Discourage the use of LANGUAGE_VERSION directive.

Pinning the language version is rarely the right thing to do.
This changes makes it harder to do so by requiring an additional
directive with a name that indicates to developers that they are
likely doing something dangerous.

compiler/test-infrastructure/tests/org/jetbrains/kotlin/test/builders/LanguageVersionSettingsBuilder.kt

@@ -63,7 +63,24 @@ class LanguageVersionSettingsBuilder {
             this.languageVersion = languageVersion
         }
         val languageVersionDirective = directives.singleOrZeroValue(LanguageSettingsDirectives.LANGUAGE_VERSION)
+        val allowDangerousLanguageVersionTesting =
+            directives.contains(LanguageSettingsDirectives.ALLOW_DANGEROUS_LANGUAGE_VERSION_TESTING)
         if (languageVersionDirective != null) {
+            if (!allowDangerousLanguageVersionTesting) {
+                error(
+                    """
+                        The LANGUAGE_VERSION directive is prone to limiting test to a specific language version,
+                        which will become obsolete at some point and the test won't check things like feature
+                        intersection with newer releases.
+
+                        For language feature testing, use `// !LANGUAGE: [+-]FeatureName` directive instead,
+                        where FeatureName is an entry of the enum `LanguageFeature`
+
+                        If you are really sure you need to pin language versions, use the LANGUAGE_VERSION
+                        directive in combination with the ALLOW_DANGEROUS_LANGUAGE_VERSION_TESTING directive.
+                    """.trimIndent()
+                )
+            }
             languageVersion = languageVersionDirective
             if (languageVersion < LanguageVersion.fromVersionString(this.apiVersion.versionString)!!) {
                 error(

compiler/test-infrastructure/tests/org/jetbrains/kotlin/test/directives/LanguageSettingsDirectives.kt

@@ -26,6 +26,21 @@ object LanguageSettingsDirectives : SimpleDirectivesContainer() {
         parser = this::parseLanguageVersion
     )
 
+    val ALLOW_DANGEROUS_LANGUAGE_VERSION_TESTING by directive(
+        description = """
+            Allows the use of the LANGUAGE_VERSION directive. However, before you use it, please
+            make sure that you actually do need to pin language versions.
+
+            The LANGUAGE_VERSION directive is prone to limiting test to a specific language version,
+            which will become obsolete at some point and the test won't check things like feature
+            intersection with newer releases.
+
+            For language feature testing, use `// !LANGUAGE: [+-]FeatureName` directive instead,
+            where FeatureName is an entry of the enum `LanguageFeature`
+        """.trimIndent()
+    )
+
+
     // --------------------- Analysis Flags ---------------------
 
     val OPT_IN by stringDirective(

plugins/fir-plugin-prototype/testData/firLoadK2Compiled/simple-lang-ver-2.1.kt

@@ -1,4 +1,5 @@
 // API_VERSION: 2.0
+// ALLOW_DANGEROUS_LANGUAGE_VERSION_TESTING
 // LANGUAGE_VERSION: 2.1
 package test
 

plugins/fir-plugin-prototype/testData/firLoadK2Compiled/simple.kt

@@ -1,4 +1,5 @@
 // API_VERSION: 2.0
+// ALLOW_DANGEROUS_LANGUAGE_VERSION_TESTING
 // LANGUAGE_VERSION: 2.0
 package test