Support recent KUP requirements

See KT-75078 for the requirements itself.

Closes #298

Author: fzhinkin

build.gradle.kts

@@ -104,8 +104,23 @@ allprojects {
                 apiVersion = KotlinVersion.fromVersion(it)
             }
 
+            freeCompilerArgs.addAll("-Xreport-all-warnings", "-Xrender-internal-diagnostic-names")
+
             progressiveMode = true
-            allWarningsAsErrors = true
+
+            if (getAllWarningsAsErrorsValue(project)) {
+                allWarningsAsErrors = true
+            } else {
+                freeCompilerArgs.addAll("-Wextra", "-Xuse-fir-experimental-checkers")
+            }
+
+            getAdditionalKotlinCompilerOptions(project).let {
+                freeCompilerArgs.addAll(it)
+            }
+        }
+        doFirst {
+            logger.info("Added Kotlin compiler flags: ${compilerOptions.freeCompilerArgs.get().joinToString(", ")}")
+            logger.info("allWarningsAsErrors=${compilerOptions.allWarningsAsErrors.get()}")
         }
     }
 }

buildSrc/src/main/kotlin/KotlinCommunity.kt

@@ -75,4 +75,33 @@ fun getOverriddenKotlinNativeVersion(project: Project): String? {
         project.logger.info("""Configured Kotlin Native distribution version: '$nativeVersion' for project ${project.name}""")
     }
     return nativeVersion
-}
+}
+
+/**
+ * Parses additional compiler options specified using the `kotlin_additional_cli_options` property.
+ *
+ * @return a list of additional options, or an empty list if none were specified
+ */
+fun getAdditionalKotlinCompilerOptions(project: Project): List<String> {
+    val opts = project.providers.gradleProperty("kotlin_additional_cli_options").orNull
+    val parsedOpts = opts?.split(' ').orEmpty()
+    if (parsedOpts.isNotEmpty()) {
+        project.logger.info("""Configured Kotlin compiler options: '$opts' for project ${project.name}""")
+    }
+    return parsedOpts
+}
+
+/**
+ * Check if `allWarningsAsErrors` was configured to be set using the `kotlin_Werror_override` property.
+ *
+ * @return `true` if `kotlin_Werror_override` was set to `enable` or it was not specified at all, `false` if it was set to `disable`.
+ */
+fun getAllWarningsAsErrorsValue(project: Project): Boolean {
+    val werrorOverride = project.providers.gradleProperty("kotlin_Werror_override").orNull ?: return true
+    project.logger.info("""Configured kotlin_Werror_override: '$werrorOverride' for project ${project.name}""")
+    return when (werrorOverride) {
+        "enable" -> true
+        "disable" -> false
+        else -> error("Unexpected value for 'kotlin_Werror_override' property: $werrorOverride")
+    }
+}