[LL FIR] AbstractDependentCopyTest: supports custom text in copy

This provides an ability to test more real-world cases.
E.g., we can emulate completion behavior

^KT-60987

analysis/low-level-api-fir/src/org/jetbrains/kotlin/analysis/low/level/api/fir/api/LowLevelFirApiFacadeForResolveOnAir.kt

@@ -26,6 +26,7 @@ import org.jetbrains.kotlin.analysis.low.level.api.fir.util.codeFragment
 import org.jetbrains.kotlin.analysis.low.level.api.fir.util.errorWithFirSpecificEntries
 import org.jetbrains.kotlin.analysis.low.level.api.fir.util.isScriptStatement
 import org.jetbrains.kotlin.analysis.low.level.api.fir.util.originalDeclaration
+import org.jetbrains.kotlin.analysis.low.level.api.fir.util.originalKtFile
 import org.jetbrains.kotlin.utils.exceptions.errorWithAttachment
 import org.jetbrains.kotlin.analysis.utils.printer.parentOfType
 import org.jetbrains.kotlin.analysis.utils.printer.parentsOfType
@@ -190,6 +191,8 @@ object LowLevelFirApiFacadeForResolveOnAir {
         require(originalFirResolveSession is LLFirResolvableResolveSession)
         require(elementToAnalyze !is KtFile) { "KtFile for dependency element not supported" }
 
+        elementToAnalyze.containingKtFile.originalKtFile = originalKtFile
+
         val minimalCopiedDeclaration = elementToAnalyze.getNonLocalContainingOrThisDeclarationCodeFragmentAware {
             it.isApplicableForOnAirResolve()
         }

analysis/low-level-api-fir/testdata/dependentCopy/function.context.txt

@@ -0,0 +1,17 @@
+Tower Data Context:
+    Element 0
+        Scope: FirDefaultStarImportingScope
+    Element 1
+        Scope: FirDefaultSimpleImportingScope
+    Element 2
+        Scope: FirExplicitStarImportingScope
+    Element 3
+        Scope: FirDefaultSimpleImportingScope
+    Element 4
+        Scope: FirDefaultSimpleImportingScope
+    Element 5
+        Scope: FirPackageMemberScope
+    Element 6
+        Scope: FirExplicitSimpleImportingScope
+    Element 7
+        Scope: FirLocalScope

analysis/low-level-api-fir/testdata/dependentCopy/function.copy.txt

@@ -0,0 +1,5 @@
+fun foo() {
+    val i = unusedInOriginal()
+}
+
+fun unusedInOriginal() = 42

analysis/low-level-api-fir/testdata/dependentCopy/function.fir.txt

@@ -0,0 +1,8 @@
+KT element: KtNamedFunction
+FIR element: FirSimpleFunctionImpl
+FIR source kind: KtRealSourceElementKind
+
+FIR element rendered:
+public final [ResolvedTo(BODY_RESOLVE)] fun foo(): R|kotlin/Unit| {
+    [ResolvedTo(BODY_RESOLVE)] lval i: R|kotlin/Int| = R|/unusedInOriginal|()
+}

analysis/low-level-api-fir/testdata/dependentCopy/function.kt

@@ -0,0 +1,5 @@
+fun<caret> foo() {
+    "original text"
+}
+
+fun unusedInOriginal() = 42

analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/AbstractDependentCopyTest.kt

@@ -11,8 +11,11 @@ import org.jetbrains.kotlin.analysis.low.level.api.fir.test.base.AbstractLowLeve
 import org.jetbrains.kotlin.analysis.test.framework.services.expressionMarkerProvider
 import org.jetbrains.kotlin.psi.KtElement
 import org.jetbrains.kotlin.psi.KtFile
+import org.jetbrains.kotlin.psi.KtPsiFactory
 import org.jetbrains.kotlin.test.services.TestModuleStructure
 import org.jetbrains.kotlin.test.services.TestServices
+import kotlin.io.path.exists
+import kotlin.io.path.readText
 
 /**
  * This test class is supposed to specify the behavior of [getFirResolveSessionForDependentCopy].
@@ -28,7 +31,18 @@ abstract class AbstractDependentCopyTest : AbstractLowLevelApiSingleFileTest() {
             registeredDirectives = moduleStructure.allDirectives,
         )
 
-        val fileCopy = ktFile.copy() as KtFile
+        val specialContentForCopiedFile = getTestDataFileSiblingPath(
+            extension = "copy.txt",
+            testPrefix = null,
+        ).takeIf { it.exists() }?.readText()
+
+        val fileCopy = KtPsiFactory(ktFile.project).createFile(
+            ktFile.name,
+            specialContentForCopiedFile ?: ktFile.text,
+        )
+
+        fileCopy.originalFile = ktFile
+
         val sameElementInCopy = PsiTreeUtil.findSameElementInCopy(element, fileCopy)
         resolveWithClearCaches(ktFile) { originalSession ->
             val dependentSession = getFirResolveSessionForDependentCopy(

analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/SourceDependentCopyContextTestGenerated.java

@@ -35,4 +35,10 @@ public class SourceDependentCopyContextTestGenerated extends AbstractSourceDepen
     public void testClassBody() throws Exception {
         runTest("analysis/low-level-api-fir/testdata/dependentCopy/classBody.kt");
     }
+
+    @Test
+    @TestMetadata("function.kt")
+    public void testFunction() throws Exception {
+        runTest("analysis/low-level-api-fir/testdata/dependentCopy/function.kt");
+    }
 }

analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/SourceDependentCopyFirTestGenerated.java

@@ -35,4 +35,10 @@ public class SourceDependentCopyFirTestGenerated extends AbstractSourceDependent
     public void testClassBody() throws Exception {
         runTest("analysis/low-level-api-fir/testdata/dependentCopy/classBody.kt");
     }
+
+    @Test
+    @TestMetadata("function.kt")
+    public void testFunction() throws Exception {
+        runTest("analysis/low-level-api-fir/testdata/dependentCopy/function.kt");
+    }
 }