🚥 Introduce a new Lexical query lookupControlStructure to the SwiftLexicalLookup Library

[lokesh-tr]

⚠️ Under Progress ⚠️

• Explanation:
  Introduces a new Lexical query lookupControlStructure to the SwiftLexicalLookup Library which answers "which function / closure / init / deinit / accessor gets terminated by this return / throw statement?", also answers "which loop does this break / continue statement correspond to?"
• Scope:
  Purely Additive. Shouldn't break anything. New API exposed under @_spi(Experimental).
• Issues:
  This API will be used by SourceKit-LSP to fix (Highlight related identifiers for control flow keywords sourcekit-lsp#2400).
• Original PRs:
  Link to the SourceKit-LSP PR: 🚥 Implement Document Highlight for all return / throws (function/closure/accessor/init/deinit) and break / continue (for/while/repeat...while) sourcekit-lsp#2403
• Risk:
  No Real Risks to the best of my knowledge. This is my first contribution to SwiftSyntax, it's possible that I overlooked something. If that's the case, enlighten me :)
• Testing:
  Tests and Docs are yet to be written. After some time, will polish this PR description as well.
• Reviewers:
  Discussed with @ahoppen on some fun things that i can implement in sourcekit-lsp, he suggested this 😁. Open for anyone to review, the current version makes the feature work, might require some polishing.

[ahoppen]

Nice 😍 Very excited to get my hands on this.

The overall direction looks good to me. A couple things we should ensure:

• We handle labelled break statements correctly, eg. the following. We should have lookupLabeledStmts to help with this.

LOOP: while true {
  switch x {
  case 1: break LOOP
  default: break
  }
}

• Double check that all syntax nodes are added correctly, I think subscripts are currently missing but there might be other.

[ahoppen]

CC @MAJKFL if you’re interested in helping review this and contributing your expertise in name lookup edge cases. 😉

[rintaro]

Thank you!
Overall, I think this is a great addition to SwiftLexicalLookup.

I know this PR is still in draft, but I left some comments anyway.

[rintaro]

I don't feel comfortable with having this method on SyntaxProtocol.
Instead, how about making

@_spi(Experimental) public protocol ControlTransferStmtSyntax {
  var controlTransferTarget: Syntax? { get }
}

And make break, continue, return, throw, and fallthrough conform to it?

[rintaro]

Unfortunately, break and continue target is not that simple. You'd need to handle labels e.g. break LOOP.
Also, they can target other non-loop labeled statements , e.g.

OUTER: do {
  for val in values {
    if val.condition {
      break OUTER
    }
  }
}

In the actual compiler, the rule is implemented in swift::findBreakOrContinueStmtTarget

[rintaro]

The target of throw can be do-catch statements.

[rintaro]

Actually, SyntaxProtoco.lookupCatchNode() should be usable for throw case.