NebulaLauncher/Nebula.SourceGenerators/SourceHelper.cs

using System.Text;
using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp.Syntax;
using Microsoft.CodeAnalysis.Text;

namespace Nebula.SourceGenerators;

public static class SourceHelper
{
    public static void GenerateAttribute(string attributeName, string usage,
        IncrementalGeneratorInitializationContext context)
    {
        var attributeSourceCode = $@"// <auto-generated/>

namespace SourceGen
{{
    [System.AttributeUsage(System.AttributeTargets.{usage})]
    public class {attributeName} : System.Attribute
    {{
    }}
}}";

        context.RegisterPostInitializationOutput(ctx => ctx.AddSource(
            $"{attributeName}.g.cs",
            SourceText.From(attributeSourceCode, Encoding.UTF8)));
    }
    
    public static bool HasAttribute(ISymbol type, string attributeName)
    {
        foreach (var attribute in type.GetAttributes())
            if (attribute.AttributeClass?.ToDisplayString() == attributeName)
                return true;

        return false;
    }

    public static (ClassDeclarationSyntax, bool reportAttributeFound) GetClassDeclarationForSourceGen(
        GeneratorSyntaxContext context, string AttributeName)
    {
        var classDeclarationSyntax = (ClassDeclarationSyntax)context.Node;

        foreach (var attributeListSyntax in classDeclarationSyntax.AttributeLists)
        foreach (var attributeSyntax in attributeListSyntax.Attributes)
        {
            if (context.SemanticModel.GetSymbolInfo(attributeSyntax).Symbol is not IMethodSymbol attributeSymbol)
                continue;

            var attributeName = attributeSymbol.ContainingType.ToDisplayString();

            if (attributeName == AttributeName)
                return (classDeclarationSyntax, true);
        }

        return (classDeclarationSyntax, false);
    }
}