Better compiler error message for checked annotations

compiler-plugin/compiler-plugin-k2/src/main/core/kotlinx/rpc/codegen/checkers/FirCheckedAnnotationCheckers.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2023-2024 JetBrains s.r.o and contributors. Use of this source code is governed by the Apache 2.0 license.
+ * Copyright 2023-2025 JetBrains s.r.o and contributors. Use of this source code is governed by the Apache 2.0 license.
  */
 
 package kotlinx.rpc.codegen.checkers
@@ -231,6 +231,7 @@ object FirCheckedAnnotationHelper {
                 source = source,
                 context = context,
                 reporter = reporter,
+                typeArgumentIndex = i,
             )
         }
 
@@ -304,6 +305,7 @@ object FirCheckedAnnotationHelper {
         source: KtSourceElement?,
         context: CheckerContext,
         reporter: DiagnosticReporter,
+        typeArgumentIndex: Int,
     ) {
         for (annotationClass in annotations) {
             val hasCheckedAnnotation = hasCheckedAnnotation(
@@ -316,7 +318,9 @@ object FirCheckedAnnotationHelper {
                 reporter.reportOn(
                     source = source,
                     factory = FirRpcDiagnostics.CHECKED_ANNOTATION_VIOLATION,
-                    a = annotationClass.defaultType(),
+                    a = typeArgumentIndex,
+                    b = annotationClass.defaultType(),
+                    c = classSymbol,
                     context = context,
                 )
             }

compiler-plugin/compiler-plugin-k2/src/main/core/kotlinx/rpc/codegen/checkers/diagnostics/FirRpcDiagnostics.kt

@@ -11,7 +11,9 @@ import org.jetbrains.kotlin.diagnostics.SourceElementPositioningStrategies
 import org.jetbrains.kotlin.diagnostics.error0
 import org.jetbrains.kotlin.diagnostics.error1
 import org.jetbrains.kotlin.diagnostics.error2
+import org.jetbrains.kotlin.diagnostics.error3
 import org.jetbrains.kotlin.diagnostics.rendering.RootDiagnosticRendererFactory
+import org.jetbrains.kotlin.fir.symbols.FirBasedSymbol
 import org.jetbrains.kotlin.fir.types.ConeKotlinType
 import org.jetbrains.kotlin.name.Name
 import org.jetbrains.kotlin.psi.KtAnnotated
@@ -27,7 +29,7 @@ object FirRpcDiagnostics {
     val MISSING_RPC_ANNOTATION by error0<KtAnnotated>()
     val MISSING_SERIALIZATION_MODULE by error0<KtAnnotated>()
     val WRONG_RPC_ANNOTATION_TARGET by error1<KtAnnotated, ConeKotlinType>()
-    val CHECKED_ANNOTATION_VIOLATION by error1<KtAnnotated, ConeKotlinType>()
+    val CHECKED_ANNOTATION_VIOLATION by error3<KtAnnotated, Int, ConeKotlinType, FirBasedSymbol<*>>()
     val NON_SUSPENDING_REQUEST_WITHOUT_STREAMING_RETURN_TYPE by error0<KtElement>()
     val AD_HOC_POLYMORPHISM_IN_RPC_SERVICE by error2<KtElement, Int, Name>()
     val TYPE_PARAMETERS_IN_RPC_FUNCTION by error0<KtElement>(SourceElementPositioningStrategies.TYPE_PARAMETERS_LIST)

compiler-plugin/compiler-plugin-k2/src/main/core/kotlinx/rpc/codegen/checkers/diagnostics/RpcDiagnosticRendererFactory.kt

@@ -35,9 +35,12 @@ object RpcDiagnosticRendererFactory : BaseDiagnosticRendererFactory() {
 
         put(
             factory = FirRpcDiagnostics.CHECKED_ANNOTATION_VIOLATION,
-            message = "Type argument marked with {0} annotation " +
-                    "must be annotated with {0} or an annotation annotated with {0}.",
-            rendererA = FirDiagnosticRenderers.RENDER_TYPE,
+            message = "{0} type argument is marked with @{1} annotation, but inferred type is {2}. " +
+                    "Provide a type that is marked with @{1} annotation explicitly " +
+                    "or remove the annotation from the type argument declaration.",
+            rendererA = Renderer { it.indexPositionSpelled() },
+            rendererB = FirDiagnosticRenderers.RENDER_TYPE,
+            rendererC = FirDiagnosticRenderers.SYMBOL,
         )
 
         put(
@@ -64,6 +67,18 @@ object RpcDiagnosticRendererFactory : BaseDiagnosticRendererFactory() {
     }
 }
 
+private fun Int.indexPositionSpelled(): String {
+    val padded = this + 1
+    val suffix = when (padded % 10) {
+        1 -> "st"
+        2 -> "nd"
+        3 -> "rd"
+        else -> "th"
+    }
+
+    return "$padded$suffix"
+}
+
 class RpcStrictModeDiagnosticRendererFactory(
     private val diagnostics: FirRpcStrictModeDiagnostics,
 ) : BaseDiagnosticRendererFactory() {