FIR: Add fake root prefix for IDE resolution

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/FirQualifiedNameResolver.kt

@@ -20,6 +20,8 @@ import org.jetbrains.kotlin.fir.types.FirTypeProjection
 import org.jetbrains.kotlin.name.FqName
 import org.jetbrains.kotlin.name.Name
 
+const val ROOT_PREFIX_FOR_IDE_RESOLUTION_MODE = "_root_ide_package_"
+
 class FirQualifiedNameResolver(private val components: BodyResolveComponents) {
     private val session = components.session
     private var qualifierStack = mutableListOf<NameWithTypeArguments>()
@@ -66,6 +68,11 @@ class FirQualifiedNameResolver(private val components: BodyResolveComponents) {
         }
         val symbolProvider = session.symbolProvider
         var qualifierParts = qualifierStack.asReversed().map { it.name.asString() }
+
+        val fakeRootIdePrefixIsPresent = qualifierParts.firstOrNull() == ROOT_PREFIX_FOR_IDE_RESOLUTION_MODE
+        if (fakeRootIdePrefixIsPresent) {
+            qualifierParts = qualifierParts.drop(1)
+        }
         var resolved: PackageOrClass?
         do {
             resolved = resolveToPackageOrClass(
@@ -78,6 +85,11 @@ class FirQualifiedNameResolver(private val components: BodyResolveComponents) {
 
         if (resolved != null) {
             qualifierPartsToDrop = qualifierParts.size - 1
+            // we would need to drop fake package too
+            if (fakeRootIdePrefixIsPresent) {
+                qualifierPartsToDrop += 1
+            }
+
             return buildResolvedQualifier {
                 this.source = source
                 packageFqName = resolved.packageFqName

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/providers/impl/FirQualifierResolverImpl.kt

@@ -7,6 +7,7 @@ package org.jetbrains.kotlin.fir.resolve.providers.impl
 
 import org.jetbrains.kotlin.fir.FirSession
 import org.jetbrains.kotlin.fir.NoMutableState
+import org.jetbrains.kotlin.fir.ROOT_PREFIX_FOR_IDE_RESOLUTION_MODE
 import org.jetbrains.kotlin.fir.resolve.FirQualifierResolver
 import org.jetbrains.kotlin.fir.resolve.symbolProvider
 import org.jetbrains.kotlin.fir.symbols.impl.FirClassifierSymbol
@@ -29,6 +30,10 @@ class FirQualifierResolverImpl(val session: FirSession) : FirQualifierResolver()
     }
 
     override fun resolveSymbol(parts: List<FirQualifierPart>): FirClassifierSymbol<*>? {
+        if (parts.firstOrNull()?.name?.asString() == ROOT_PREFIX_FOR_IDE_RESOLUTION_MODE) {
+            return resolveSymbol(parts.drop(1))
+        }
+
         val firProvider = session.symbolProvider
 
         if (parts.isNotEmpty()) {