Simplify capture representations (swiftlang#360)

Introduce CaptureList, make CaptureStructure semi-vestigial

Author: milseman

Sources/_StringProcessing/Regex/DSLTree.swift

@@ -451,6 +451,88 @@ public struct CaptureTransform: Hashable, CustomStringConvertible {
 // These wrapper types are required because even @_spi-marked public APIs can't
 // include symbols from implementation-only dependencies.
 
+extension DSLTree.Node {
+  func _addCaptures(
+    to list: inout CaptureList,
+    optionalNesting nesting: Int
+  ) {
+    let addOptional = nesting+1
+    switch self {
+    case let .orderedChoice(children):
+      for child in children {
+        child._addCaptures(to: &list, optionalNesting: addOptional)
+      }
+
+    case let .concatenation(children):
+      for child in children {
+        child._addCaptures(to: &list, optionalNesting: nesting)
+      }
+
+    case let .capture(name, _, child):
+      list.append(.init(
+        name: name,
+        type: child.valueCaptureType?.base,
+        optionalDepth: nesting))
+      child._addCaptures(to: &list, optionalNesting: nesting)
+
+    case let .nonCapturingGroup(kind, child):
+      assert(!kind.ast.isCapturing)
+      child._addCaptures(to: &list, optionalNesting: nesting)
+
+    case let .conditional(cond, trueBranch, falseBranch):
+      switch cond.ast {
+      case .group(let g):
+        AST.Node.group(g)._addCaptures(to: &list, optionalNesting: nesting)
+      default:
+        break
+      }
+
+      trueBranch._addCaptures(to: &list, optionalNesting: addOptional)
+      falseBranch._addCaptures(to: &list, optionalNesting: addOptional)
+
+
+    case let .quantification(amount, _, child):
+      var optNesting = nesting
+      if amount.ast.bounds.atLeast == 0 {
+        optNesting += 1
+      }
+      child._addCaptures(to: &list, optionalNesting: optNesting)
+
+    case let .regexLiteral(re):
+      return re.ast._addCaptures(to: &list, optionalNesting: nesting)
+
+    case let .absentFunction(abs):
+      switch abs.ast.kind {
+      case .expression(_, _, let child):
+        child._addCaptures(to: &list, optionalNesting: nesting)
+      case .clearer, .repeater, .stopper:
+        break
+      @unknown default:
+        fatalError()
+      }
+
+    case let .convertedRegexLiteral(n, _):
+      return n._addCaptures(to: &list, optionalNesting: nesting)
+
+    case .matcher:
+      break
+
+    case .transform(_, let child):
+      child._addCaptures(to: &list, optionalNesting: nesting)
+
+    case .customCharacterClass, .atom, .trivia, .empty,
+        .quotedLiteral, .consumer, .characterPredicate:
+      break
+    }
+  }
+
+  var _captureList: CaptureList {
+    var list = CaptureList()
+    self._addCaptures(to: &list, optionalNesting: 0)
+    return list
+  }
+}
+
 extension DSLTree {
   /// Presents a wrapped version of `DSLTree.Node` that can provide an internal
   /// `_TreeNode` conformance.