[FIR] Fixes for RedundantReturnUnitType checker

compiler/fir/analysis-tests/testData/extendedCheckers/RedundantReturnUnitTypeChecker.kt

@@ -1,4 +1,12 @@
+data class My(val x: Unit)
+
+interface I {
+    val x: Unit
+}
+
 class A {
+    fun too(): @Annotation Unit {}
+
     fun foo(): <!REDUNDANT_RETURN_UNIT_TYPE!>Unit<!>
     {
     }

compiler/fir/analysis-tests/testData/extendedCheckers/RedundantReturnUnitTypeChecker.txt

@@ -1,9 +1,30 @@
 FILE: RedundantReturnUnitTypeChecker.kt
+    public final data class My : R|kotlin/Any| {
+        public constructor(x: R|kotlin/Unit|): R|My| {
+            super<R|kotlin/Any|>()
+        }
+
+        public final val x: R|kotlin/Unit| = R|<local>/x|
+            public get(): R|kotlin/Unit|
+
+        public final fun component1(): R|kotlin/Unit|
+
+        public final fun copy(x: R|kotlin/Unit| = this@R|/My|.R|/My.x|): R|My|
+
+    }
+    public abstract interface I : R|kotlin/Any| {
+        public abstract val x: R|kotlin/Unit|
+            public get(): R|kotlin/Unit|
+
+    }
     public final class A : R|kotlin/Any| {
         public constructor(): R|A| {
             super<R|kotlin/Any|>()
         }
 
+        public final fun too(): @R|kotlin/Annotation|() R|kotlin/Unit| {
+        }
+
         public final fun foo(): R|kotlin/Unit| {
         }
 

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/extended/RedundantReturnUnitTypeChecker.kt

@@ -5,6 +5,7 @@
 
 package org.jetbrains.kotlin.fir.analysis.checkers.extended
 
+import org.jetbrains.kotlin.fir.FirFakeSourceElement
 import org.jetbrains.kotlin.fir.analysis.checkers.context.CheckerContext
 import org.jetbrains.kotlin.fir.analysis.checkers.declaration.FirBasicDeclarationChecker
 import org.jetbrains.kotlin.fir.analysis.diagnostics.DiagnosticReporter
@@ -19,8 +20,12 @@ object RedundantReturnUnitType : FirBasicDeclarationChecker() {
         if (declaration !is FirSimpleFunction) return
         if (declaration.body is FirSingleExpressionBlock) return
         if (declaration.returnTypeRef.source == null) return
+        if (declaration.source is FirFakeSourceElement<*>) return
 
-        if (declaration.returnTypeRef.isUnit) {
+        val returnType = declaration.returnTypeRef
+        if (returnType.annotations.isNotEmpty()) return
+
+        if (returnType.isUnit) {
             reporter.report(declaration.returnTypeRef.source, FirErrors.REDUNDANT_RETURN_UNIT_TYPE)
         }
     }

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/collectors/components/AbstractDiagnosticCollectorComponent.kt

@@ -20,4 +20,4 @@ abstract class AbstractDiagnosticCollectorComponent(private val collector: Abstr
     protected fun runCheck(block: (DiagnosticReporter) -> Unit) {
         collector.runCheck(block)
     }
-}
+}