Is there literally no other code in the application?

Rust has plenty of situations where you do unsafe things but wrap that in safe APIs. If you’re returning regions of that mmapped file, for example, a lifetime can be associated to those references to ensure that those are valid for the duration of the file being mmapped in the program.

It can be used to ensure that if you need to write back to that mmapped file (inside the same program) that there are no existing references to it, because those would be invalid after an update to the file. You need to do the same in C, but there are no guardrails you can build in C to make that same assurance.

> If you snapshot your entire heap into an mmapped file for fast I/O,

I've never heard of this trick. And my first reaction is "That would be a nightmare of memory unsafety if I did it in C++"

What's it used for? IPC?

I think emacs (used to?) do something awful like this. https://lwn.net/Articles/707615/

I'd call mmaping data structures into memory an advanced systems programming trick which can result in a nice performance boost but which also has some severe drawbacks (portability across big/little endian architectures and internal pointers being two examples).

I know some very skilled C++ and Rust developers who can pull it off. If you're at that skill level, Rust is not going to get in your way because you're just going to use unsafe and throw some sanitizers and fuzzers at it. I wouldn't trust myself to implement it.

You have to combine it with other techniques, e.g. journaling to make it safe, but this is not always necessary (e.g. when using large read-only data-structures)