perf(transformer): nullish coalescing operator transform use SparseStack (#5942)

Use `SparseStack` (introduced in #5940) to store the stack of blocks which may need a `var _temp;` statement added to them. This reduces the memory required for the stack, on assumption that most blocks won't need a `var` statement.

crates/oxc_transformer/src/es2020/mod.rs

@@ -31,6 +31,13 @@ impl<'a> ES2020<'a> {
 }
 
 impl<'a> Traverse<'a> for ES2020<'a> {
+    #[inline] // Inline because it's no-op in release mode
+    fn exit_program(&mut self, program: &mut Program<'a>, ctx: &mut TraverseCtx<'a>) {
+        if self.options.nullish_coalescing_operator {
+            self.nullish_coalescing_operator.exit_program(program, ctx);
+        }
+    }
+
     fn enter_statements(
         &mut self,
         statements: &mut Vec<'a, Statement<'a>>,

crates/oxc_transformer/src/es2020/nullish_coalescing_operator.rs

@@ -35,22 +35,31 @@ use oxc_span::SPAN;
 use oxc_syntax::operator::{AssignmentOperator, BinaryOperator, LogicalOperator};
 use oxc_traverse::{Ancestor, Traverse, TraverseCtx};
 
-use crate::context::Ctx;
+use crate::{context::Ctx, helpers::stack::SparseStack};
 
 pub struct NullishCoalescingOperator<'a> {
     _ctx: Ctx<'a>,
-    var_declarations: std::vec::Vec<Vec<'a, VariableDeclarator<'a>>>,
+    var_declarations: SparseStack<Vec<'a, VariableDeclarator<'a>>>,
 }
 
 impl<'a> NullishCoalescingOperator<'a> {
     pub fn new(ctx: Ctx<'a>) -> Self {
-        Self { _ctx: ctx, var_declarations: vec![] }
+        Self { _ctx: ctx, var_declarations: SparseStack::new() }
     }
 }
 
 impl<'a> Traverse<'a> for NullishCoalescingOperator<'a> {
-    fn enter_statements(&mut self, _stmts: &mut Vec<'a, Statement<'a>>, ctx: &mut TraverseCtx<'a>) {
+    #[inline] // Inline because it's no-op in release mode
-        self.var_declarations.push(ctx.ast.vec());
+    fn exit_program(&mut self, _program: &mut Program<'a>, _ctx: &mut TraverseCtx<'a>) {
+        debug_assert!(self.var_declarations.is_empty());
+    }
+
+    fn enter_statements(
+        &mut self,
+        _stmts: &mut Vec<'a, Statement<'a>>,
+        _ctx: &mut TraverseCtx<'a>,
+    ) {
+        self.var_declarations.push(None);
     }
 
     fn exit_statements(
@@ -59,9 +68,7 @@ impl<'a> Traverse<'a> for NullishCoalescingOperator<'a> {
         ctx: &mut TraverseCtx<'a>,
     ) {
         if let Some(declarations) = self.var_declarations.pop() {
-            if declarations.is_empty() {
+            debug_assert!(!declarations.is_empty());
-                return;
-            }
             let variable = ctx.ast.alloc_variable_declaration(
                 SPAN,
                 VariableDeclarationKind::Var,
@@ -151,8 +158,7 @@ impl<'a> Traverse<'a> for NullishCoalescingOperator<'a> {
         } else {
             let kind = VariableDeclarationKind::Var;
             self.var_declarations
-                .last_mut()
+                .get_mut_or_init(|| ctx.ast.vec())
-                .unwrap()
                 .push(ctx.ast.variable_declarator(SPAN, kind, id, None, false));
         }
 

crates/oxc_transformer/src/helpers/stack.rs

@@ -49,7 +49,7 @@ impl<T> SparseStack<T> {
         if has_value {
             debug_assert!(!self.values.is_empty());
             // SAFETY: Last `self.has_values` is only `true` if there's a corresponding value in `self.values`.
-            // This invariant is maintained in `push`, `take`, and `get_or_init`.
+            // This invariant is maintained in `push`, `take`, `get_or_init`, and `get_mut_or_init`.
             // We maintain it here too because we just popped from `self.has_values`, so that `true`
             // has been consumed at the same time we consume its corresponding value from `self.values`.
             let value = unsafe { self.values.pop().unwrap_unchecked() };
@@ -70,7 +70,7 @@ impl<T> SparseStack<T> {
 
             debug_assert!(!self.values.is_empty());
             // SAFETY: Last `self.has_values` is only `true` if there's a corresponding value in `self.values`.
-            // This invariant is maintained in `push`, `pop`, and `get_or_init`.
+            // This invariant is maintained in `push`, `pop`, `get_or_init`, and `get_mut_or_init`.
             // We maintain it here too because we just set last `self.has_values` to `false`
             // at the same time as we consume the corresponding value from `self.values`.
             let value = unsafe { self.values.pop().unwrap_unchecked() };
@@ -94,12 +94,31 @@ impl<T> SparseStack<T> {
 
         debug_assert!(!self.values.is_empty());
         // SAFETY: Last `self.has_values` is only `true` if there's a corresponding value in `self.values`.
-        // This invariant is maintained in `push`, `pop`, and `take`.
+        // This invariant is maintained in `push`, `pop`, `take`, and `get_mut_or_init`.
         // Here either last `self.has_values` was already `true`, or it's just been set to `true`
         // and a value pushed to `self.values` above.
         unsafe { self.values.last().unwrap_unchecked() }
     }
 
+    /// Initialize the value for top entry on the stack, if it has no value already.
+    /// Return mutable reference to value.
+    ///
+    /// # Panics
+    /// Panics if the stack is empty.
+    pub fn get_mut_or_init<I: FnOnce() -> T>(&mut self, init: I) -> &mut T {
+        let has_value = self.has_values.last_mut().unwrap();
+        if !*has_value {
+            *has_value = true;
+            self.values.push(init());
+        }
+
+        // SAFETY: Last `self.has_values` is only `true` if there's a corresponding value in `self.values`.
+        // This invariant is maintained in `push`, `pop`, `take`, and `get_or_init`.
+        // Here either last `self.has_values` was already `true`, or it's just been set to `true`
+        // and a value pushed to `self.values` above.
+        unsafe { self.values.last_mut().unwrap_unchecked() }
+    }
+
     /// Get number of entries on the stack.
     #[inline]
     pub fn len(&self) -> usize {
@@ -108,7 +127,6 @@ impl<T> SparseStack<T> {
 
     /// Returns `true` if stack is empty.
     #[inline]
-    #[expect(dead_code)]
     pub fn is_empty(&self) -> bool {
         self.has_values.is_empty()
     }

crates/oxc_transformer/src/lib.rs

@@ -132,6 +132,7 @@ impl<'a> Traverse<'a> for Transformer<'a> {
     fn exit_program(&mut self, program: &mut Program<'a>, ctx: &mut TraverseCtx<'a>) {
         self.x1_react.exit_program(program, ctx);
         self.x0_typescript.exit_program(program, ctx);
+        self.x2_es2020.exit_program(program, ctx);
         self.x3_es2015.exit_program(program, ctx);
     }