diff --git a/compiler/fir/raw-fir/light-tree2fir/src/org/jetbrains/kotlin/fir/lightTree/converter/DeclarationsConverter.kt b/compiler/fir/raw-fir/light-tree2fir/src/org/jetbrains/kotlin/fir/lightTree/converter/DeclarationsConverter.kt
index 9cf599e809a..53d17ab6622 100644
--- a/compiler/fir/raw-fir/light-tree2fir/src/org/jetbrains/kotlin/fir/lightTree/converter/DeclarationsConverter.kt
+++ b/compiler/fir/raw-fir/light-tree2fir/src/org/jetbrains/kotlin/fir/lightTree/converter/DeclarationsConverter.kt
@@ -1967,7 +1967,7 @@ class DeclarationsConverter(
     private val extensionFunctionAnnotation = buildAnnotationCall {
         annotationTypeRef = buildResolvedTypeRef {
             type = ConeClassLikeTypeImpl(
-                ConeClassLikeLookupTagImpl(ClassId.fromString(EXTENSION_FUNCTION_ANNOTATION)),
+                ConeClassLikeLookupTagImpl(EXTENSION_FUNCTION_ANNOTATION),
                 emptyArray(),
                 false
             )
diff --git a/compiler/fir/raw-fir/psi2fir/src/org/jetbrains/kotlin/fir/builder/RawFirBuilder.kt b/compiler/fir/raw-fir/psi2fir/src/org/jetbrains/kotlin/fir/builder/RawFirBuilder.kt
index 16d3a3ba257..fff2e5dc3d5 100644
--- a/compiler/fir/raw-fir/psi2fir/src/org/jetbrains/kotlin/fir/builder/RawFirBuilder.kt
+++ b/compiler/fir/raw-fir/psi2fir/src/org/jetbrains/kotlin/fir/builder/RawFirBuilder.kt
@@ -38,7 +38,6 @@ import org.jetbrains.kotlin.fir.types.impl.FirTypeArgumentListImpl
 import org.jetbrains.kotlin.fir.types.impl.FirTypePlaceholderProjection
 import org.jetbrains.kotlin.lexer.KtTokens.*
 import org.jetbrains.kotlin.name.CallableId
-import org.jetbrains.kotlin.name.LocalCallableIdConstructor
 import org.jetbrains.kotlin.name.ClassId
 import org.jetbrains.kotlin.name.Name
 import org.jetbrains.kotlin.psi.*
@@ -2225,7 +2224,7 @@ open class RawFirBuilder(
     private val extensionFunctionAnnotation = buildAnnotationCall {
         annotationTypeRef = buildResolvedTypeRef {
             type = ConeClassLikeTypeImpl(
-                ConeClassLikeLookupTagImpl(ClassId.fromString(EXTENSION_FUNCTION_ANNOTATION)),
+                ConeClassLikeLookupTagImpl(EXTENSION_FUNCTION_ANNOTATION),
                 emptyArray(),
                 isNullable = false
             )
diff --git a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/Utils.kt b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/Utils.kt
index d11bef39459..7e407939201 100644
--- a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/Utils.kt
+++ b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/Utils.kt
@@ -70,12 +70,12 @@ fun FirSourceElement.getWholeQualifierSourceIfPossible(stepsToWholeQualifier: In
     return when (this) {
         is FirRealPsiSourceElement<*> -> {
             val qualifiersChain = generateSequence(psi) { it.parent }
-            val wholeQualifier = qualifiersChain.drop(stepsToWholeQualifier).first()
+            val wholeQualifier = qualifiersChain.elementAt(stepsToWholeQualifier)
             wholeQualifier.toFirPsiSourceElement() as FirRealPsiSourceElement
         }
         is FirLightSourceElement -> {
             val qualifiersChain = generateSequence(lighterASTNode) { treeStructure.getParent(it) }
-            val wholeQualifier = qualifiersChain.drop(stepsToWholeQualifier).first()
+            val wholeQualifier = qualifiersChain.elementAt(stepsToWholeQualifier)
             wholeQualifier.toFirLightSourceElement(
                 treeStructure,
                 startOffset = this.startOffset,
diff --git a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/types/FirTypeUtils.kt b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/types/FirTypeUtils.kt
index c0e36bb7802..e2d7352370a 100644
--- a/compiler/fir/tree/src/org/jetbrains/kotlin/fir/types/FirTypeUtils.kt
+++ b/compiler/fir/tree/src/org/jetbrains/kotlin/fir/types/FirTypeUtils.kt
@@ -72,13 +72,13 @@ val FirFunctionTypeRef.parametersCount: Int
     else
         valueParameters.size
 
-const val EXTENSION_FUNCTION_ANNOTATION = "kotlin/ExtensionFunctionType"
+val EXTENSION_FUNCTION_ANNOTATION = ClassId.fromString("kotlin/ExtensionFunctionType")
 
 val FirAnnotationCall.isExtensionFunctionAnnotationCall: Boolean
     get() = (this as? FirAnnotationCall)?.let { annotationCall ->
         (annotationCall.annotationTypeRef as? FirResolvedTypeRef)?.let { typeRef ->
             (typeRef.type as? ConeClassLikeType)?.let {
-                it.lookupTag.classId.asString() == EXTENSION_FUNCTION_ANNOTATION
+                it.lookupTag.classId == EXTENSION_FUNCTION_ANNOTATION
             }
         }
     } == true
diff --git a/core/compiler.common/src/org/jetbrains/kotlin/name/ClassId.java b/core/compiler.common/src/org/jetbrains/kotlin/name/ClassId.java
index 091d1a44500..c27da1ad63d 100644
--- a/core/compiler.common/src/org/jetbrains/kotlin/name/ClassId.java
+++ b/core/compiler.common/src/org/jetbrains/kotlin/name/ClassId.java
@@ -105,8 +105,16 @@ public final class ClassId {
 
     @NotNull
     public static ClassId fromString(@NotNull String string, boolean isLocal) {
-        String packageName = StringsKt.substringBeforeLast(string, '/', "").replace('/', '.');
-        String className = StringsKt.substringAfterLast(string, '/', string);
+        int lastSlashIndex = string.lastIndexOf("/");
+        String packageName;
+        String className;
+        if (lastSlashIndex == -1) {
+            packageName = "";
+            className = string;
+        } else {
+            packageName = string.substring(0, lastSlashIndex).replace('/', '.');
+            className = string.substring(lastSlashIndex + 1);
+        }
         return new ClassId(new FqName(packageName), new FqName(className), isLocal);
     }
 
diff --git a/core/compiler.common/src/org/jetbrains/kotlin/name/NameUtils.kt b/core/compiler.common/src/org/jetbrains/kotlin/name/NameUtils.kt
index 8780bbeb09f..eb904e69553 100644
--- a/core/compiler.common/src/org/jetbrains/kotlin/name/NameUtils.kt
+++ b/core/compiler.common/src/org/jetbrains/kotlin/name/NameUtils.kt
@@ -42,7 +42,7 @@ object NameUtils {
             // NB `uppercase` uses Locale.ROOT and is locale-independent.
             // See Javadoc on java.lang.String.toUpperCase() for more details.
             if (Character.isJavaIdentifierStart(str[0]))
-                str.substring(0, 1).uppercase() + str.substring(1)
+                str[0].uppercase() + str.substring(1)
             else
                 "_$str"