[Macro test] Handle source locations in macro diagnostics better.

"Relink" the folded syntax node back into the primary syntax node. When
we do this, we get consistent source locations that do not require any
adjustment.

Test this by adding a Fix-It to the silly AddBlocker macro, replacing
the `+` with a `-`.

Author: DougGregor

lib/ASTGen/Sources/ASTGen/Diagnostics.swift

@@ -5,7 +5,6 @@ import SwiftSyntax
 fileprivate func emitDiagnosticParts(
   diagEnginePtr: UnsafeMutablePointer<UInt8>,
   sourceFileBuffer: UnsafeMutableBufferPointer<UInt8>,
-  nodeStartOffset: Int?,
   message: String,
   severity: DiagnosticSeverity,
   position: AbsolutePosition,
@@ -22,18 +21,8 @@ fileprivate func emitDiagnosticParts(
 
   // Form a source location for the given absolute position
   func sourceLoc(
-    at origPosition: AbsolutePosition
+    at position: AbsolutePosition
   ) -> UnsafeMutablePointer<UInt8>? {
-    // FIXME: Our tree is very confused about absolute offsets. Work around
-    // the issue in a very hacky way.
-    let position: AbsolutePosition
-    if let nodeStartOffset = nodeStartOffset,
-        origPosition.utf8Offset < nodeStartOffset {
-      position = origPosition + SourceLength(utf8Length: nodeStartOffset)
-    } else {
-      position = origPosition
-    }
-
     if let sourceFileBase = sourceFileBuffer.baseAddress,
       position.utf8Offset >= 0 &&
         position.utf8Offset < sourceFileBuffer.count {
@@ -99,7 +88,6 @@ fileprivate func emitDiagnosticParts(
 func emitDiagnostic(
   diagEnginePtr: UnsafeMutablePointer<UInt8>,
   sourceFileBuffer: UnsafeMutableBufferPointer<UInt8>,
-  nodeStartOffset: Int? = nil,
   diagnostic: Diagnostic
 ) {
   // Collect all of the Fix-It changes based on their Fix-It ID.
@@ -113,7 +101,6 @@ func emitDiagnostic(
   emitDiagnosticParts(
     diagEnginePtr: diagEnginePtr,
     sourceFileBuffer: sourceFileBuffer,
-    nodeStartOffset: nodeStartOffset,
     message: diagnostic.diagMessage.message,
     severity: diagnostic.diagMessage.severity,
     position: diagnostic.position,
@@ -126,7 +113,6 @@ func emitDiagnostic(
     emitDiagnosticParts(
       diagEnginePtr: diagEnginePtr,
       sourceFileBuffer: sourceFileBuffer,
-      nodeStartOffset: nodeStartOffset,
       message: note.message,
       severity: .note, position: note.position,
       fixItChanges: fixItChangesByID[note.noteMessage.fixItID] ?? []

lib/ASTGen/Sources/ASTGen/Macros.swift

@@ -182,7 +182,6 @@ func evaluateMacro(
       emitDiagnostic(
         diagEnginePtr: diagEnginePtr,
         sourceFileBuffer: .init(mutating: sourceFile.pointee.buffer),
-        nodeStartOffset: parentSyntax.position.utf8Offset,
         diagnostic: diag
       )
     }

test/Macros/Inputs/syntax_macro_definitions.swift

@@ -63,6 +63,10 @@ struct SimpleDiagnosticMessage: DiagnosticMessage {
   let severity: DiagnosticSeverity
 }
 
+extension SimpleDiagnosticMessage: FixItMessage {
+  var fixItID: MessageID { diagnosticID }
+}
+
 public struct AddBlocker: ExpressionMacro {
   public static func expansion(
     of node: MacroExpansionExprSyntax, in context: inout MacroExpansionContext
@@ -77,12 +81,13 @@ public struct AddBlocker: ExpressionMacro {
       context.diagnose(error.asDiagnostic)
     }
 
+    // Link the folded argument back into the tree.
+    var node = node.withArgumentList(node.argumentList.replacing(childAt: 0, with: node.argumentList.first!.withExpression(foldedArgument.as(ExprSyntax.self)!)))
+
    class AddVisitor: SyntaxVisitor {
       var diagnostics: [Diagnostic] = []
-      let startPosition: AbsolutePosition
 
-      init(startPosition: AbsolutePosition) {
-        self.startPosition = startPosition
+      init() {
         super.init(viewMode: .sourceAccurate)
       }
 
@@ -91,18 +96,38 @@ public struct AddBlocker: ExpressionMacro {
       ) -> SyntaxVisitorContinueKind {
         if let binOp = node.operatorOperand.as(BinaryOperatorExprSyntax.self) {
           if binOp.operatorToken.text == "+" {
+            let messageID = MessageID(domain: "silly", id: "addblock")
             diagnostics.append(
               Diagnostic(
                 node: Syntax(node.operatorOperand),
-                position: startPosition + SourceLength(utf8Length: binOp.operatorToken.position.utf8Offset),
                 message: SimpleDiagnosticMessage(
-                  message: "blocked an add",
-                  diagnosticID: MessageID(domain: "silly", id: "addblock"),
+                  message: "blocked an add; did you mean to subtract?",
+                  diagnosticID: messageID,
                   severity: .error
                 ),
                 highlights: [
                   Syntax(node.leftOperand.withoutTrivia()),
                   Syntax(node.rightOperand.withoutTrivia())
+                ],
+                fixIts: [
+                  FixIt(
+                    message: SimpleDiagnosticMessage(
+                      message: "use '-'",
+                      diagnosticID: messageID,
+                      severity: .error
+                    ),
+                    changes: [
+                      FixIt.Change.replace(
+                        oldNode: Syntax(binOp.operatorToken.withoutTrivia()),
+                        newNode: Syntax(
+                          TokenSyntax(
+                            .spacedBinaryOperator("-"),
+                            presence: .present
+                          )
+                        )
+                      )
+                    ]
+                  ),
                 ]
               )
             )
@@ -113,8 +138,8 @@ public struct AddBlocker: ExpressionMacro {
       }
     }
 
-    let visitor = AddVisitor(startPosition: argument.position)
-    visitor.walk(Syntax(foldedArgument))
+    let visitor = AddVisitor()
+    visitor.walk(Syntax(node))
 
     for diag in visitor.diagnostics {
       context.diagnose(diag)

test/Macros/macro_expand.swift

@@ -53,7 +53,6 @@ macro addBlocker<T>(_ value: T) -> T = MacroDefinition.AddBlocker
 func testAddBlocker(a: Int, b: Int, c: Int) {
   _ = #addBlocker(a * b * c)
 #if TEST_DIAGNOSTICS
-  // FIXME: Highlight locations are wrong.
-  _ = #addBlocker(a + b * c) // expected-error{{blocked an add}}
+  _ = #addBlocker(a + b * c) // expected-error{{blocked an add; did you mean to subtract?}}{{21-22=-}}
 #endif
 }