FIR IDE: Add completion for top level functions and properties

idea/idea-fir/src/org/jetbrains/kotlin/idea/completion/KotlinFirCompletionContributor.kt

@@ -85,7 +85,7 @@ private object KotlinFirCompletionProvider : CompletionProvider<CompletionParame
  * TODO refactor it, try to split into several classes, or decompose it into several classes.
  */
 private class KotlinCommonCompletionProvider(
-    prefixMatcher: PrefixMatcher,
+    private val prefixMatcher: PrefixMatcher,
     private val indexHelper: IndexHelper
 ) {
     private val lookupElementFactory = KotlinFirLookupElementFactory()
@@ -93,6 +93,9 @@ private class KotlinCommonCompletionProvider(
     private val scopeNameFilter: KtScopeNameFilter =
         { name -> !name.isSpecial && prefixMatcher.prefixMatches(name.identifier) }
 
+    private val shouldCompleteTopLevelCallablesFromIndex: Boolean
+        get() = prefixMatcher.prefix.isNotEmpty()
+
     private fun KtAnalysisSession.addSymbolToCompletion(completionResultSet: CompletionResultSet, expectedType: KtType?, symbol: KtSymbol) {
         if (symbol !is KtNamedSymbol) return
         with(lookupElementFactory) {
@@ -199,6 +202,14 @@ private class KotlinCommonCompletionProvider(
         availableNonExtensions.forEach { addSymbolToCompletion(result, expectedType, it) }
         extensionsWhichCanBeCalled.forEach { addSymbolToCompletion(result, expectedType, it) }
 
+        if (shouldCompleteTopLevelCallablesFromIndex) {
+            val topLevelCallables = indexHelper.getTopLevelCallables(scopeNameFilter)
+            topLevelCallables.asSequence()
+                .map { it.getSymbol() }
+                .forEach { addSymbolToCompletion(result, expectedType, it) }
+        }
+
+
         collectTypesCompletion(result, implicitScopes, expectedType)
     }
 }

idea/idea-frontend-fir/idea-fir-low-level-api/src/org/jetbrains/kotlin/idea/fir/low/level/api/IndexHelper.kt

@@ -9,6 +9,7 @@ import com.intellij.openapi.progress.ProgressManager
 import com.intellij.openapi.project.Project
 import com.intellij.psi.PsiElement
 import com.intellij.psi.search.GlobalSearchScope
+import com.intellij.psi.stubs.StringStubIndexExtension
 import com.intellij.psi.stubs.StubIndex
 import com.intellij.psi.stubs.StubIndexKey
 import org.jetbrains.kotlin.name.CallableId
@@ -16,10 +17,7 @@ import org.jetbrains.kotlin.idea.stubindex.*
 import org.jetbrains.kotlin.name.ClassId
 import org.jetbrains.kotlin.name.FqName
 import org.jetbrains.kotlin.name.Name
-import org.jetbrains.kotlin.psi.KtClassOrObject
-import org.jetbrains.kotlin.psi.KtFunction
-import org.jetbrains.kotlin.psi.KtNamedFunction
-import org.jetbrains.kotlin.psi.KtProperty
+import org.jetbrains.kotlin.psi.*
 
 /*
 * Move to another module
@@ -86,6 +84,20 @@ public class IndexHelper(val project: Project, private val scope: GlobalSearchSc
             .toList()
     }
 
+    fun getTopLevelCallables(nameFilter: (Name) -> Boolean): Collection<KtCallableDeclaration> {
+        fun sequenceOfElements(index: StringStubIndexExtension<out KtCallableDeclaration>): Sequence<KtCallableDeclaration> =
+            index.getAllKeys(project).asSequence()
+                .onEach { ProgressManager.checkCanceled() }
+                .filter { fqName -> nameFilter(getShortName(fqName)) }
+                .flatMap { fqName -> index[fqName, project, scope] }
+                .filter { it.receiverTypeReference == null }
+
+        val functions = sequenceOfElements(KotlinTopLevelFunctionFqnNameIndex.getInstance())
+        val properties = sequenceOfElements(KotlinTopLevelPropertyFqnNameIndex.getInstance())
+
+        return (functions + properties).toList()
+    }
+
     companion object {
         private fun CallableId.asStringForIndexes(): String =
             (if (packageName.isRoot) callableName.asString() else toString()).replace('/', '.')