In nodesDescription only merge tokens if they occur consecutively in the source code

This was motiviated by the following discussion, where the issue becomes more apparent: swiftlang#1643 (comment).

Author: ahoppen

Sources/SwiftParserDiagnostics/MissingNodesError.swift

@@ -67,7 +67,12 @@ fileprivate enum NodesDescriptionPart {
   }
 
   static func descriptionParts(for nodes: some Sequence<Syntax>) -> [NodesDescriptionPart] {
+    // For each token that we modified (e.g. made it present), map to the original token in `nodes`
+    var originalTokens: [TokenSyntax: TokenSyntax] = [:]
+
+    // Accumulates the result
     var parts: [NodesDescriptionPart] = []
+
     for missingNode in nodes {
       if let token = missingNode.as(TokenSyntax.self) {
         let newPart: NodesDescriptionPart
@@ -77,10 +82,12 @@ fileprivate enum NodesDescriptionPart {
           let (rawKind, text) = token.tokenKind.decomposeToRaw()
           if let text = text, !text.isEmpty {
             let presentToken = token.with(\.presence, .present)
+            originalTokens[presentToken] = token
             newPart = .tokensWithDefaultText([presentToken])
           } else if let defaultText = rawKind.defaultText {
             let newKind = TokenKind.fromRaw(kind: rawKind, text: String(syntaxText: defaultText))
             let presentToken = token.with(\.tokenKind, newKind).with(\.presence, .present)
+            originalTokens[presentToken] = token
             newPart = .tokensWithDefaultText([presentToken])
           } else {
             newPart = .tokenWithoutDefaultText(token)
@@ -89,7 +96,14 @@ fileprivate enum NodesDescriptionPart {
 
         switch (parts.last, newPart) {
         case (.tokensWithDefaultText(let previousTokens), .tokensWithDefaultText(let newTokens)):
-          parts[parts.count - 1] = .tokensWithDefaultText(previousTokens + newTokens)
+          // Merge `tokensWithDefaultText` if they occur consecutively in the tree
+          if let lastPrevious = previousTokens.last,
+            originalTokens[lastPrevious, default: lastPrevious].nextToken(viewMode: .all) == token
+          {
+            parts[parts.count - 1] = .tokensWithDefaultText(previousTokens + newTokens)
+          } else {
+            parts.append(newPart)
+          }
         default:
           parts.append(newPart)
         }

Tests/SwiftParserTest/translated/RecoveryTests.swift

@@ -1989,7 +1989,7 @@ final class RecoveryTests: XCTestCase {
       diagnostics: [
         DiagnosticSpec(
           message: "expected ':' to begin inheritance clause",
-          fixIts: ["replace '()' with ':'"]
+          fixIts: ["replace '(' and ')' with ':'"]
         )
       ],
       fixedSource: """
@@ -2006,7 +2006,7 @@ final class RecoveryTests: XCTestCase {
       diagnostics: [
         DiagnosticSpec(
           message: "expected ':' to begin inheritance clause",
-          fixIts: ["replace '()' with ':'"]
+          fixIts: ["replace '(' and ')' with ':'"]
         )
       ],
       fixedSource: """
@@ -2023,7 +2023,7 @@ final class RecoveryTests: XCTestCase {
       diagnostics: [
         DiagnosticSpec(
           message: "expected ':' to begin inheritance clause",
-          fixIts: ["replace '()' with ':'"]
+          fixIts: ["replace '(' and ')' with ':'"]
         )
       ],
       fixedSource: """
@@ -2040,7 +2040,7 @@ final class RecoveryTests: XCTestCase {
       diagnostics: [
         DiagnosticSpec(
           message: "expected ':' to begin inheritance clause",
-          fixIts: ["replace '()' with ':'"]
+          fixIts: ["replace '(' and ')' with ':'"]
         )
       ],
       fixedSource: """
@@ -2057,7 +2057,7 @@ final class RecoveryTests: XCTestCase {
       diagnostics: [
         DiagnosticSpec(
           message: "expected ':' to begin inheritance clause",
-          fixIts: ["replace '()' with ':'"]
+          fixIts: ["replace '(' and ')' with ':'"]
         )
       ],
       fixedSource: """