feat: warn if experimental language features are used (#624)

Closes #108

### Summary of Changes

Warn if experimental language features are used. For the moment, this is
only indexed accesses.

---------

Co-authored-by: megalinter-bot <129584137+megalinter-bot@users.noreply.github.com>

Author: lars-reimann

src/language/validation/experimentalLanguageFeature.ts

@@ -0,0 +1,11 @@
+import { SdsIndexedAccess } from '../generated/ast.js';
+import { ValidationAcceptor } from 'langium';
+
+export const CODE_EXPERIMENTAL_LANGUAGE_FEATURE = 'experimental/language-feature';
+
+export const indexedAccessesShouldBeUsedWithCaution = (node: SdsIndexedAccess, accept: ValidationAcceptor): void => {
+    accept('warning', 'Indexed accesses are experimental and may change without prior notice.', {
+        node,
+        code: CODE_EXPERIMENTAL_LANGUAGE_FEATURE,
+    });
+};

src/language/validation/safe-ds-validator.ts

@@ -63,6 +63,7 @@ import {
 import { placeholderShouldBeUsed } from './other/declarations/placeholders.js';
 import { segmentParameterShouldBeUsed, segmentResultMustBeAssignedExactlyOnce } from './other/declarations/segments.js';
 import { lambdaParameterMustNotHaveConstModifier } from './other/expressions/lambdas.js';
+import { indexedAccessesShouldBeUsedWithCaution } from './experimentalLanguageFeature.js';
 
 /**
  * Register custom validation checks.
@@ -107,6 +108,7 @@ export const registerValidationChecks = function (services: SafeDsServices) {
         SdsEnumVariant: [enumVariantMustContainUniqueNames, enumVariantParameterListShouldNotBeEmpty],
         SdsExpressionLambda: [expressionLambdaMustContainUniqueNames],
         SdsFunction: [functionMustContainUniqueNames, functionResultListShouldNotBeEmpty],
+        SdsIndexedAccess: [indexedAccessesShouldBeUsedWithCaution],
         SdsLambda: [lambdaParameterMustNotHaveConstModifier],
         SdsMemberAccess: [memberAccessNullSafetyShouldBeNeeded(services)],
         SdsModule: [moduleDeclarationsMustMatchFileKind, moduleWithDeclarationsMustStatePackage],

src/resources/builtins/safeds/lang/coreAnnotations.sdsstub

@@ -90,10 +90,12 @@ annotation Deprecated(
 ])
 annotation Experimental
 
+@Experimental
 @Description("The function has no side effects and returns the same results for the same arguments.")
 @Target([AnnotationTarget.Function])
 annotation Pure
 
+@Experimental
 @Description("The function has no side effects.")
 @Target([AnnotationTarget.Function])
 annotation NoSideEffects

src/resources/builtins/safeds/lang/documentation.sdsstub

@@ -12,6 +12,7 @@ annotation Since(
     version: String
 )
 
+@Experimental
 @Description("This parameter should only be used by expert users.")
 @Target([AnnotationTarget.Parameter])
 annotation Expert

tests/resources/validation/experimental language feature/indexed access/main.sdstest

@@ -0,0 +1,9 @@
+package tests.validation.experimentalLanguageFeature.indexedAccess
+
+pipeline myPipeline {
+    // $TEST$ warning "Indexed accesses are experimental and may change without prior notice."
+    »[1, 2][1]«;
+
+    // $TEST$ warning "Indexed accesses are experimental and may change without prior notice."
+    »{"a": "b"}["a"]«;
+}