You can also use generic functions that take struct arguments instead of delegates to get something like STL. I wish there was some syntactic sugar in the language for that, too - basically, struct lambdas.

There is a bit of that with static lambdas, function pointers (unsafe), but still isn't quite what you want.

Well, until relatively recently, they wouldn't be able to desugar what I really want into underlying bits. But now that we have verifiable ref structs with ref fields in them in C# 11, it's a straightforward transformation. Might be worth a proposal, actually...