feat(traverse): record current block scope (#8007)

Record "block" scope ID along with "hoist" scope ID in `Traverse`.

"Block" scope is the scope where a `let` statement would be inserted above current position in AST.

Block scope and current scope differ from each other inside classes. For example, if want to create a `let` temp var for `foo()` or `bar()` in example below, should use block scope not current scope. Current scope is the class itself, but the `let` statement will be inserted *outside* the class.

```js
class C {
  [foo()]: bar();
}
```

All transforms which create `let` bindings should use block scope not current scope. We should add `VarDeclarationsStore::insert_let` method which uses block scope, to accompany `insert_var` (which uses hoist scope).

This is implemented in a rather hacky way, and we should improve it later. Notably, we're not considering `for` statements as block scopes because we currently have no way to insert `let` statements into them if they don't have a body block.

crates/oxc_traverse/scripts/lib/walk.mjs

@@ -129,6 +129,23 @@ function generateWalkForStruct(type, types) {
       `;
       exitScopeCode += 'ctx.set_current_hoist_scope_id(previous_hoist_scope_id);';
     }
+
+    // TODO: Type names shouldn't be hard-coded here. Block scopes should be signalled by attrs in AST.
+    let isBlockScope = [
+      'Program',
+      'BlockStatement',
+      'Function',
+      'ArrowFunctionExpression',
+      'StaticBlock',
+      'TSModuleDeclaration',
+    ].includes(type.name);
+    if (isBlockScope) {
+      enterScopeCode += `
+        let previous_block_scope_id = ctx.current_block_scope_id();
+        ctx.set_current_block_scope_id(current_scope_id);
+      `;
+      exitScopeCode += 'ctx.set_current_block_scope_id(previous_block_scope_id);';
+    }
   }
 
   const fieldsCodes = visitedFields.map((field, index) => {

crates/oxc_traverse/src/context/mod.rs

@@ -188,6 +188,14 @@ impl<'a> TraverseCtx<'a> {
         self.scoping.current_hoist_scope_id()
     }
 
+    /// Get current block scope ID.
+    ///
+    /// Shortcut for `ctx.scoping.current_block_scope_id`.
+    #[inline]
+    pub fn current_block_scope_id(&self) -> ScopeId {
+        self.scoping.current_block_scope_id()
+    }
+
     /// Get current scope flags.
     ///
     /// Shortcut for `ctx.scoping.current_scope_flags`.
@@ -659,4 +667,10 @@ impl<'a> TraverseCtx<'a> {
     pub(crate) fn set_current_hoist_scope_id(&mut self, scope_id: ScopeId) {
         self.scoping.set_current_hoist_scope_id(scope_id);
     }
+
+    /// Shortcut for `ctx.scoping.set_current_block_scope_id`, to make `walk_*` methods less verbose.
+    #[inline]
+    pub(crate) fn set_current_block_scope_id(&mut self, scope_id: ScopeId) {
+        self.scoping.set_current_block_scope_id(scope_id);
+    }
 }

crates/oxc_traverse/src/context/scoping.rs

@@ -27,6 +27,7 @@ pub struct TraverseScoping {
     uid_names: Option<FxHashSet<CompactStr>>,
     current_scope_id: ScopeId,
     current_hoist_scope_id: ScopeId,
+    current_block_scope_id: ScopeId,
 }
 
 // Public methods
@@ -43,6 +44,12 @@ impl TraverseScoping {
         self.current_hoist_scope_id
     }
 
+    /// Get current block scope ID
+    #[inline]
+    pub(crate) fn current_block_scope_id(&self) -> ScopeId {
+        self.current_block_scope_id
+    }
+
     /// Get current scope flags
     #[inline]
     pub fn current_scope_flags(&self) -> ScopeFlags {
@@ -390,6 +397,7 @@ impl TraverseScoping {
             // Dummy values. Both immediately overwritten in `walk_program`.
             current_scope_id: ScopeId::new(0),
             current_hoist_scope_id: ScopeId::new(0),
+            current_block_scope_id: ScopeId::new(0),
         }
     }
 
@@ -410,6 +418,12 @@ impl TraverseScoping {
         self.current_hoist_scope_id = scope_id;
     }
 
+    /// Set current block scope ID
+    #[inline]
+    pub(crate) fn set_current_block_scope_id(&mut self, scope_id: ScopeId) {
+        self.current_block_scope_id = scope_id;
+    }
+
     /// Get `uid_names`.
     ///
     /// Iterate through all symbols and unresolved references in AST and identify any var names

crates/oxc_traverse/src/generated/walk.rs

@@ -39,6 +39,8 @@ pub(crate) unsafe fn walk_program<'a, Tr: Traverse<'a>>(
     ctx.set_current_scope_id(current_scope_id);
     let previous_hoist_scope_id = ctx.current_hoist_scope_id();
     ctx.set_current_hoist_scope_id(current_scope_id);
+    let previous_block_scope_id = ctx.current_block_scope_id();
+    ctx.set_current_block_scope_id(current_scope_id);
     let pop_token = ctx
         .push_stack(Ancestor::ProgramHashbang(ancestor::ProgramWithoutHashbang(node, PhantomData)));
     if let Some(field) =
@@ -62,6 +64,7 @@ pub(crate) unsafe fn walk_program<'a, Tr: Traverse<'a>>(
     ctx.pop_stack(pop_token);
     ctx.set_current_scope_id(previous_scope_id);
     ctx.set_current_hoist_scope_id(previous_hoist_scope_id);
+    ctx.set_current_block_scope_id(previous_block_scope_id);
     traverser.exit_program(&mut *node, ctx);
 }
 
@@ -1425,6 +1428,8 @@ pub(crate) unsafe fn walk_block_statement<'a, Tr: Traverse<'a>>(
         .get()
         .unwrap();
     ctx.set_current_scope_id(current_scope_id);
+    let previous_block_scope_id = ctx.current_block_scope_id();
+    ctx.set_current_block_scope_id(current_scope_id);
     let pop_token = ctx.push_stack(Ancestor::BlockStatementBody(
         ancestor::BlockStatementWithoutBody(node, PhantomData),
     ));
@@ -1435,6 +1440,7 @@ pub(crate) unsafe fn walk_block_statement<'a, Tr: Traverse<'a>>(
     );
     ctx.pop_stack(pop_token);
     ctx.set_current_scope_id(previous_scope_id);
+    ctx.set_current_block_scope_id(previous_block_scope_id);
     traverser.exit_block_statement(&mut *node, ctx);
 }
 
@@ -2267,6 +2273,8 @@ pub(crate) unsafe fn walk_function<'a, Tr: Traverse<'a>>(
     ctx.set_current_scope_id(current_scope_id);
     let previous_hoist_scope_id = ctx.current_hoist_scope_id();
     ctx.set_current_hoist_scope_id(current_scope_id);
+    let previous_block_scope_id = ctx.current_block_scope_id();
+    ctx.set_current_block_scope_id(current_scope_id);
     let pop_token =
         ctx.push_stack(Ancestor::FunctionId(ancestor::FunctionWithoutId(node, PhantomData)));
     if let Some(field) = &mut *((node as *mut u8).add(ancestor::OFFSET_FUNCTION_ID)
@@ -2308,6 +2316,7 @@ pub(crate) unsafe fn walk_function<'a, Tr: Traverse<'a>>(
     ctx.pop_stack(pop_token);
     ctx.set_current_scope_id(previous_scope_id);
     ctx.set_current_hoist_scope_id(previous_hoist_scope_id);
+    ctx.set_current_block_scope_id(previous_block_scope_id);
     traverser.exit_function(&mut *node, ctx);
 }
 
@@ -2401,6 +2410,8 @@ pub(crate) unsafe fn walk_arrow_function_expression<'a, Tr: Traverse<'a>>(
     ctx.set_current_scope_id(current_scope_id);
     let previous_hoist_scope_id = ctx.current_hoist_scope_id();
     ctx.set_current_hoist_scope_id(current_scope_id);
+    let previous_block_scope_id = ctx.current_block_scope_id();
+    ctx.set_current_block_scope_id(current_scope_id);
     let pop_token = ctx.push_stack(Ancestor::ArrowFunctionExpressionTypeParameters(
         ancestor::ArrowFunctionExpressionWithoutTypeParameters(node, PhantomData),
     ));
@@ -2434,6 +2445,7 @@ pub(crate) unsafe fn walk_arrow_function_expression<'a, Tr: Traverse<'a>>(
     ctx.pop_stack(pop_token);
     ctx.set_current_scope_id(previous_scope_id);
     ctx.set_current_hoist_scope_id(previous_hoist_scope_id);
+    ctx.set_current_block_scope_id(previous_block_scope_id);
     traverser.exit_arrow_function_expression(&mut *node, ctx);
 }
 
@@ -2655,6 +2667,8 @@ pub(crate) unsafe fn walk_static_block<'a, Tr: Traverse<'a>>(
     ctx.set_current_scope_id(current_scope_id);
     let previous_hoist_scope_id = ctx.current_hoist_scope_id();
     ctx.set_current_hoist_scope_id(current_scope_id);
+    let previous_block_scope_id = ctx.current_block_scope_id();
+    ctx.set_current_block_scope_id(current_scope_id);
     let pop_token = ctx
         .push_stack(Ancestor::StaticBlockBody(ancestor::StaticBlockWithoutBody(node, PhantomData)));
     walk_statements(
@@ -2665,6 +2679,7 @@ pub(crate) unsafe fn walk_static_block<'a, Tr: Traverse<'a>>(
     ctx.pop_stack(pop_token);
     ctx.set_current_scope_id(previous_scope_id);
     ctx.set_current_hoist_scope_id(previous_hoist_scope_id);
+    ctx.set_current_block_scope_id(previous_block_scope_id);
     traverser.exit_static_block(&mut *node, ctx);
 }
 
@@ -4917,6 +4932,8 @@ pub(crate) unsafe fn walk_ts_module_declaration<'a, Tr: Traverse<'a>>(
     ctx.set_current_scope_id(current_scope_id);
     let previous_hoist_scope_id = ctx.current_hoist_scope_id();
     ctx.set_current_hoist_scope_id(current_scope_id);
+    let previous_block_scope_id = ctx.current_block_scope_id();
+    ctx.set_current_block_scope_id(current_scope_id);
     if let Some(field) = &mut *((node as *mut u8).add(ancestor::OFFSET_TS_MODULE_DECLARATION_BODY)
         as *mut Option<TSModuleDeclarationBody>)
     {
@@ -4926,6 +4943,7 @@ pub(crate) unsafe fn walk_ts_module_declaration<'a, Tr: Traverse<'a>>(
     ctx.pop_stack(pop_token);
     ctx.set_current_scope_id(previous_scope_id);
     ctx.set_current_hoist_scope_id(previous_hoist_scope_id);
+    ctx.set_current_block_scope_id(previous_block_scope_id);
     traverser.exit_ts_module_declaration(&mut *node, ctx);
 }