[compiler][wip] Support finally clauses

[everettbu]

Mirror of facebook/react#35607
Original author: josephsavona

---

First pass at adding support for finally clauses. I took a simple approach where we reify the JS semantics around return shadowing in the HIR and output. Ie, we create a temporary variable that will be the return value, and change returns in the try/catch to assign to that value and then break to the finalizer. In the finalizer returns work as normal, but we put a final extra "if temporary set, return it" statement. It would be more ideal to fully restore the return shadowing, but given that finally clauses are relatively rare this seems like a reasonable compromise.

I need to do more analysis to make sure the approach is correct.

---

Stack created with Sapling. Best reviewed with ReviewStack.

• -> #35607
• #35298
• #35596
• #35573
• #35595
• #35539

[greptile-apps]

Greptile Summary

This PR adds support for finally clauses in try statements to the React compiler by making both catch and finally blocks optional in the HIR representation. The implementation uses a straightforward approach where control flow is directed through the finalizer block when present.

Key Changes

• Updated TryTerminal type to have optional handler and finalizer fields instead of required handler
• Modified BuildHIR.ts to construct finalizer blocks and properly route control flow through them
• Updated all visitor patterns and traversal logic to handle optional handler and finalizer blocks
• Added codegen support to emit try-catch-finally statements with optional clauses
• Enhanced test coverage with multiple try-finally scenarios including returns, breaks, and continues

Issues Found

• Critical Bug: ValidateNoJSXInTryStatement.ts does not add finalizer blocks to activeTryBlocks, which means JSX created inside finally clauses will not be validated. This is inconsistent with the validation for try and catch blocks.

Confidence Score: 3/5

• This PR has a logical error in validation that will allow invalid JSX in finally blocks
• The implementation is well-structured and mostly correct, but contains a critical bug in ValidateNoJSXInTryStatement.ts where finally blocks are not tracked for JSX validation. This means JSX constructed in finally clauses won't be caught by validation, which defeats the purpose of the error boundary validation.
• Pay close attention to ValidateNoJSXInTryStatement.ts - it has a logical error that needs to be fixed

Important Files Changed

Filename	Overview
compiler/packages/babel-plugin-react-compiler/src/Validation/ValidateNoJSXInTryStatement.ts	Added logic to validate JSX in try blocks, but finalizer blocks are not tracked for validation
compiler/packages/babel-plugin-react-compiler/src/HIR/BuildHIR.ts	Added support for finally clauses in try statements with proper control flow handling
compiler/packages/babel-plugin-react-compiler/src/ReactiveScopes/CodegenReactiveFunction.ts	Updated codegen to emit optional catch clause and finalizer in try statements
compiler/packages/babel-plugin-react-compiler/src/ReactiveScopes/BuildReactiveFunction.ts	Added scheduling and traversal logic for finalizer blocks in try statements

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[greptile-apps]

logic: finalizer block not added to activeTryBlocks, so JSX in finally won't be validated

Suggested change

	if (block.terminal.kind === 'try') {
	activeTryBlocks.push(block.terminal.handler);
	// Add the try block itself to activeTryBlocks
	activeTryBlocks.push(block.terminal.block);
	// Also add handler if present
	if (block.terminal.handler !== null) {
	activeTryBlocks.push(block.terminal.handler);
	}
	}
	if (block.terminal.kind === 'try') {
	// Add the try block itself to activeTryBlocks
	activeTryBlocks.push(block.terminal.block);
	// Also add handler if present
	if (block.terminal.handler !== null) {
	activeTryBlocks.push(block.terminal.handler);
	}
	// Also add finalizer if present
	if (block.terminal.finalizer !== null) {
	activeTryBlocks.push(block.terminal.finalizer);
	}
	}

Prompt To Fix With AI

This is a comment left during a code review.
Path: compiler/packages/babel-plugin-react-compiler/src/Validation/ValidateNoJSXInTryStatement.ts
Line: 57:64

Comment:
**logic:** finalizer block not added to `activeTryBlocks`, so JSX in finally won't be validated

```suggestion
    if (block.terminal.kind === 'try') {
      // Add the try block itself to activeTryBlocks
      activeTryBlocks.push(block.terminal.block);
      // Also add handler if present
      if (block.terminal.handler !== null) {
        activeTryBlocks.push(block.terminal.handler);
      }
      // Also add finalizer if present
      if (block.terminal.finalizer !== null) {
        activeTryBlocks.push(block.terminal.finalizer);
      }
    }
```

How can I resolve this? If you propose a fix, please make it concise.

[greptile-apps]

_{264 files reviewed, 1 comment}

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[greptile-apps]

logic: finalizer block not added to activeTryBlocks, JSX in finally won't be validated

Suggested change

	if (block.terminal.kind === 'try') {
	activeTryBlocks.push(block.terminal.handler);
	// Add the try block itself to activeTryBlocks
	activeTryBlocks.push(block.terminal.block);
	// Also add handler if present
	if (block.terminal.handler !== null) {
	activeTryBlocks.push(block.terminal.handler);
	}
	if (block.terminal.kind === 'try') {
	// Add the try block itself to activeTryBlocks
	activeTryBlocks.push(block.terminal.block);
	// Also add handler if present
	if (block.terminal.handler !== null) {
	activeTryBlocks.push(block.terminal.handler);
	}
	// Also add finalizer if present
	if (block.terminal.finalizer !== null) {
	activeTryBlocks.push(block.terminal.finalizer);
	}
	}

Prompt To Fix With AI

This is a comment left during a code review.
Path: compiler/packages/babel-plugin-react-compiler/src/Validation/ValidateNoJSXInTryStatement.ts
Line: 57:63

Comment:
**logic:** finalizer block not added to `activeTryBlocks`, JSX in finally won't be validated

```suggestion
    if (block.terminal.kind === 'try') {
      // Add the try block itself to activeTryBlocks
      activeTryBlocks.push(block.terminal.block);
      // Also add handler if present
      if (block.terminal.handler !== null) {
        activeTryBlocks.push(block.terminal.handler);
      }
      // Also add finalizer if present
      if (block.terminal.finalizer !== null) {
        activeTryBlocks.push(block.terminal.finalizer);
      }
    }
```

How can I resolve this? If you propose a fix, please make it concise.