Add name for diagnostics for syntax node child

Author: ahoppen

Sources/SwiftSyntax/Misc.swift.gyb

@@ -39,6 +39,19 @@ extension Syntax {
 % for node in NON_BASE_SYNTAX_NODES:
     case .${node.swift_syntax_kind}(let node):
       return node
+% end
+    }
+  }
+
+  public func childNameForDiagnostics(_ index: SyntaxChildrenIndex) -> String? {
+    switch self.as(SyntaxEnum.self) {
+    case .token(let node):
+      return node.childNameForDiagnostics(index)
+    case .unknown(let node):
+      return node.childNameForDiagnostics(index)
+% for node in NON_BASE_SYNTAX_NODES:
+    case .${node.swift_syntax_kind}(let node):
+      return node.childNameForDiagnostics(index)
 % end
     }
   }

Sources/SwiftSyntax/Syntax.swift

@@ -131,6 +131,10 @@ public protocol SyntaxProtocol: CustomStringConvertible,
   /// Converts the given `Syntax` node to this type. Returns `nil` if the
   /// conversion is not possible.
   init?(_ syntaxNode: Syntax)
+
+  /// Return a name with which the child at the given `index` can be referred to
+  /// in diagnostics.
+  func childNameForDiagnostics(_ index: SyntaxChildrenIndex) -> String?
 }
 
 extension SyntaxProtocol {

Sources/SwiftSyntax/SyntaxBaseNodes.swift.gyb

@@ -120,6 +120,10 @@ public struct ${node.name}: ${node.name}Protocol, SyntaxHashable {
   public func asProtocol(_: ${node.name}Protocol.Protocol) -> ${node.name}Protocol {
     return Syntax(self).asProtocol(${node.name}Protocol.self)!
   }
+
+  public func childNameForDiagnostics(_ index: SyntaxChildrenIndex) -> String? {
+    return Syntax(self).childNameForDiagnostics(index)
+  }
 }
 
 extension ${node.name}: CustomReflectable {

Sources/SwiftSyntax/SyntaxChildren.swift

@@ -20,7 +20,7 @@ public struct SyntaxChildrenIndexData: Comparable {
   fileprivate let offset: UInt32
   /// The index of the node in the parent.
   /// See `AbsoluteSyntaxPosition.indexInParent`
-  fileprivate let indexInParent: UInt32
+  let indexInParent: UInt32
   /// Unique value for a node within its own tree.
   /// See `SyntaxIdentifier.indexIntree`
   fileprivate let indexInTree: SyntaxIndexInTree
@@ -61,7 +61,7 @@ public struct SyntaxChildrenIndex: Comparable, ExpressibleByNilLiteral {
   /// SyntaxChildrenIndex` an enumbecause the optional value can be
   /// force-unwrapped when we know that an index is not the end index, saving a
   /// switch case comparison.
-  fileprivate let data: SyntaxChildrenIndexData?
+  let data: SyntaxChildrenIndexData?
 
   fileprivate init(offset: UInt32, indexInParent: UInt32,
                    indexInTree: SyntaxIndexInTree) {

Sources/SwiftSyntax/SyntaxCollections.swift.gyb

@@ -217,6 +217,10 @@ public struct ${node.name}: SyntaxCollection, SyntaxHashable {
       self = withTrailingTrivia(newValue ?? [])
     }
   }
+
+  public func childNameForDiagnostics(_ index: SyntaxChildrenIndex) -> String? {
+    return nil
+  }
 }
 
 /// Conformance for `${node.name}` to the `BidirectionalCollection` protocol.

Sources/SwiftSyntax/SyntaxNodes.swift.gyb.template

@@ -171,6 +171,21 @@ public struct ${node.name}: ${base_type}Protocol, SyntaxHashable {
     return ${node.name}(newData)
   }
 %     end
+
+  public func childNameForDiagnostics(_ index: SyntaxChildrenIndex) -> String? {
+    switch index.data?.indexInParent {
+%     for (index, child) in enumerate(node.children):
+    case ${index}:
+%       if child.name_for_diagnostics:
+      return "${child.name_for_diagnostics}"
+%       else:
+      return nil
+%       end
+%     end
+    default:
+      fatalError("Invalid index")
+    }
+  }
 }
 
 extension ${node.name}: CustomReflectable {

Sources/SwiftSyntax/SyntaxOtherNodes.swift

@@ -39,6 +39,10 @@ public struct UnknownSyntax: SyntaxProtocol, SyntaxHashable {
     let data = SyntaxData.forRoot(raw)
     self.init(data)
   }
+
+  public func childNameForDiagnostics(_ index: SyntaxChildrenIndex) -> String? {
+    return nil
+  }
 }
 
 extension UnknownSyntax: CustomReflectable {
@@ -192,6 +196,10 @@ public struct TokenSyntax: SyntaxProtocol, SyntaxHashable {
   public var totalLength: SourceLength {
     return raw.totalLength
   }
+
+  public func childNameForDiagnostics(_ index: SyntaxChildrenIndex) -> String? {
+    return nil
+  }
 }
 
 extension TokenSyntax: CustomReflectable {