Sort class members to ensure deterministic builds

Class methods and fields are currently sorted at serialization (see
DescriptorSerializer.sort) and at deserialization (see
DeserializedMemberScope.OptimizedImplementation#addMembers). Therefore,
the contents of the generated stub files are sorted in incremental
builds but not in clean builds.

The consequence is that the contents of the generated stub files may not
be consistent across a clean build and an incremental build, making the
build non-deterministic and dependent tasks run unnecessarily (see
KT-40882).

To work around that, this commit sorts class methods and fields when
outputting stub files.

Bug: KT-40882 (there are actually 2 issues in here; this commit fixes
     the first one)
Test: New DeterministicBuildIT + Updated existing test expectation files

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/DeterministicBuildIT.kt

@@ -0,0 +1,93 @@
+/*
+ * Copyright 2010-2020 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.gradle
+
+import org.jetbrains.kotlin.gradle.util.allJavaFiles
+import org.junit.Test
+import java.io.File
+import kotlin.test.assertEquals
+
+/** Tests that the outputs of a build are deterministic. */
+class DeterministicBuildIT : BaseGradleIT() {
+
+    @Test
+    fun `test KaptGenerateStubsTask - KT-40882`() = with(
+        Project("simple", directoryPrefix = "kapt2")
+    ) {
+        setupWorkingDir()
+        projectDir
+            .resolve("src/main/java/Foo.kt")
+            .writeText(
+                """
+                class Foo : Bar {
+                    // The fields and methods are ordered such that any sorting by KGP will be detected.
+                    val fooField1 = 1
+                    val fooField3 = 3
+                    val fooField2 = 2
+                    fun fooMethod1() {}
+                    fun fooMethod3() {}
+                    fun fooMethod2() {}
+                }
+                """.trimIndent()
+            )
+        projectDir
+            .resolve("src/main/java/Bar.kt")
+            .writeText(
+                """
+                interface Bar {
+                    val barField1 = 1
+                    val barField3 = 3
+                    val barField2 = 2
+                    fun barMethod1() {}
+                    fun barMethod3() {}
+                    fun barMethod2() {}
+                }
+                """.trimIndent()
+            )
+
+        val buildAndSnapshotStubFiles: () -> Map<File, String> = {
+            lateinit var stubFiles: Map<File, String>
+            build(":kaptGenerateStubsKotlin") {
+                assertSuccessful()
+                assertTasksExecuted(":kaptGenerateStubsKotlin")
+                stubFiles = fileInWorkingDir("build/tmp/kapt3/stubs").allJavaFiles().map {
+                    it to it.readText()
+                }.toMap()
+            }
+            stubFiles
+        }
+
+        // Run the first build
+        val stubFilesAfterFirstBuild = buildAndSnapshotStubFiles()
+
+        // Make a change
+        projectDir.resolve("src/main/java/Foo.kt").also {
+            it.writeText(
+                """
+                class Foo : Bar {
+                    val fooField1 = 1
+                    val fooField3 = 3
+                    val fooField2 = 2
+                    fun fooMethod1() { println("Method body changed!") }
+                    fun fooMethod3() {}
+                    fun fooMethod2() {}
+                }
+                """.trimIndent()
+            )
+        }
+
+        // Run the second build
+        val stubFilesAfterSecondBuild = buildAndSnapshotStubFiles()
+
+        // Check that the build outputs are deterministic
+        assertEquals(stubFilesAfterFirstBuild.size, stubFilesAfterSecondBuild.size)
+        for (file in stubFilesAfterFirstBuild.keys) {
+            val fileContentsAfterFirstBuild = stubFilesAfterFirstBuild[file]
+            val fileContentsAfterSecondBuild = stubFilesAfterSecondBuild[file]
+            assertEquals(fileContentsAfterFirstBuild, fileContentsAfterSecondBuild)
+        }
+    }
+}

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/Kapt3IT.kt

@@ -398,9 +398,9 @@ open class Kapt3IT : Kapt3BaseIT() {
             val actual = getErrorMessages()
             // try as 0 starting lines first, then as 1 starting line
             try {
-                Assert.assertEquals(genJavaErrorString(8, 16), actual)
+                Assert.assertEquals(genJavaErrorString(8, 15), actual)
             } catch (e: AssertionError) {
-                Assert.assertEquals(genJavaErrorString(9, 17), actual)
+                Assert.assertEquals(genJavaErrorString(9, 16), actual)
             }
         }