feat: warn if statement has no effect (#787)

Closes #664

### Summary of Changes

Show a warning if a statement has no effect.

packages/safe-ds-lang/src/language/flow/safe-ds-call-graph-computer.ts

@@ -61,13 +61,19 @@ export class SafeDsCallGraphComputer {
      */
     private readonly callCache: WorkspaceCache<string, SdsCall[]>;
 
+    /**
+     * Stores the call graph for the callable with the given ID if it is called without substitutions.
+     */
+    private readonly callGraphCache: WorkspaceCache<string, CallGraph>;
+
     constructor(services: SafeDsServices) {
         this.astNodeLocator = services.workspace.AstNodeLocator;
         this.nodeMapper = services.helpers.NodeMapper;
         this.partialEvaluator = services.evaluation.PartialEvaluator;
         this.typeComputer = services.types.TypeComputer;
 
         this.callCache = new WorkspaceCache(services.shared);
+        this.callGraphCache = new WorkspaceCache(services.shared);
     }
 
     /**
@@ -98,6 +104,22 @@ export class SafeDsCallGraphComputer {
      * of any containing callables, i.e. the context of the call/callable.
      */
     getCallGraph(node: SdsCall | SdsCallable, substitutions: ParameterSubstitutions = NO_SUBSTITUTIONS): CallGraph {
+        // Cache the result if no substitutions are given
+        if (isEmpty(substitutions)) {
+            const key = this.getNodeId(node);
+            return this.callGraphCache.get(key, () => {
+                return this.doGetCallGraph(node, substitutions);
+            });
+        } else {
+            /* c8 ignore next 2 */
+            return this.doGetCallGraph(node, substitutions);
+        }
+    }
+
+    private doGetCallGraph(
+        node: SdsCall | SdsCallable,
+        substitutions: ParameterSubstitutions = NO_SUBSTITUTIONS,
+    ): CallGraph {
         if (isSdsCall(node)) {
             const call = this.createSyntheticCallForCall(node, substitutions);
             return this.getCallGraphWithRecursionCheck(call, []);

packages/safe-ds-lang/src/language/validation/other/statements/statements.ts

@@ -0,0 +1,57 @@
+import {
+    isSdsAssignment,
+    isSdsExpressionStatement,
+    isSdsWildcard,
+    SdsExpression,
+    SdsStatement,
+} from '../../../generated/ast.js';
+import { ValidationAcceptor } from 'langium';
+import { SafeDsServices } from '../../../safe-ds-module.js';
+import { getAssignees } from '../../../helpers/nodeProperties.js';
+
+export const CODE_STATEMENT_HAS_NO_EFFECT = 'statement/has-no-effect';
+
+export const statementMustDoSomething = (services: SafeDsServices) => {
+    const statementDoesSomething = statementDoesSomethingProvider(services);
+
+    return (node: SdsStatement, accept: ValidationAcceptor): void => {
+        if (!statementDoesSomething(node)) {
+            accept('warning', 'This statement does nothing.', {
+                node,
+                code: CODE_STATEMENT_HAS_NO_EFFECT,
+            });
+        }
+    };
+};
+
+const statementDoesSomethingProvider = (services: SafeDsServices) => {
+    const statementExpressionDoesSomething = statementExpressionDoesSomethingProvider(services);
+
+    return (node: SdsStatement): boolean => {
+        if (isSdsAssignment(node)) {
+            return !getAssignees(node).every(isSdsWildcard) || statementExpressionDoesSomething(node.expression);
+        } else if (isSdsExpressionStatement(node)) {
+            return statementExpressionDoesSomething(node.expression);
+        } else {
+            /* c8 ignore next 2 */
+            return false;
+        }
+    };
+};
+
+const statementExpressionDoesSomethingProvider = (services: SafeDsServices) => {
+    const callGraphComputer = services.flow.CallGraphComputer;
+    const purityComputer = services.purity.PurityComputer;
+
+    return (node: SdsExpression | undefined): boolean => {
+        if (!node) {
+            /* c8 ignore next 2 */
+            return false;
+        }
+
+        return (
+            callGraphComputer.getAllContainedCalls(node).some((it) => callGraphComputer.isRecursive(it)) ||
+            !purityComputer.isPureExpression(node)
+        );
+    };
+};

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

@@ -175,6 +175,7 @@ import {
     resultMustHaveTypeHint,
     yieldTypeMustMatchResultType,
 } from './types.js';
+import { statementMustDoSomething } from './other/statements/statements.js';
 
 /**
  * Register custom validation checks.
@@ -339,6 +340,7 @@ export const registerValidationChecks = function (services: SafeDsServices) {
             segmentResultListShouldNotBeEmpty,
             segmentShouldBeUsed(services),
         ],
+        SdsStatement: [statementMustDoSomething(services)],
         SdsTemplateString: [templateStringMustHaveExpressionBetweenTwoStringParts],
         SdsTypeArgumentList: [typeArgumentListsShouldBeUsedWithCaution],
         SdsTypeParameter: [

packages/safe-ds-lang/tests/resources/validation/skip-old/assignments/hasNoEffect.sdstest → packages/safe-ds-lang/tests/resources/validation/other/statements/assignments/has no effect/main.sdstest

@@ -1,4 +1,6 @@
-step myFunction() -> a: Int {
+package tests.validation.other.statements.assignments.hasNoEffect
+
+segment mySegment() -> a: Int {
     // $TEST$ warning "This statement does nothing."
     »_ = 1 + 2;«
 

packages/safe-ds-lang/tests/resources/validation/skip-old/expressionStatements/hasNoEffect.sdstest → packages/safe-ds-lang/tests/resources/validation/other/statements/expression statements/has no effect/hasNoEffect.sdstest

@@ -1,30 +1,35 @@
-package tests.validation.statements.expressionStatements.hasNoEffect
+package tests.validation.other.statements.expressionStatements.hasNoEffect
 
+@Impure([ImpurityReason.Other])
 fun impureFunction()
-@Pure fun pureFunction() -> a: Int
+
+@Pure
+fun pureFunction() -> a: Int
 
 class MyClass() {
+    @Impure([ImpurityReason.Other])
     fun impureFunction()
+
     @Pure fun pureFunction()
 }
 
-step pureStep() {
+segment pureSegment() {
     val a = pureFunction();
 }
 
-step impureStep() {
+segment impureSegment() {
     impureFunction();
 }
 
-step recursiveA() {
+segment recursiveA() {
     recursiveB();
 }
 
-step recursiveB() {
+segment recursiveB() {
     recursiveA();
 }
 
-step myStep() {
+segment mySegment() {
     // $TEST$ warning "This statement does nothing."
     »1 + 2;«
     // $TEST$ warning "This statement does nothing."
@@ -51,15 +56,18 @@ step myStep() {
         »MyClass().impureFunction();«
     };
 
+    // $TEST$ warning "This statement does nothing."
+    »pureSegment();«
+
+    // $TEST$ no warning "This statement does nothing."
+    »impureSegment();«
+
     // $TEST$ warning "This statement does nothing."
     »(() {
         pureFunction();
         MyClass().pureFunction();
     })();«
 
-    // $TEST$ warning "This statement does nothing."
-    »pureStep();«
-
     // $TEST$ no warning "This statement does nothing."
     »(() {
         impureFunction();
@@ -70,9 +78,6 @@ step myStep() {
         MyClass().impureFunction();
     })();«
 
-    // $TEST$ no warning "This statement does nothing."
-    »impureStep();«
-
     // $TEST$ no warning "This statement does nothing."
     »recursiveA();«
 }