Native: fix error message for C union with nested struct with bitfields

tryRenderStructOrUnion can return null for a struct or union to indicate
that passing it by value to or from C is not supported (e.g. it has
bitfields).

It incorrectly handled the case when passing a nested struct or union by
value was unsupported -- instead of null, it returned incorrect struct
declaration.

As a result, when passing such a complex struct or union by value to or
from C, the backend generated C stubs with syntax errors, which resulted
into obscure compilation error ("unable to compile C bridge").

This commit fixes this case in tryRenderStructOrUnion function, and thus
makes the compilation error message more understandable.

^KT-55030

kotlin-native/Interop/StubGenerator/src/main/kotlin/org/jetbrains/kotlin/native/interop/gen/StructRendering.kt

@@ -113,7 +113,7 @@ private fun tryRenderVar(type: Type, name: String): String? = when (type) {
     is IntegerType -> "${type.spelling} $name"
     is FloatingType -> "${type.spelling} $name"
     is VectorType -> "${type.spelling} $name"
-    is RecordType -> "${tryRenderStructOrUnion(type.decl.def!!)} $name"
+    is RecordType -> tryRenderStructOrUnion(type.decl.def!!)?.let { "$it $name" }
     is EnumType -> tryRenderVar(type.def.baseType, name)
     is PointerType -> "void* $name"
     is ConstArrayType -> tryRenderVar(type.elemType, "$name[${type.length}]")

kotlin-native/Interop/StubGenerator/src/test/kotlin/org/jetbrains/kotlin/native/interop/gen/StructRenderingTests.kt

@@ -0,0 +1,55 @@
+/*
+ * Copyright 2010-2023 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.native.interop.gen
+
+import org.jetbrains.kotlin.native.interop.indexer.StructDef
+import org.jetbrains.kotlin.native.interop.indexer.buildNativeIndex
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class StructRenderingTests : InteropTestsBase() {
+    // See https://youtrack.jetbrains.com/issue/KT-55030#focus=Comments-27-6742454.0-0
+    @Test
+    fun kt55030() {
+        val files = TempFiles("kt55030")
+        val header = files.file("header.h", """
+            union SupportedValue {
+                struct {
+                    int a;
+                } b;
+                int c;
+            };
+
+            union UnsupportedValue {
+                struct {
+                    int a : 5;
+                } b;
+                int c;
+            };
+        """.trimIndent())
+        val defFile = files.file("kt55030.def", """
+            headers = ${header.name}
+        """.trimIndent())
+
+
+        val library = buildNativeLibraryFrom(defFile, files.directory)
+        val index = buildNativeIndex(library, verbose = false).index
+
+        fun getUnionDef(name: String): StructDef {
+            return index.structs.find { it.spelling == "union $name" }!!.def!!
+        }
+
+        assertEquals(
+                null,
+                tryRenderStructOrUnion(getUnionDef("UnsupportedValue"))
+        )
+
+        assertEquals(
+                "union { struct  { int a; } b; int c; }",
+                tryRenderStructOrUnion(getUnionDef("SupportedValue"))
+        )
+    }
+}