feat(traverse): support automatically variable hoisting during genera…

…ting bindings

crates/oxc_traverse/scripts/lib/walk.mjs

@@ -109,11 +109,26 @@ function generateWalkForStruct(type, types) {
     // but we don't take that into account.
     // Visitor should not do that though, so maybe it's OK.
     // In final version, we should not make `scope_id` fields `Cell`s to prevent this.
+
+    // const Var = Self::Top.bits() | Self::Function.bits() | Self::ClassStaticBlock.bits() | Self::TsModuleBlock.bits();
+    let isVarHoistingScope = type.name == 'Function' || ['Top', 'Function', 'ClassStaticBlock', 'TsModuleBlock']
+      .some(flag => scopeArgs.flags.includes(flag));
+
     enterScopeCode = `
       let previous_scope_id = ctx.current_scope_id();
-      ctx.set_current_scope_id((*(${makeFieldCode(scopeIdField)})).get().unwrap());
+      ${isVarHoistingScope ? `let previous_hoist_scope_id = ctx.current_hoist_scope_id();` : ''}
+      let current_scope_id = (*(${makeFieldCode(scopeIdField)})).get().unwrap();
+      ctx.set_current_scope_id(current_scope_id);
     `;
+
+    if (isVarHoistingScope) {
+      enterScopeCode += `ctx.set_current_hoist_scope_id(current_scope_id);`;
+    }
+
     exitScopeCode = `ctx.set_current_scope_id(previous_scope_id);`;
+    if (isVarHoistingScope) {
+      exitScopeCode += `ctx.set_current_hoist_scope_id(previous_hoist_scope_id);`;
+    }
   }
 
   const fieldsCodes = visitedFields.map((field, index) => {

crates/oxc_traverse/src/context/mod.rs

@@ -3,8 +3,8 @@ use oxc_ast::{
     ast::{Expression, IdentifierReference, Statement},
     AstBuilder,
 };
-use oxc_semantic::{NodeId, ScopeTree, SymbolTable};
-use oxc_span::{Atom, CompactStr, Span, SPAN};
+use oxc_semantic::{ScopeTree, SymbolTable};
+use oxc_span::{Atom, CompactStr, Span};
 use oxc_syntax::{
     reference::{ReferenceFlags, ReferenceId},
     scope::{ScopeFlags, ScopeId},
@@ -186,6 +186,14 @@ impl<'a> TraverseCtx<'a> {
         self.scoping.current_scope_id()
     }
 
+    /// Get current scope ID.
+    ///
+    /// Shortcut for `ctx.scoping.current_hoist_scope_id`.
+    #[inline]
+    pub fn current_hoist_scope_id(&self) -> ScopeId {
+        self.scoping.current_hoist_scope_id()
+    }
+
     /// Get current scope flags.
     ///
     /// Shortcut for `ctx.scoping.current_scope_flags`.
@@ -356,12 +364,7 @@ impl<'a> TraverseCtx<'a> {
         // Get name for UID
         let name = self.generate_uid_name(name);
         let name_atom = self.ast.atom(&name);
-
-        // Add binding to scope
-        let symbol_id =
-            self.symbols_mut().create_symbol(SPAN, name.clone(), flags, scope_id, NodeId::DUMMY);
-        self.scopes_mut().add_binding(scope_id, name, symbol_id);
-
+        let symbol_id = self.scoping.add_binding(scope_id, name, flags);
         BoundIdentifier::new(name_atom, symbol_id)
     }
 
@@ -421,6 +424,15 @@ impl<'a> TraverseCtx<'a> {
         self.generate_uid_based_on_node(node, self.current_scope_id(), flags)
     }
 
+    /// Generate UID in current hoist scope.
+    ///
+    /// See also comments on [`TraverseScoping::generate_uid_name`] for important information
+    /// on how UIDs are generated. There are some potential "gotchas".
+    #[inline]
+    pub fn generate_uid_in_current_hoist_scope(&mut self, name: &str) -> BoundIdentifier<'a> {
+        self.generate_uid(name, self.current_hoist_scope_id(), SymbolFlags::FunctionScopedVariable)
+    }
+
     /// Create a reference bound to a `SymbolId`.
     ///
     /// This is a shortcut for `ctx.scoping.create_bound_reference`.
@@ -623,4 +635,10 @@ impl<'a> TraverseCtx<'a> {
     pub(crate) fn set_current_scope_id(&mut self, scope_id: ScopeId) {
         self.scoping.set_current_scope_id(scope_id);
     }
+
+    /// Shortcut for `ctx.scoping.set_current_hoist_scope_id`, to make `walk_*` methods less verbose.
+    #[inline]
+    pub(crate) fn set_current_hoist_scope_id(&mut self, scope_id: ScopeId) {
+        self.scoping.set_current_hoist_scope_id(scope_id);
+    }
 }

crates/oxc_traverse/src/context/scoping.rs

@@ -26,6 +26,7 @@ pub struct TraverseScoping {
     symbols: SymbolTable,
     uid_names: Option<FxHashSet<CompactStr>>,
     current_scope_id: ScopeId,
+    current_hoist_scope_id: ScopeId,
 }
 
 // Public methods
@@ -40,6 +41,12 @@ impl TraverseScoping {
         self.current_scope_id
     }
 
+    /// Get current var hoisting scope ID
+    #[inline]
+    pub(crate) fn current_hoist_scope_id(&self) -> ScopeId {
+        self.current_hoist_scope_id
+    }
+
     /// Get current scope flags
     #[inline]
     pub fn current_scope_flags(&self) -> ScopeFlags {
@@ -164,6 +171,21 @@ impl TraverseScoping {
         self.scopes.delete_scope(scope_id);
     }
 
+    /// Add binding to [`ScopeTree`] and [`SymbolTable`].
+    #[inline]
+    pub(crate) fn add_binding(
+        &mut self,
+        scope_id: ScopeId,
+        name: CompactStr,
+        flags: SymbolFlags,
+    ) -> SymbolId {
+        let symbol_id =
+            self.symbols.create_symbol(SPAN, name.clone(), flags, scope_id, NodeId::DUMMY);
+        self.scopes.add_binding(scope_id, name, symbol_id);
+
+        symbol_id
+    }
+
     /// Generate binding.
     ///
     /// Creates a symbol with the provided name and flags and adds it to the specified scope.
@@ -174,8 +196,6 @@ impl TraverseScoping {
         flags: SymbolFlags,
     ) -> BoundIdentifier<'a> {
         let owned_name = name.to_compact_str();
-
-        // Add binding to scope
         let symbol_id =
             self.symbols.create_symbol(SPAN, owned_name.clone(), flags, scope_id, NodeId::DUMMY);
         self.scopes.add_binding(scope_id, owned_name, symbol_id);
@@ -388,6 +408,8 @@ impl TraverseScoping {
             uid_names: None,
             // Dummy value. Immediately overwritten in `walk_program`.
             current_scope_id: ScopeId::new(0),
+            // Dummy value. Immediately overwritten in `walk_program`.
+            current_hoist_scope_id: ScopeId::new(0),
         }
     }
 
@@ -397,6 +419,12 @@ impl TraverseScoping {
         self.current_scope_id = scope_id;
     }
 
+    /// Set current hoist scope ID
+    #[inline]
+    pub(crate) fn set_current_hoist_scope_id(&mut self, scope_id: ScopeId) {
+        self.current_hoist_scope_id = scope_id;
+    }
+
     /// Get `uid_names`.
     ///
     /// Iterate through all symbols and unresolved references in AST and identify any var names