Add a new visitor: SyntaxTransformVisitor.

This visitor returns a transformed Syntax node rather than exposing a pre-post visitor.

Author: zoecarver

Sources/SwiftSyntax/SyntaxData.swift

@@ -213,7 +213,7 @@ class SyntaxBox: CustomStringConvertible,
 ///
 /// SyntaxData is an implementation detail, and should not be exposed to clients
 /// of SwiftSyntax.
-struct SyntaxData {
+public struct SyntaxData {
   private enum ParentOrArena {
     // For non-root nodes.
     case parent(SyntaxBox)

Sources/SwiftSyntax/SyntaxVisitor.swift.gyb

@@ -141,3 +141,75 @@ open class SyntaxVisitor {
     }
   }
 }
+
+public protocol SyntaxTransformVisitor {
+  associatedtype ResultType = Void
+  
+  func visit(_ token: TokenSyntax) -> ResultType?
+  func visit(_ node: UnknownSyntax) -> ResultType?
+  
+% for node in SYNTAX_NODES:
+%   if is_visitable(node):
+  /// Visiting `${node.name}` specifically.
+  ///   - Parameter node: the node we are visiting.
+  ///   - Returns: nil by default.
+  func visit(_ node: ${node.name}) -> ResultType?
+%   end
+% end
+}
+
+extension SyntaxTransformVisitor {
+  public func visit(_ token: TokenSyntax) -> ResultType? { nil }
+  public func visit(_ node: UnknownSyntax) -> ResultType? { nil }
+
+% for node in SYNTAX_NODES:
+%   if is_visitable(node):
+  /// Visiting `${node.name}` specifically.
+  ///   - Parameter node: the node we are visiting.
+  ///   - Returns: nil by default.
+  public func visit(_ node: ${node.name}) -> ResultType? {
+%   if node.is_base():
+      // Avoid calling into visitChildren if possible.
+      if !node.raw.layoutView!.children.isEmpty {
+        return visitChildren(node).first
+      }
+      return nil
+%   else:
+      // Avoid calling into visitChildren if possible.
+      if !node.raw.layoutView!.children.isEmpty {
+        return visitChildren(node).first
+      }
+      return nil
+%   end
+  }
+%   end
+% end
+
+  public func visit(_ data: SyntaxData) -> ResultType? {
+    switch data.raw.kind {
+    case .token:
+      let node = TokenSyntax(data)
+      return visit(node)
+    case .unknown:
+      let node = UnknownSyntax(data)
+      return visit(node)
+    // The implementation of every generated case goes into its own function. This
+    // circumvents an issue where the compiler allocates stack space for every
+    // case statement next to each other in debug builds, causing it to allocate
+    // ~50KB per call to this function. rdar://55929175
+  % for node in NON_BASE_SYNTAX_NODES:
+    case .${node.swift_syntax_kind}:
+      let node = ${node.name}(data)
+      return visit(node)
+  % end
+    }
+  }
+
+  public func visitChildren<SyntaxType: SyntaxProtocol>(_ node: SyntaxType) -> [ResultType] {
+    let syntaxNode = Syntax(node)
+        return NonNilRawSyntaxChildren(syntaxNode, viewMode: .sourceAccurate).compactMap { childRaw in
+      let childData = SyntaxData(childRaw, parent: syntaxNode)
+      return visit(childData)
+    }
+  }
+}