3 ms·

Aside from the aspect of restrict pointers being a footgun for C programmers who aren't used to them and apart from being able to convert C to unsafe Rust with

Aside from the aspect of restrict pointers being a footgun for C programmers who aren't used to them and apart from being able to convert C to unsafe Rust with e.g. Corrode, what use cases (within the scope of things that one has to use unsafe Rust for as opposed to writing C-like code) are there for pointers that are allowed to alias?

steveklabnik 8y ago
Well, &T is allowed to alias too. Having unchecked pointers is important for Rust to be a systems programming language. But, since they're unchecked, they may alias. That's just the nature of not being able to make guarantees.
- hsivonen 8y ago
  That doesn't explain it from the use case perspective. E.g. dereferencing a pointer with asterisk operator requires the pointer to be aligned, so unchecked things can still have requirements that need to be met.
  - Jweb_Guru 8y ago
    I think the primary reason why Rust doesn't have explicit unsafe restrict pointers is that there's virtually no demand for them (that I've seen, anyway). Unlike raw pointers with no aliasing restrictions, restrict is used super rarely in C because it's so hard to figure out when it's safe--I think it was only added to make C competitive with Fortran in certain benchmarks. Restrict means you can never alias, so storing one in a data structure is just tempting fate without the aid of the compiler, and if you just need it temporarily casting a * mut to &mut works fine. Rust programs certainly use restrict a lot more than any C program does, and I don't think adding an additional layer of "restrict without a lifetime" would be terribly beneficial in most cases.
yoklov 8y ago
I think it makes unsafe code less error prone to write, but additionally it allows you to interop with C APIs that have aliasing pointers. Also, if they were restrict, it would be UB in rust, which I think is something Rust tries to avoid where possible.

steveklabnik 8y ago

Well, &T is allowed to alias too. Having unchecked pointers is important for Rust to be a systems programming language. But, since they're unchecked, they may alias. That's just the nature of not being able to make guarantees.

hsivonen 8y ago
That doesn't explain it from the use case perspective. E.g. dereferencing a pointer with asterisk operator requires the pointer to be aligned, so unchecked things can still have requirements that need to be met.
- Jweb_Guru 8y ago
  I think the primary reason why Rust doesn't have explicit unsafe restrict pointers is that there's virtually no demand for them (that I've seen, anyway). Unlike raw pointers with no aliasing restrictions, restrict is used super rarely in C because it's so hard to figure out when it's safe--I think it was only added to make C competitive with Fortran in certain benchmarks. Restrict means you can never alias, so storing one in a data structure is just tempting fate without the aid of the compiler, and if you just need it temporarily casting a * mut to &mut works fine. Rust programs certainly use restrict a lot more than any C program does, and I don't think adding an additional layer of "restrict without a lifetime" would be terribly beneficial in most cases.

yoklov 8y ago

I think it makes unsafe code less error prone to write, but additionally it allows you to interop with C APIs that have aliasing pointers. Also, if they were restrict, it would be UB in rust, which I think is something Rust tries to avoid where possible.