[Macros] Note when diagnostics came from the implementation of a macro

Author: DougGregor

lib/ASTGen/Sources/ASTGen/Diagnostics.swift

@@ -88,7 +88,8 @@ fileprivate func emitDiagnosticParts(
 func emitDiagnostic(
   diagEnginePtr: UnsafeMutablePointer<UInt8>,
   sourceFileBuffer: UnsafeMutableBufferPointer<UInt8>,
-  diagnostic: Diagnostic
+  diagnostic: Diagnostic,
+  messageSuffix: String? = nil
 ) {
   // Collect all of the Fix-It changes based on their Fix-It ID.
   var fixItChangesByID: [MessageID : [FixIt.Change]] = [:]
@@ -101,7 +102,7 @@ func emitDiagnostic(
   emitDiagnosticParts(
     diagEnginePtr: diagEnginePtr,
     sourceFileBuffer: sourceFileBuffer,
-    message: diagnostic.diagMessage.message,
+    message: diagnostic.diagMessage.message + (messageSuffix ?? ""),
     severity: diagnostic.diagMessage.severity,
     position: diagnostic.position,
     highlights: diagnostic.highlights,

lib/ASTGen/Sources/ASTGen/Macros.swift

@@ -176,13 +176,13 @@ func evaluateMacro(
     }
 
     // Emit diagnostics accumulated in the context.
+    let macroName = parentExpansion.macro.withoutTrivia().description
     for diag in context.diagnostics {
-      // FIXME: Consider tacking on a note that says that this diagnostic
-      // came from a macro expansion.
       emitDiagnostic(
         diagEnginePtr: diagEnginePtr,
         sourceFileBuffer: .init(mutating: sourceFile.pointee.buffer),
-        diagnostic: diag
+        diagnostic: diag,
+        messageSuffix: " (from macro '\(macroName)')"
       )
     }
 

test/Macros/macro_expand.swift

@@ -53,6 +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
-  _ = #addBlocker(a + b * c) // expected-error{{blocked an add; did you mean to subtract?}}{{21-22=-}}
+  _ = #addBlocker(a + b * c) // expected-error{{blocked an add; did you mean to subtract? (from macro 'addBlocker')}}{{21-22=-}}
 #endif
 }