[FIR] Implement NULL_FOR_NONNULL_TYPE diagnostics, fix tests

idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDataClassConverters.kt

@@ -739,6 +739,12 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
+    add(FirErrors.NULL_FOR_NONNULL_TYPE) { firDiagnostic ->
+        NullForNonnullTypeImpl(
+            firDiagnostic as FirPsiDiagnostic<*>,
+            token,
+        )
+    }
     add(FirErrors.INAPPLICABLE_LATEINIT_MODIFIER) { firDiagnostic ->
         InapplicableLateinitModifierImpl(
             firDiagnostic.a,

idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnostics.kt

@@ -532,6 +532,10 @@ sealed class KtFirDiagnostic<PSI: PsiElement> : KtDiagnosticWithPsi<PSI> {
         abstract val actualType: KtType
     }
 
+    abstract class NullForNonnullType : KtFirDiagnostic<PsiElement>() {
+        override val diagnosticClass get() = NullForNonnullType::class
+    }
+
     abstract class InapplicableLateinitModifier : KtFirDiagnostic<KtModifierListOwner>() {
         override val diagnosticClass get() = InapplicableLateinitModifier::class
         abstract val reason: String

idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnosticsImpl.kt

@@ -851,6 +851,13 @@ internal class ArgumentTypeMismatchImpl(
     override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
 }
 
+internal class NullForNonnullTypeImpl(
+    firDiagnostic: FirPsiDiagnostic<*>,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.NullForNonnullType(), KtAbstractFirDiagnostic<PsiElement> {
+    override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
+}
+
 internal class InapplicableLateinitModifierImpl(
     override val reason: String,
     firDiagnostic: FirPsiDiagnostic<*>,

idea/testData/checker/FunctionReturnTypes.fir.kt

@@ -27,7 +27,7 @@ fun foo(expr: StringBuilder): Int {
 
 fun unitShort() : Unit = Unit
 fun unitShortConv() : Unit = <error descr="[RETURN_TYPE_MISMATCH] Return type mismatch: expected kotlin/Unit, actual kotlin/Int">1</error>
-fun unitShortNull() : Unit = <error descr="[RETURN_TYPE_MISMATCH] Return type mismatch: expected kotlin/Unit, actual kotlin/Nothing?">null</error>
+fun unitShortNull() : Unit = <error descr="[NULL_FOR_NONNULL_TYPE] ">null</error>
 
 fun intEmpty() : Int {}
 fun intShortInfer() = 1

idea/testData/checker/NullAsAnnotationArgument.fir.kt

@@ -3,8 +3,8 @@ package test
 
 annotation class A(val value: String)
 
-@A(<error descr="[ARGUMENT_TYPE_MISMATCH] Argument type mismatch: actual type is kotlin/Nothing? but kotlin/String was expected">null</error>)
+@A(<error descr="[NULL_FOR_NONNULL_TYPE] ">null</error>)
 fun foo() {}
 
-@A(<error descr="[ARGUMENT_TYPE_MISMATCH] Argument type mismatch: actual type is kotlin/Nothing? but kotlin/String was expected">null</error>)
+@A(<error descr="[NULL_FOR_NONNULL_TYPE] ">null</error>)
 class B