Unsafe Rust: An Intro and Open Questions

To me, it would be better if libraries using unsafe code were marked.

- "I'm so l33t I don't need the compiler to check me." (Don't hire those guys.)

- "Safe code is too slow". (File bugs on the compiler's optimizer.)

- "Porting this to safe code would require a redesign". (See the Rust port of DOOM.)

Most of the real needs for "Unsafe" in Rust come from

- The need to interface with external code, including system calls.

- Forced type conversion ("casting")

- Memory allocation.

The first one is mostly a problem with expressive power in the foreign function interface. Can you express what "int read(int fd, char buf[], size_t len)" means in the foreign function definition syntax? Rust's foreign function syntax isn't expressive enough to do that.[1] You can't tell Rust that "len" is the length of "buf". Being able to do that would help reduce the need for unsafe code. Most of the POSIX/Linux API can be described with relatively simple syntax that allows you to associate size info with C arrays. (I once proposed this as an extension to C. It's technically possible but politically too difficult.)[2]

If your external interface still requires unsafe code after that, you're probably talking to something that has elaborate foreign data structures visible to the caller. Those really are unsafe. They also usually need a rewrite anyway. (OpenSSL comes to mind.)

Forced type conversion, or casting, is traditionally a problem. Most of the trouble comes from C, where casts bypass all type checking. In practice, much casting is safe. If a type is fully mapped to the underlying bits (i.e. all possible bit value are valid for the type), then allowing a cast is safe. If you cast 4 bytes to a 32-bit unsigned integer, the result is always a valid 32-bit unsigned integer. Conversions like that should be explicit, but are not memory-unsafe. On the other hand, casting to a pointer is always unsafe. Again, with a bit more expressive power, the need for unsafe code can be reduced.

Memory allocation is hard. However, more of it could be done in safe code. Suppose Rust had a type "space", which is simply an array of bytes, treated as write-only. Constructors take in an array of "space" of the desired type, create a valid local structure with the initialized values, and then perform an operation which copies the structure to the array of "space" and changes its type to the type of the structure. This is safe construction. As an optimization, the compiler can observe that if no reads are made from the local structure prior to converting the "space", the extra local copy is unnecessary.

"Space" would still have Rust scope and lifetime, so all that machinery remains hidden. But it's convenient to separate it from construction. Raw memory allocation is complex and unsafe, but separated from the type system, it's a coherent closed system that doesn't get modified much. It's a good candidate for formal proof of correctness - not too big, and critical to system operation.

Operations such as expanding vectors seem to include unsafe code. That's worth a hard look. If you had the "space" concept, and the operation that moves a struct into a "safe" array and converts the type, it should be possible to do operations such as growing an array without unsafe code.

For Rust 2, it's worth looking at how the need for unsafe code can be reduced. Ultimately, everything should be either memory safe or have a machine proof of memory correctness at the instruction level.

[1] https://doc.rust-lang.org/book/ffi.html [2] http://www.animats.com/papers/languages/safearraysforc43.pdf

The first is similar to the c extension you described - building data structures with dynamically-sized arrays eg a bitfield followed by population_count(bitfield) entries. It would be great to have some way to express this without having to pay a whole usize for DST eg:

    struct Node{
        keys: u8,
        vals: [ShortPointer; popcount(keys)],
    }

    fn join_step(state: &mut Vec, ..) {
        ...
        for values in primitive.eval_from_join(&arguments[..], &state[..]).into_iter() {
            // promise the borrow checker that we will pop values before we exit this scope
            let values = unsafe { ::std::mem::transmute::, &'a Vec>(&values) };
            push_all(state, values);
            if join.constraints[ix].iter().all(|constraint| constraint.is_satisfied_by(&state[..])) {
                join_step(state, ...)
            }
            pop_all(state, values);
        }
    }

Here is a really simplified version that illustrates the problem - http://is.gd/8AZvZe