From 896ff860f9b860046e9968284490abc2db84ef6c Mon Sep 17 00:00:00 2001
From: Boshen <1430279+Boshen@users.noreply.github.com>
Date: Thu, 28 Nov 2024 10:37:41 +0000
Subject: [PATCH] fix(minifier): do not fold if statement block with lexical
 declaration (#7519)

---
 crates/oxc_allocator/src/vec.rs               | 11 +++++-----
 .../peephole_minimize_conditions.rs           | 21 +++++++++++++++----
 2 files changed, 22 insertions(+), 10 deletions(-)

diff --git a/crates/oxc_allocator/src/vec.rs b/crates/oxc_allocator/src/vec.rs
index 057333aae5c36..bb098c9c66fd3 100644
--- a/crates/oxc_allocator/src/vec.rs
+++ b/crates/oxc_allocator/src/vec.rs
@@ -223,12 +223,11 @@ impl<'alloc, T> ops::Index<usize> for Vec<'alloc, T> {
     }
 }
 
-// Unused right now.
-// impl<'alloc, T> ops::IndexMut<usize> for Vec<'alloc, T> {
-// fn index_mut(&mut self, index: usize) -> &mut Self::Output {
-// self.0.index_mut(index)
-// }
-// }
+impl<'alloc, T> ops::IndexMut<usize> for Vec<'alloc, T> {
+    fn index_mut(&mut self, index: usize) -> &mut Self::Output {
+        self.0.index_mut(index)
+    }
+}
 
 #[cfg(any(feature = "serialize", test))]
 impl<'alloc, T> Serialize for Vec<'alloc, T>
diff --git a/crates/oxc_minifier/src/ast_passes/peephole_minimize_conditions.rs b/crates/oxc_minifier/src/ast_passes/peephole_minimize_conditions.rs
index d29c6c8341f03..9f94bd52d2401 100644
--- a/crates/oxc_minifier/src/ast_passes/peephole_minimize_conditions.rs
+++ b/crates/oxc_minifier/src/ast_passes/peephole_minimize_conditions.rs
@@ -68,15 +68,19 @@ impl<'a> PeepholeMinimizeConditions {
     /// Duplicate logic to DCE part.
     fn try_fold_if_block_one(&mut self, if_stmt: &mut IfStatement<'a>, ctx: &mut TraverseCtx<'a>) {
         if let Statement::BlockStatement(block) = &mut if_stmt.consequent {
-            if block.body.len() == 1 {
+            if block.body.len() == 1
+                && !matches!(&block.body[0], Statement::VariableDeclaration(decl) if !decl.kind.is_var())
+            {
                 self.changed = true;
-                if_stmt.consequent = ctx.ast.move_statement(block.body.first_mut().unwrap());
+                if_stmt.consequent = ctx.ast.move_statement(&mut block.body[0]);
             }
         }
         if let Some(Statement::BlockStatement(block)) = &mut if_stmt.alternate {
-            if block.body.len() == 1 {
+            if block.body.len() == 1
+                && !matches!(&block.body[0], Statement::VariableDeclaration(decl) if !decl.kind.is_var())
+            {
                 self.changed = true;
-                if_stmt.alternate = Some(ctx.ast.move_statement(block.body.first_mut().unwrap()));
+                if_stmt.alternate = Some(ctx.ast.move_statement(&mut block.body[0]));
             }
         }
     }
@@ -241,6 +245,15 @@ mod test {
         fold_same("function f(){foo()}");
         fold_same("switch(x){case y: foo()}");
         fold_same("try{foo()}catch(ex){bar()}finally{baz()}");
+
+        // Dot not fold `let` and `const`.
+        // Lexical declaration cannot appear in a single-statement context.
+        fold_same("if (foo) { const bar = 1 } else { const baz = 1 }");
+        fold_same("if (foo) { let bar = 1 } else { let baz = 1 }");
+        fold(
+            "if (foo) { var bar = 1 } else { var baz = 1 }",
+            "if (foo) var bar = 1; else var baz = 1;",
+        );
     }
 
     /** Try to minimize returns */