> [...] That's not a big problem [...]

Depends somewhat on your expectations, I suppose. Compared to Python, Java, sure, but Rust off course strives to offer "zero-cost" high level concepts.

I think the critique is in the same realm of C++'s std::function. Convenience, sure, but far from zero-cost.

To the point it got replaced by std::function_ref() in C++26.