Add diagnostic for invalid property targeted attributes

Author: Sergio0694

src/CommunityToolkit.Mvvm.SourceGenerators/AnalyzerReleases.Shipped.md

@@ -48,3 +48,4 @@ Rule ID | Category | Severity | Notes
 MVVMTK0032 | CommunityToolkit.Mvvm.SourceGenerators.INotifyPropertyChangedGenerator | Warning | See https://aka.ms/mvvmtoolkit/errors/mvvmtk0032
 MVVMTK0033 | CommunityToolkit.Mvvm.SourceGenerators.ObservableObjectGenerator | Warning | See https://aka.ms/mvvmtoolkit/errors/mvvmtk0033
 MVVMTK0034 | CommunityToolkit.Mvvm.SourceGenerators.ObservablePropertyGenerator | Warning | See https://aka.ms/mvvmtoolkit/errors/mvvmtk0034
+MVVMTK0035 | CommunityToolkit.Mvvm.SourceGenerators.ObservablePropertyGenerator | Error | See https://aka.ms/mvvmtoolkit/errors/mvvmtk0035

src/CommunityToolkit.Mvvm.SourceGenerators/ComponentModel/ObservablePropertyGenerator.Execute.cs

@@ -214,9 +214,17 @@ public static bool TryGetInfo(
                     // lack of IntelliSense when constructing attributes over the field, but this is the best we can do from this end anyway.
                     SymbolInfo attributeSymbolInfo = semanticModel.GetSymbolInfo(attribute, token);
 
+                    // Check if the attribute type can be resolved, and emit a diagnostic if that is not the case. This happens if eg. the
+                    // attribute type is spelled incorrectly, or if a user is missing the using directive for the attribute type being used.
                     if ((attributeSymbolInfo.Symbol ?? attributeSymbolInfo.CandidateSymbols.SingleOrDefault()) is not ISymbol attributeSymbol ||
                         (attributeSymbol as INamedTypeSymbol ?? attributeSymbol.ContainingType) is not INamedTypeSymbol attributeTypeSymbol)
                     {
+                        builder.Add(
+                            InvalidPropertyTargetedAttributeOnObservablePropertyField,
+                            attribute,
+                            fieldSymbol,
+                            attribute.Name);
+
                         continue;
                     }
 

src/CommunityToolkit.Mvvm.SourceGenerators/Diagnostics/DiagnosticDescriptors.cs

@@ -541,7 +541,7 @@ internal static class DiagnosticDescriptors
         description:
             "Classes with no base types should prefer inheriting from ObservableObject instead of using attributes to generate INotifyPropertyChanged code, as that will " +
             "reduce the binary size of the application (the attributes are only meant to support cases where the annotated types are already inheriting from a different type).",
-        helpLinkUri: "https://aka.ms/mvvmtoolkit/errors/mvvmtk0032");
+        helpLinkUri: "https://aka.ms/mvvmtoolkit/errors/mvvmtk0033");
 
     /// <summary>
     /// Gets a <see cref="DiagnosticDescriptor"/> indicating when a field with <c>[ObservableProperty]</c> is being directly referenced.
@@ -557,5 +557,21 @@ internal static class DiagnosticDescriptors
         defaultSeverity: DiagnosticSeverity.Warning,
         isEnabledByDefault: true,
         description: "Fields with [ObservableProperty] should not be directly referenced, and the generated properties should be used instead.",
-        helpLinkUri: "https://aka.ms/mvvmtoolkit/errors/mvvmtk0033");
+        helpLinkUri: "https://aka.ms/mvvmtoolkit/errors/mvvmtk0034");
+
+    /// <summary>
+    /// Gets a <see cref="DiagnosticDescriptor"/> indicating when a field with <c>[ObservableProperty]</c> is using an invalid attribute targeting the property.
+    /// <para>
+    /// Format: <c>"The field {0} annotated with [ObservableProperty] is using attribute "{1}" which was not recognized as a valid type (are you missing a using directive?)"</c>.
+    /// </para>
+    /// </summary>
+    public static readonly DiagnosticDescriptor InvalidPropertyTargetedAttributeOnObservablePropertyField = new DiagnosticDescriptor(
+        id: "MVVMTK0035",
+        title: "Invalid property targeted attribute type",
+        messageFormat: "The field {0} annotated with [ObservableProperty] is using attribute \"{1}\" which was not recognized as a valid type (are you missing a using directive?)",
+        category: typeof(ObservablePropertyGenerator).FullName,
+        defaultSeverity: DiagnosticSeverity.Error,
+        isEnabledByDefault: true,
+        description: "All attributes targeting the generated property for a field annotated with [ObservableProperty] must correctly be resolved to valid types.",
+        helpLinkUri: "https://aka.ms/mvvmtoolkit/errors/mvvmtk0035");
 }