On the other hand, Rust closures do compute the capture list, and then let the type system reason about the safety of those captures, which C++ lambdas (or blocks with the clang blocks extension) don't.

Ah okay, so rust will only ever capture the minimal set of what's necessarily used in the lambda? Different from using [&] or similar in C++ where you capture everything.

Yeah, in C++ you can sort of get away without explicit capture of all variables too by doing [=] or [&]. Though I suppose it's still explicit in a way, just saves the need to list everything.