TargetTypeTracker.cs

public class TargetTypeTracker : ISyntaxContextReceiver
{
    public IImmutableList<TypeDeclarationSyntax> TypesNeedingDtoGening =
        ImmutableList.Create<TypeDeclarationSyntax>();

    public void OnVisitSyntaxNode(GeneratorSyntaxContext context)
    {
        if (context.Node is TypeDeclarationSyntax cdecl)
            if (cdecl.IsDecoratedWithAttribute("generatemappeddto"))
                TypesNeedingDtoGening = TypesNeedingDtoGening.Add(
                    context.Node as TypeDeclarationSyntax);
    }
}

internal static class SourceGenExtns
{
    internal static bool IsDecoratedWithAttribute(
        this TypeDeclarationSyntax cdecl, string attributeName) =>
        cdecl.AttributeLists
            .SelectMany(x => x.Attributes)
            .Any(x => x.Name.ToString().ToLower() == attributeName);
}