Introduce FixIt.Change.textualReplacement

Introduce a new case to FixIt.Change to express an unstructured edit,
which replaces some range of source text (in a given file) with some
other source text. This is needed for some edits that aren't easily
mapped to the syntax tree, or when coming from other tools (such as
the compiler) that don't express these fixes in terms of syntax in the
first place.

Author: DougGregor

Sources/SwiftCompilerPluginMessageHandling/Diagnostics.swift

@@ -150,6 +150,9 @@ extension PluginMessage.Diagnostic {
           case .replaceChild(let replaceChildData):
             range = sourceManager.range(replaceChildData.replacementRange, in: replaceChildData.parent)
             text = replaceChildData.newChild.description
+          case .textualReplacement(replacementRange: let replacementRange, sourceFile: let sourceFile, newText: let newText):
+            range = sourceManager.range(replacementRange, in: sourceFile)
+            text = newText
           #if RESILIENT_LIBRARIES
           @unknown default:
             fatalError()

Sources/SwiftDiagnostics/FixIt.swift

@@ -77,6 +77,14 @@ public struct FixIt: Sendable {
     case replaceTrailingTrivia(token: TokenSyntax, newTrivia: Trivia)
     /// Replace the child node of the given parent node at the given replacement range with the given new child node
     case replaceChild(data: any ReplacingChildData)
+    /// Replace the text within the given range in a source file with new text.
+    ///
+    /// Generally, one should use other cases to replace specific syntax nodes
+    /// or trivia, because it more easily leads to a correct result. However,
+    /// this case provides a fallback for textual replacement that ignores
+    /// syntactic structure. After applying a textual replacement, there is no
+    /// way to get back to a syntax tree without reparsing.
+    case textualReplacement(replacementRange: Range<AbsolutePosition>, sourceFile: SourceFileSyntax, newText: String)
   }
 
   /// A description of what this Fix-It performs.
@@ -157,6 +165,9 @@ private extension FixIt.Change {
         range: replacingChildData.replacementRange,
         replacement: replacingChildData.newChild.description
       )
+
+    case .textualReplacement(replacementRange: let replacementRange, sourceFile: _, newText: let newText):
+      return SourceEdit(range: replacementRange, replacement: newText)
     }
   }
 }

Sources/SwiftSyntaxMacrosGenericTestSupport/Assertions.swift

@@ -632,6 +632,11 @@ fileprivate extension FixIt.Change {
         range: start..<end,
         replacement: replacingChildData.newChild.description
       )
+
+    case .textualReplacement(replacementRange: let range, sourceFile: let sourceFile, newText: let newText):
+      let start = expansionContext.position(of: range.lowerBound, anchoredAt: sourceFile)
+      let end = expansionContext.position(of: range.upperBound, anchoredAt: sourceFile)
+      return SourceEdit(range: start..<end, replacement: newText)
     }
   }
 }

Tests/SwiftDiagnosticsTest/FixItTests.swift

@@ -10,12 +10,21 @@
 //
 //===----------------------------------------------------------------------===//
 
+import SwiftDiagnostics
 import SwiftParser
 import SwiftParserDiagnostics
 import SwiftSyntax
 import XCTest
 import _SwiftSyntaxTestSupport
 
+struct SimpleFixItMessage: FixItMessage {
+  let message: String
+
+  var fixItID: MessageID {
+    MessageID(domain: "here", id: "this")
+  }
+}
+
 final class FixItTests: XCTestCase {
   func testEditsForFixIt() throws {
     let markedSource = "protocol 1️⃣Multi2️⃣ 3️⃣ident 4️⃣{}"
@@ -39,4 +48,18 @@ final class FixItTests: XCTestCase {
     XCTAssertNotEqual(changes.count, edits.count)
     XCTAssertEqual(expectedEdits, edits)
   }
+
+  func testTextualReplacement() throws {
+    let five = AbsolutePosition(utf8Offset: 5)
+    let fifteen = AbsolutePosition(utf8Offset: 15)
+    let change = FixIt(
+      message: SimpleFixItMessage(message: "fix it please"),
+      changes: [
+        .textualReplacement(replacementRange: five..<fifteen, sourceFile: "func myFunction() { }", newText: "yours")
+      ]
+    )
+    XCTAssertEqual(change.edits.count, 1)
+    XCTAssertEqual(change.edits[0].range, five..<fifteen)
+    XCTAssertEqual(change.edits[0].replacement, "yours")
+  }
 }