[K2/N] Read KDoc strings from LightTree nodes

^KT-56090

Merge-request: KT-MR-9613
Merged-by: Vladimir Sukharev <Vladimir.Sukharev@jetbrains.com>

compiler/fir/fir-serialization/src/org/jetbrains/kotlin/fir/serialization/FirKLibSerializerExtension.kt

@@ -5,6 +5,8 @@
 
 package org.jetbrains.kotlin.fir.serialization
 
+import com.intellij.lang.LighterASTNode
+import com.intellij.openapi.util.Ref
 import org.jetbrains.kotlin.fir.FirSession
 import org.jetbrains.kotlin.fir.declarations.*
 import org.jetbrains.kotlin.fir.resolve.providers.firProvider
@@ -13,6 +15,7 @@ import org.jetbrains.kotlin.fir.symbols.impl.FirCallableSymbol
 import org.jetbrains.kotlin.fir.symbols.impl.FirClassLikeSymbol
 import org.jetbrains.kotlin.fir.types.ConeErrorType
 import org.jetbrains.kotlin.fir.types.ConeFlexibleType
+import org.jetbrains.kotlin.lexer.KtTokens
 import org.jetbrains.kotlin.library.metadata.KlibMetadataProtoBuf
 import org.jetbrains.kotlin.library.metadata.KlibMetadataSerializerProtocol
 import org.jetbrains.kotlin.metadata.ProtoBuf
@@ -96,10 +99,12 @@ class FirKLibSerializerExtension(
         }
     }
 
-    private fun FirDeclaration.findKDocString(): String? {
-        // TODO: KT-56090
-        return null
-    }
+    private fun FirDeclaration.findKDocString(): String? =
+        source?.let {
+            val kidsRef = Ref<Array<LighterASTNode?>>()
+            it.treeStructure.getChildren(it.lighterASTNode, kidsRef)
+            kidsRef.get().singleOrNull { it?.tokenType == KtTokens.DOC_COMMENT }?.toString()
+        }
 
     @Suppress("Reformat")
     private fun <

kotlin-native/backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/backend/konan/FirNativeSerializer.kt

@@ -68,7 +68,7 @@ internal fun PhaseContext.firSerializer(
                 FirKLibSerializerExtension(
                         session, metadataVersion,
                         ConstValueProviderImpl(input.components),
-                        allowErrorTypes = false, exportKDoc = false
+                        allowErrorTypes = false, exportKDoc = shouldExportKDoc()
                 ),
                 configuration.languageVersionSettings,
         )

native/native.tests/testData/ObjCExport/kdocExport/kdocExport.kt

@@ -1,5 +1,3 @@
-// KT-56090: [K2/N] Emit DocString klib extensions for ObjCExport
-// MUTED_WHEN: K2
 /*
  * 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.