Generate diagnostic when null conditional expressions are used but rewrite is not configured

src/EntityFrameworkCore.Projectables.Generator/AnalyzerReleases.Unshipped.md

@@ -2,3 +2,4 @@
 
 Rule ID | Category | Severity | Notes
 --------|----------|----------|--------------------
+EFP0002 |  Design  |  Error   | 

src/EntityFrameworkCore.Projectables.Generator/Diagnostics.cs

@@ -17,5 +17,13 @@ namespace EntityFrameworkCore.Projectables.Generator
             DiagnosticSeverity.Error,
             isEnabledByDefault: true);
 
+        public static readonly DiagnosticDescriptor NullConditionalRewriteUnsupported = new(
+            id: "EFP0002",
+            title: "Method or property is not configured to support null-conditional expressions.",
+            messageFormat: "'{0}' has a null-conditional expression exposed but is not configured to rewrite this. Consider configuring a strategy using the NullConditionalRewriteSupport property on the Projectable attribute",
+            category: "Design",
+            DiagnosticSeverity.Error,
+            isEnabledByDefault: true);
+
     }
 }

src/EntityFrameworkCore.Projectables.Generator/ExpressionSyntaxRewriter.cs

@@ -15,13 +15,15 @@ namespace EntityFrameworkCore.Projectables.Generator
         readonly INamedTypeSymbol _targetTypeSymbol;
         readonly SemanticModel _semanticModel;
         readonly NullConditionalRewriteSupport _nullConditionalRewriteSupport;
+        readonly GeneratorExecutionContext _context;
         readonly Stack<ExpressionSyntax> _conditionalAccessExpressionsStack = new();
 
-        public ExpressionSyntaxRewriter(INamedTypeSymbol targetTypeSymbol, SemanticModel semanticModel, NullConditionalRewriteSupport nullConditionalRewriteSupport)
+        public ExpressionSyntaxRewriter(INamedTypeSymbol targetTypeSymbol, SemanticModel semanticModel, NullConditionalRewriteSupport nullConditionalRewriteSupport, GeneratorExecutionContext context)
         {
             _targetTypeSymbol = targetTypeSymbol;
             _semanticModel = semanticModel;
             _nullConditionalRewriteSupport = nullConditionalRewriteSupport;
+            _context = context;
         }
 
         public override SyntaxNode? VisitConditionalAccessExpression(ConditionalAccessExpressionSyntax node)
@@ -30,6 +32,12 @@ namespace EntityFrameworkCore.Projectables.Generator
 
             _conditionalAccessExpressionsStack.Push(targetExpression);
 
+            if (_nullConditionalRewriteSupport == NullConditionalRewriteSupport.None)
+            {
+                var diagnostic = Diagnostic.Create(Diagnostics.NullConditionalRewriteUnsupported, node.GetLocation(), node);
+                _context.ReportDiagnostic(diagnostic);
+            }
+
             return _nullConditionalRewriteSupport switch {
                 NullConditionalRewriteSupport.Ignore => Visit(node.WhenNotNull),
                 NullConditionalRewriteSupport.Rewrite =>

src/EntityFrameworkCore.Projectables.Generator/ProjectableInterpreter.cs

@@ -51,7 +51,7 @@ namespace EntityFrameworkCore.Projectables.Generator
                 .Cast<NullConditionalRewriteSupport>()
                 .FirstOrDefault();
 
-            var expressionSyntaxRewriter = new ExpressionSyntaxRewriter(memberSymbol.ContainingType, semanticModel, nullConditionalRewriteSupport);
+            var expressionSyntaxRewriter = new ExpressionSyntaxRewriter(memberSymbol.ContainingType, semanticModel, nullConditionalRewriteSupport, context);
             var parameterSyntaxRewriter = new ParameterSyntaxRewriter(semanticModel);
             var returnTypeSyntaxRewriter = new ReturnTypeSyntaxRewriter(semanticModel);
 

tests/EntityFrameworkCore.Projectables.Generator.Tests/ProjectionExpressionGeneratorTests.cs

@@ -487,6 +487,27 @@ namespace Foo {
             return Verifier.Verify(result.GeneratedTrees[0].ToString());
         }
 
+        [Fact]
+        public void NullableMemberBinding_WithoutSupport_IsBeingReported()
+        {
+            var compilation = CreateCompilation(@"
+using System;
+using System.Linq;
+using EntityFrameworkCore.Projectables;
+
+namespace Foo {
+    static class C {
+        [Projectable(NullConditionalRewriteSupport = NullConditionalRewriteSupport.None)]
+        public static int? GetLength(this string input) => input?.Length;
+    }
+}
+");
+            var result = RunGenerator(compilation);
+
+            var diagnostic = Assert.Single(result.Diagnostics);
+            Assert.Equal("EFP0002", diagnostic.Id);
+        }
+
         [Fact]
         public Task NullableMemberBinding_WithIgnoreSupport_IsBeingRewritten()
         {