Improve debug description of syntax nodes

This improves the description generated by the `debugDescription` function to something like the following which is easier to read IMO

```
SourceFileSyntax
├─CodeBlockItemListSyntax
│ ╰─CodeBlockItemSyntax
│   ╰─MacroExpansionExprSyntax
│     ├─pound
│     ├─identifier("stringify")
│     ├─leftParen
│     ├─TupleExprElementListSyntax
│     │ ╰─TupleExprElementSyntax
│     │   ╰─InfixOperatorExprSyntax
│     │     ├─IntegerLiteralExprSyntax
│     │     │ ╰─integerLiteral("2")
│     │     ├─BinaryOperatorExprSyntax
│     │     │ ╰─binaryOperator("+")
│     │     ╰─IntegerLiteralExprSyntax
│     │       ╰─integerLiteral("3")
│     ╰─rightParen
╰─eof
```

Since we now have this nice representation, it also removes the conformances to `CustomReflectable` (deleting 3000 lines of code :hooray:) and doing some new `CustomReflectable` hackery to make the debugger just print `debugDescription` and not try to print the node’s children itself. This way you an now do `po node` in the debugger to get the debug description from above, instead of having to do `e print(node.recursiveDescription)`

Author: ahoppen

CodeGeneration/Sources/generate-swiftsyntax/templates/swiftsyntax/SyntaxNodeFile.swift

@@ -280,27 +280,6 @@ func syntaxNode(emitKind: String) -> SourceFileSyntax {
           }
         }
       }
-
-      try! ExtensionDeclSyntax("extension \(raw: node.name): CustomReflectable") {
-        let children = DictionaryExprSyntax {
-          for child in node.children {
-            DictionaryElementSyntax(
-              leadingTrivia: .newline,
-              keyExpression: ExprSyntax(#""\#(raw: child.swiftName)""#),
-              valueExpression: child.isOptional
-                ? ExprSyntax("\(raw: child.swiftName).map(Syntax.init)?.asProtocol(SyntaxProtocol.self) as Any")
-                : ExprSyntax("Syntax(\(raw: child.swiftName)).asProtocol(SyntaxProtocol.self)")
-            )
-          }
-        }
-        DeclSyntax(
-          """
-          public var customMirror: Mirror {
-            return Mirror(self, children: \(children))
-          }
-          """
-        )
-      }
     }
   }
 }

Sources/SwiftSyntax/Syntax.swift

@@ -161,7 +161,7 @@ public extension SyntaxHashable {
 /// protocol to provide common functionality for all syntax nodes.
 /// DO NOT CONFORM TO THIS PROTOCOL YOURSELF!
 public protocol SyntaxProtocol: CustomStringConvertible,
-  CustomDebugStringConvertible, TextOutputStreamable
+  CustomDebugStringConvertible, TextOutputStreamable, CustomReflectable
 {
 
   /// Retrieve the generic syntax node that is represented by this node.
@@ -598,6 +598,12 @@ public extension SyntaxProtocol {
     debugDescription()
   }
 
+  var customMirror: Mirror {
+    // Suppress printing of children when doing `po node` in the debugger.
+    // `debugDescription` already prints them in a nicer way.
+    return Mirror(self, children: [:])
+  }
+
   /// Same as `debugDescription` but includes all children.
   var recursiveDescription: String {
     debugDescription(includeChildren: true)
@@ -615,11 +621,11 @@ public extension SyntaxProtocol {
   ///   - indentLevel: The starting indent level, 0 by default. Each level is 2
   ///   spaces.
   func debugDescription(
-    includeChildren: Bool = false,
+    includeChildren: Bool = true,
     includeTrivia: Bool = false,
     converter: SourceLocationConverter? = nil,
     mark: SyntaxProtocol? = nil,
-    indentLevel: Int = 0
+    indentString: String = ""
   ) -> String {
     var str = ""
     debugWrite(
@@ -628,7 +634,7 @@ public extension SyntaxProtocol {
       includeTrivia: includeTrivia,
       converter: converter,
       mark: mark,
-      indentLevel: indentLevel
+      indentString: indentString
     )
     return str
   }
@@ -639,7 +645,7 @@ public extension SyntaxProtocol {
     includeTrivia: Bool,
     converter: SourceLocationConverter? = nil,
     mark: SyntaxProtocol? = nil,
-    indentLevel: Int
+    indentString: String
   ) {
     if let mark = mark, self.id == mark.id {
       target.write("*** ")
@@ -660,11 +666,6 @@ public extension SyntaxProtocol {
     }
 
     let allChildren = children(viewMode: .all)
-    if includeChildren {
-      if !allChildren.isEmpty {
-        target.write(" children=\(allChildren.count)")
-      }
-    }
 
     if let converter = converter {
       let range = sourceRange(converter: converter)
@@ -680,18 +681,19 @@ public extension SyntaxProtocol {
     }
 
     if includeChildren {
-      let childIndentLevel = indentLevel + 1
       for (num, child) in allChildren.enumerated() {
+        let isLastChild = num == allChildren.count - 1
         target.write("\n")
-        target.write(String(repeating: " ", count: childIndentLevel * 2))
-        target.write("\(num): ")
+        target.write(indentString)
+        target.write(isLastChild ? "╰─" : "├─")
+        let childIndentString = indentString + (isLastChild ? "  " : "│ ")
         child.debugWrite(
           to: &target,
           includeChildren: includeChildren,
           includeTrivia: includeTrivia,
           converter: converter,
           mark: mark,
-          indentLevel: childIndentLevel
+          indentString: childIndentString
         )
       }
     }