Create a dedicated type for the rootId of a tree

Passing a `UInt` around has always felt a little unsafe and didn’t give any semantic meaning that describes what the `UInt` represents.

Author: ahoppen

Sources/SwiftSyntax/SyntaxChildren.swift

@@ -102,7 +102,7 @@ public struct SyntaxChildrenIndex: Hashable, Comparable, ExpressibleByNilLiteral
 
 fileprivate extension AbsoluteSyntaxInfo {
   /// Construct `AbsoluteSyntaxInfo` from the given index data and a `rootId`.
-  init(index: SyntaxChildrenIndexData, rootId: UInt) {
+  init(index: SyntaxChildrenIndexData, rootId: RootID) {
     let position = AbsoluteSyntaxPosition(
       offset: index.offset,
       indexInParent: index.indexInParent
@@ -152,7 +152,7 @@ struct RawSyntaxChildren: BidirectionalCollection {
   }
 
   /// The rootId of the tree the child nodes belong to
-  private let rootId: UInt
+  private let rootId: RootID
   /// The number of children in `parent`. Cached to avoid reaching into `parent` for every index
   /// advancement
   // FIXME: Do we need this cached?

Sources/SwiftSyntax/SyntaxData.swift

@@ -93,6 +93,18 @@ public struct SyntaxIndexInTree: Comparable, Hashable {
   }
 }
 
+/// Uniquely identifies a syntax tree.
+struct RootID: Hashable {
+  /// The pointer value of the root.
+  ///
+  /// Since the root ID might outlive the tree, it is not unsafe to access this.
+  private var id: UInt
+
+  fileprivate init(_ pointer: UnsafeRawPointer) {
+    self.id = UInt(bitPattern: pointer)
+  }
+}
+
 /// Provides a stable and unique identity for ``Syntax`` nodes.
 ///
 /// Note that two nodes might have the same contents even if their IDs are
@@ -111,7 +123,7 @@ public struct SyntaxIdentifier: Hashable {
   /// same instance. This guarantees that the trees with the same 'rootId' have
   /// exact the same structure. But, two trees with exactly the same structure
   /// might still have different 'rootId's.
-  let rootId: UInt
+  let rootId: RootID
   /// Unique value for a node within its own tree.
   @_spi(RawSyntax)
   public let indexInTree: SyntaxIndexInTree
@@ -128,7 +140,7 @@ public struct SyntaxIdentifier: Hashable {
 
   static func forRoot(_ raw: RawSyntax) -> SyntaxIdentifier {
     return .init(
-      rootId: UInt(bitPattern: raw.pointer),
+      rootId: RootID(raw.pointer),
       indexInTree: .zero
     )
   }
@@ -163,7 +175,7 @@ struct AbsoluteRawSyntax {
     return nil
   }
 
-  func replacingSelf(_ newRaw: RawSyntax, newRootId: UInt) -> AbsoluteRawSyntax {
+  func replacingSelf(_ newRaw: RawSyntax, newRootId: RootID) -> AbsoluteRawSyntax {
     let nodeId = SyntaxIdentifier(rootId: newRootId, indexInTree: info.nodeId.indexInTree)
     let newInfo = AbsoluteSyntaxInfo(position: info.position, nodeId: nodeId)
     return .init(raw: newRaw, info: newInfo)