
+        let stmt = self.ast.statement_declaration(
+            self.ast.declaration_from_variable(self.get_variable_declaration()?),
+        );
         Some(stmt)
     }
 }