> In rust the borrow checker would guard against reordering such things

It doesn't even get that far: Rust guarantees that things drop in reverse order of declaration, full stop.

One interesting wrinkle here: for struct members, Rust does the opposite of what C++ does. We debated changing it to match, but

> there might be unsafe code relying on drop-order which the borrow checker would be oblivious to.

There was no super real compelling argument to choose one direction over the other in the abstract, and "be the same as C++" was not considered important enough to risk breaking unsafe code that relied on the (what was at the time) implementation defined behavior.