Reduce noisy C# compilation errors

crates/bindings-csharp/BSATN.Codegen/Type.cs

@@ -15,11 +15,18 @@ public MemberDeclaration(ISymbol member, ITypeSymbol type, DiagReporter diag)
         {
             TypeInfo = GetTypeInfo(type);
         }
+        catch (UnresolvedTypeException)
+        {
+            // If it's an unresolved type, this error will have been already highlighted by .NET itself, no need to add noise.
+            // Just add some dummy type to avoid further errors.
+            // Note that we just use `object` here because emitting the unresolved type's name again would produce more of said noise.
+            TypeInfo = "SpacetimeDB.BSATN.Unsupported<object>";
+        }
         catch (Exception e)
         {
             diag.Report(ErrorDescriptor.UnsupportedType, (member, type, e));
-            // dummy type; can't instantiate an interface, but at least it will produce fewer noisy errors
-            TypeInfo = $"SpacetimeDB.BSATN.IReadWrite<{Type}>";
+            // dummy BSATN implementation to produce fewer noisy errors
+            TypeInfo = $"SpacetimeDB.BSATN.Unsupported<{Type}>";
         }
     }
 

crates/bindings-csharp/BSATN.Codegen/Utils.cs

@@ -72,6 +72,9 @@ IncrementalGeneratorInitializationContext context
 
     public static string MakeRwTypeParam(string typeParam) => typeParam + "RW";
 
+    public class UnresolvedTypeException(INamedTypeSymbol type)
+        : InvalidOperationException($"Could not resolve type {type}") { }
+
     public static string GetTypeInfo(ITypeSymbol type)
     {
         // We need to distinguish handle nullable reference types specially:
@@ -120,7 +123,7 @@ static string GetTypeInfoForNamedType(INamedTypeSymbol type)
         {
             if (type.TypeKind == Microsoft.CodeAnalysis.TypeKind.Error)
             {
-                throw new InvalidOperationException($"Could not resolve type {type}");
+                throw new UnresolvedTypeException(type);
             }
             if (type.TypeKind == Microsoft.CodeAnalysis.TypeKind.Enum)
             {

crates/bindings-csharp/BSATN.Runtime/BSATN/Runtime.cs

@@ -438,3 +438,17 @@ public AlgebraicType GetAlgebraicType(ITypeRegistrar registrar) =>
     public AlgebraicType GetAlgebraicType(ITypeRegistrar registrar) =>
         enumerable.GetAlgebraicType(registrar);
 }
+
+// This is a dummy type, mainly used by codegen as a diagnostics placeholder to
+// reduce amount of noisy compilation errors when a used type is not supported by BSATN.
+public readonly struct Unsupported<T> : IReadWrite<T>
+{
+    private static readonly NotSupportedException Exception =
+        new($"Type {typeof(T)} is not supported by BSATN.");
+
+    public T Read(BinaryReader reader) => throw Exception;
+
+    public void Write(BinaryWriter writer, T value) => throw Exception;
+
+    public AlgebraicType GetAlgebraicType(ITypeRegistrar registrar) => throw Exception;
+}