Untangle _RegexParser from RegexBuilder (swiftlang#299)

This makes the changes necessary for _RegexParser to be imported as an
implementation-only dependency. The change provides _StringProcessing wrappers
for all `AST` types that need to be publicly visible via SPI, and a
DSLTree.Node wrapper for internal conformance to _TreeNode.

Co-authored-by: Richard Wei <[email protected]>

Author: natecook1000

Sources/_StringProcessing/Regex/DSLTree.swift

@@ -451,88 +451,6 @@ 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.