codegen/Generator/MySourceGenerator.cs

using System.Text;
using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.Text;

namespace MySourceGenerator {
	[Generator]
	public class MySourceGenerator : ISourceGenerator {
		public void Execute(GeneratorExecutionContext context) {
			var template = context.AdditionalFiles.Where(static file => file.Path.EndsWith("HelloWorld.template.cs")).First().GetText()?.ToString();
			if (template == null) {
				return;
			}
			context.AddSource("HelleWorld.generated", SourceText.From(template, Encoding.UTF8));
		}

		public void Initialize(GeneratorInitializationContext context) {
		}
	}
}