Create actual: put top-level actual declarations in the same file

When multiple expect declarations are in the same file,
and some of them already have actual declarations for this platform,
then "Create actual fix" put other declarations into the same file
with existing actual declarations

So #KT-17058 Fixed
So #KT-21082 Fixed

idea/src/org/jetbrains/kotlin/idea/highlighter/markers/HasActualMarker.kt

@@ -21,6 +21,7 @@ import com.intellij.ide.util.DefaultPsiElementCellRenderer
 import com.intellij.psi.PsiElement
 import org.jetbrains.kotlin.descriptors.*
 import org.jetbrains.kotlin.idea.caches.resolve.findModuleDescriptor
+import org.jetbrains.kotlin.idea.caches.resolve.resolveToDescriptorIfAny
 import org.jetbrains.kotlin.idea.caches.resolve.unsafeResolveToDescriptor
 import org.jetbrains.kotlin.idea.core.toDescriptor
 import org.jetbrains.kotlin.idea.facet.implementingDescriptors
@@ -29,6 +30,7 @@ import org.jetbrains.kotlin.resolve.DescriptorToSourceUtils
 import org.jetbrains.kotlin.resolve.MultiTargetPlatform
 import org.jetbrains.kotlin.resolve.descriptorUtil.module
 import org.jetbrains.kotlin.resolve.getMultiTargetPlatform
+import org.jetbrains.kotlin.resolve.lazy.BodyResolveMode
 import org.jetbrains.kotlin.resolve.multiplatform.ExpectedActualResolver
 import java.awt.event.MouseEvent
 
@@ -87,8 +89,11 @@ private fun DeclarationDescriptor.actualsForExpected(): Collection<DeclarationDe
     return emptyList()
 }
 
-internal fun KtDeclaration.actualsForExpected(): Set<KtDeclaration> {
-    return unsafeResolveToDescriptor().actualsForExpected().mapNotNullTo(LinkedHashSet()) {
-        DescriptorToSourceUtils.descriptorToDeclaration(it) as? KtDeclaration
-    }
-}
+// null means "any platform" here
+internal fun KtDeclaration.actualsForExpected(platform: MultiTargetPlatform? = null): Set<KtDeclaration> =
+    resolveToDescriptorIfAny(BodyResolveMode.FULL)
+        ?.actualsForExpected()
+        ?.filter { platform == null || it.module.getMultiTargetPlatform() == platform }
+        ?.mapNotNullTo(LinkedHashSet()) {
+            DescriptorToSourceUtils.descriptorToDeclaration(it) as? KtDeclaration
+        } ?: emptySet()

idea/src/org/jetbrains/kotlin/idea/quickfix/expectactual/CreateActualFix.kt

@@ -32,6 +32,7 @@ import org.jetbrains.kotlin.diagnostics.DiagnosticFactory
 import org.jetbrains.kotlin.diagnostics.Errors
 import org.jetbrains.kotlin.idea.core.*
 import org.jetbrains.kotlin.idea.facet.implementingModules
+import org.jetbrains.kotlin.idea.highlighter.markers.actualsForExpected
 import org.jetbrains.kotlin.idea.project.TargetPlatformDetector
 import org.jetbrains.kotlin.idea.quickfix.KotlinQuickFixAction
 import org.jetbrains.kotlin.idea.quickfix.KotlinSingleIntentionActionFactory
@@ -40,6 +41,7 @@ import org.jetbrains.kotlin.idea.util.IdeDescriptorRenderers
 import org.jetbrains.kotlin.idea.util.application.runWriteAction
 import org.jetbrains.kotlin.lexer.KtTokens
 import org.jetbrains.kotlin.psi.*
+import org.jetbrains.kotlin.psi.psiUtil.hasExpectModifier
 import org.jetbrains.kotlin.resolve.MultiTargetPlatform
 import org.jetbrains.kotlin.resolve.getMultiTargetPlatform
 
@@ -94,6 +96,15 @@ sealed class CreateActualFix<out D : KtNamedDeclaration>(
 
     private fun getOrCreateImplementationFile(): KtFile? {
         val declaration = element as? KtNamedDeclaration ?: return null
+        val parent = declaration.parent
+        if (parent is KtFile) {
+            for (otherDeclaration in parent.declarations) {
+                if (otherDeclaration === declaration) continue
+                if (!otherDeclaration.hasExpectModifier()) continue
+                val actualDeclaration = otherDeclaration.actualsForExpected(actualPlatform).singleOrNull() ?: continue
+                return actualDeclaration.containingKtFile
+            }
+        }
         val name = declaration.name ?: return null
 
         val expectedDir = declaration.containingFile.containingDirectory