Fix ObservableValidatorValidateAllPropertiesGenerator and IMessengerRegisterAllGenerator to provide a nicer "unsupported C# language version" diagnostic instead of simply generating unsupported code and failing later in compilation

Author: zhuman

CommunityToolkit.Mvvm.SourceGenerators/ComponentModel/ObservableValidatorValidateAllPropertiesGenerator.cs

@@ -9,6 +9,7 @@
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CSharp;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
+using static CommunityToolkit.Mvvm.SourceGenerators.Diagnostics.DiagnosticDescriptors;
 
 namespace CommunityToolkit.Mvvm.SourceGenerators;
 
@@ -39,6 +40,8 @@ public void Initialize(IncrementalGeneratorInitializationContext context)
             .Select(static (item, _) => Execute.GetInfo(item))
             .WithComparer(ValidationInfo.Comparer.Default);
 
+        context.FilterWithLanguageVersion(ref validationInfo, LanguageVersion.CSharp8, UnsupportedCSharpLanguageVersionError);
+
         // Check whether the header file is needed
         IncrementalValueProvider<bool> isHeaderFileNeeded =
             validationInfo

CommunityToolkit.Mvvm.SourceGenerators/Messaging/IMessengerRegisterAllGenerator.cs

@@ -10,6 +10,7 @@
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CSharp;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
+using static CommunityToolkit.Mvvm.SourceGenerators.Diagnostics.DiagnosticDescriptors;
 
 namespace CommunityToolkit.Mvvm.SourceGenerators;
 
@@ -47,6 +48,8 @@ public void Initialize(IncrementalGeneratorInitializationContext context)
             .Select(static (item, _) => Execute.GetInfo(item.Type, item.Interfaces))
             .WithComparer(RecipientInfo.Comparer.Default);
 
+        context.FilterWithLanguageVersion(ref recipientInfo, LanguageVersion.CSharp8, UnsupportedCSharpLanguageVersionError);
+
         // Check whether the header file is needed
         IncrementalValueProvider<bool> isHeaderFileNeeded =
             recipientInfo