At least in Rust, capture granularity is per-variable, so I think closures don't contain a stack frame pointer? Though, I'm not sure why this would matter to a user evaluating Rust's closures vs. D's nested fns?

It’s a pointer to the environment, and a pointer to the function. So yes, not a stack frame pointer.

The environment is a struct containing what is captured; so for example, it would contain an &T if your closure captures a T by reference. You don’t need to walk a set of stack pointers to find it, just follow the reference directly there.

That's distinctly different from D's, which has a pointer to the enclosing stack frame.

If it is the enclosing function, it's just a single pointer to it. The "walking" comes from if you're accessing the enclosing function's enclosing function's frame.

Makes perfect sense, thanks. I’m now wondering about the trade offs of these two approaches...