I think this is a good idea, mostly because I've been of the opinion that much of the rust "safety" could be done with a linter pass in C++. These annotations should help solve the edge cases that aren't deterministic.

I've not generally been a big fan of where C++ has been going in the last couple revisions, mostly because it seems the newer features aren't fully thought out when it comes to how they interact with other features (reminds me of javascript in that regard) leaving a bunch of new footguns, looks at lambda's.

I think i've said this here before, that C++ needs its own version of "use strict;" that basically kills off some of the syntactically odd corner cases that lead to those footguns.