FIR CLI: use VFS for LT files loading

may improve performance in some cases and is consistent with handling
of PSI files

compiler/cli/src/org/jetbrains/kotlin/cli/jvm/compiler/VfsBasedProjectEnvironment.kt

@@ -107,6 +107,9 @@ open class VfsBasedProjectEnvironment(
         baseModuleData: FirModuleData,
         fileSearchScope: AbstractProjectFileSearchScope
     ) = FirJavaFacade(firSession, baseModuleData, project.createJavaClassFinder(fileSearchScope.asPsiSearchScope()))
+
+    override fun getFileText(filePath: String): String? =
+        localFileSystem.findFileByPath(filePath)?.inputStream?.reader(Charsets.UTF_8)?.readText()
 }
 
 private fun AbstractProjectFileSearchScope.asPsiSearchScope() =

compiler/cli/src/org/jetbrains/kotlin/cli/jvm/compiler/pipeline/compilerPipeline.kt

@@ -311,8 +311,14 @@ fun compileModuleToAnalyzedFir(
     val countFilesAndLines = if (performanceManager == null) null else performanceManager::addSourcesStats
 
     // raw fir
-    val commonRawFir = commonSession?.buildFirViaLightTree(input.commonSources, diagnosticsReporter, countFilesAndLines)
-    val rawFir = session.buildFirViaLightTree(input.platformSources, diagnosticsReporter, countFilesAndLines)
+    val commonRawFir = commonSession?.buildFirViaLightTree(
+        input.commonSources,
+        environment.projectEnvironment,
+        diagnosticsReporter,
+        countFilesAndLines
+    )
+    val rawFir =
+        session.buildFirViaLightTree(input.platformSources, environment.projectEnvironment, diagnosticsReporter, countFilesAndLines)
 
     // resolution
     commonSession?.apply {

compiler/fir/entrypoint/src/org/jetbrains/kotlin/fir/pipeline/buildFir.kt

@@ -5,8 +5,7 @@
 
 package org.jetbrains.kotlin.fir.pipeline
 
-import com.intellij.openapi.util.io.FileUtil
-import com.intellij.openapi.vfs.CharsetToolkit
+import com.intellij.openapi.util.text.StringUtilRt
 import org.jetbrains.kotlin.diagnostics.DiagnosticReporter
 import org.jetbrains.kotlin.fir.FirSession
 import org.jetbrains.kotlin.fir.builder.PsiHandlingMode
@@ -15,12 +14,15 @@ import org.jetbrains.kotlin.fir.declarations.FirFile
 import org.jetbrains.kotlin.fir.lightTree.LightTree2Fir
 import org.jetbrains.kotlin.fir.resolve.providers.firProvider
 import org.jetbrains.kotlin.fir.resolve.providers.impl.FirProviderImpl
+import org.jetbrains.kotlin.fir.session.environment.AbstractProjectEnvironment
 import org.jetbrains.kotlin.fir.session.sourcesToPathsMapper
 import org.jetbrains.kotlin.psi.KtFile
 import java.io.File
+import java.io.FileNotFoundException
 
 fun FirSession.buildFirViaLightTree(
     files: Collection<File>,
+    projectEnvironment: AbstractProjectEnvironment,
     diagnosticsReporter: DiagnosticReporter? = null,
     reportFilesAndLines: ((Int, Int) -> Unit)? = null
 ): List<FirFile> {
@@ -30,7 +32,8 @@ fun FirSession.buildFirViaLightTree(
     val shouldCountLines = (reportFilesAndLines != null)
     var linesCount = 0
     val firFiles = files.map { file ->
-        val code = FileUtil.loadFile(file, CharsetToolkit.UTF8, true /* code below relies on conversion */)
+        val text = projectEnvironment.getFileText(file.absolutePath) ?: throw FileNotFoundException(file.path)
+        val code = StringUtilRt.convertLineSeparators(text)
         if (shouldCountLines) {
             linesCount += code.count { it == '\n' } // assuming converted line separators
         }

compiler/fir/entrypoint/src/org/jetbrains/kotlin/fir/session/environment/AbstractProjectEnvironment.kt

@@ -60,4 +60,6 @@ interface AbstractProjectEnvironment {
         baseModuleData: FirModuleData,
         fileSearchScope: AbstractProjectFileSearchScope
     ): FirJavaFacade
+
+    fun getFileText(filePath: String): String?
 }