CLI: improve path relativization for compiler messages

On Windows, the absolute file for the root of the drive is `"C:\"`,
which differs from the absolute file for any other directory, which
doesn't end with `\`. This resulted in incorrect trimming of the first
character of the path name in `descendantRelativeTo`.

Also, do not use canonicalPath because there is no point in expanding
symbolic links here.

 #KT-40979 Fixed

compiler/tests/org/jetbrains/kotlin/integration/FilePathNormalizationTest.kt

@@ -0,0 +1,75 @@
+/*
+ * Copyright 2010-2021 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.integration
+
+import com.intellij.openapi.util.SystemInfo
+import org.jetbrains.kotlin.utils.fileUtils.descendantRelativeTo
+import java.io.File
+import kotlin.io.path.createSymbolicLinkPointingTo
+
+class FilePathNormalizationTest : KotlinIntegrationTestBase() {
+    // This test checks that path normalization logic used in MessageRenderer.PLAIN_RELATIVE_PATHS works correctly.
+    // It compiles and runs a program in a separate process to be able to test how it works from different working directories.
+    // (It could be tested in the same process by changing the user.dir manually, but that could change behavior
+    // of other tests run in parallel.)
+    fun test() {
+        val descendantRelativeTo = File::descendantRelativeTo.name
+        val program = ProgramWithDependencyOnCompiler(
+            tmpdir, """
+            import org.jetbrains.kotlin.utils.fileUtils.$descendantRelativeTo
+            import java.io.File
+
+            fun main(args: Array<String>) {
+                println(File(args[0]).$descendantRelativeTo(File(".").absoluteFile).path)
+            }
+            """.trimIndent()
+        )
+
+        program.compile()
+
+        fun doTest(cwd: File, filePath: String, expectedWithForwardSlash: String) {
+            // We use "/" below for simplicity, but the actual paths in compiler messages use the system separator.
+            val expected = expectedWithForwardSlash.replace("/", File.separator)
+            val actual = program.run(cwd, filePath)
+            assertEquals("cwd: $cwd\nfilePath: $filePath\n", expected, actual)
+        }
+
+        doTest(tmpdir, "a", "a")
+        doTest(tmpdir, "./a", "a")
+        doTest(tmpdir, "a/../b", "b")
+        doTest(tmpdir, "..", "..")
+        doTest(tmpdir, "../a", "../a")
+        doTest(tmpdir, tmpdir.path + "/a", "a")
+        doTest(tmpdir, tmpdir.path + "/./a", "a")
+
+        val root = File("/")
+        doTest(root, "test", "test")
+
+        doTest(root, tmpdir.path + "/a", (tmpdir.path + "/a").removePrefix(root.absolutePath))
+
+        doTest(
+            root, "./test",
+            if (SystemInfo.isWindows) "./test" else "test"
+        )
+
+        // Check symbolic links, but skip file systems which don't support them (e.g. Windows).
+        fun doSymbolicLinkTest(cwd: File, source: File, target: File, expected: String) {
+            val link = try {
+                source.toPath().createSymbolicLinkPointingTo(target.toPath()).toFile()
+            } catch (e: Throwable) {
+                null
+            }
+            if (link != null) {
+                doTest(cwd, link.path, expected)
+            }
+        }
+
+        doSymbolicLinkTest(tmpdir, tmpdir / "a", tmpdir / "b", "a")
+        doSymbolicLinkTest(tmpdir / "unrelated", tmpdir / "a", tmpdir / "../b", tmpdir.path + "/a")
+    }
+
+    private operator fun File.div(x: String): File = File(this, x)
+}

compiler/util/src/org/jetbrains/kotlin/utils/fileUtils.kt

@@ -37,14 +37,9 @@ fun File.withReplacedExtensionOrNull(oldExt: String, newExt: String): File? {
  * If this file does not belong to the [base] directory, it is returned unchanged.
  */
 fun File.descendantRelativeTo(base: File): File {
-    val prefix = base.canonicalPath
-    val answer = this.canonicalPath
-    return if (answer.startsWith(prefix)) {
-        val prefixSize = prefix.length
-        if (answer.length > prefixSize) {
-            File(answer.substring(prefixSize + 1))
-        } else File("")
-    } else {
-        this
-    }
-}
+    assert(base.isAbsolute) { "$base" }
+    assert(base.isDirectory) { "$base" }
+    val cwd = base.normalize()
+    val filePath = this.absoluteFile.normalize()
+    return if (filePath.startsWith(cwd)) filePath.relativeTo(cwd) else this
+}