tor/doc/HACKING/CodingStandardsRust.md

Rust Coding Standards

You MUST follow the standards laid out in .../doc/HACKING/CodingStandards.md, where applicable.

Module/Crate Declarations

Each Tor C module which is being rewritten MUST be in its own crate. See the structure of .../src/rust for examples.

In your crate, you MUST use lib.rs ONLY for pulling in external crates (e.g. extern crate libc;) and exporting public objects from other Rust modules (e.g. pub use mymodule::foo;). For example, if you create a crate in .../src/rust/yourcrate, your Rust code should live in .../src/rust/yourcrate/yourcode.rs and the public interface to it should be exported in .../src/rust/yourcrate/lib.rs.

If your code is to be called from Tor C code, you MUST define a safe ffi.rs which ONLY copies &[u8]s (i.e. byte arrays) across the FFI boundary.

For example, in a hypothetical tor_addition Rust module:

In .../src/rust/tor_addition/addition.rs:

pub fn get_sum(a: i32, b: i32) -> i32 {
    a + b
}

In .../src/rust/tor_addition/lib.rs:

pub use addition::*;

In .../src/rust/tor_addition/ffi.rs:

#[no_mangle]
pub extern "C" fn tor_get_sum(a: c_int, b: c_int) -> c_int {
    get_sum(a, b)
}

If your Rust code must call out to parts of Tor's C code, you must declare the functions you are calling in the external crate, located at .../src/rust/external.

XXX get better examples of how to declare these externs, when/how they XXX are unsafe, what they are expected to do —isis

Modules should strive to be below 500 lines (tests excluded). Single responsibility and limited dependencies should be a guiding standard.

If you have any external modules as dependencies (e.g. extern crate libc;), you MUST declare them in your crate's lib.rs and NOT in any other module.

Dependencies

In general, we use modules from only the Rust standard library whenever possible. We will review including external crates on a case-by-case basis.

Documentation

You MUST include #[deny(missing_docs)] in your crate.

For example, a one-sentence, "first person" description of function behaviour (see requirements for documentation as described in .../src/HACKING/CodingStandards.md), then an # Inputs section for inputs or initialisation values, a # Returns section for return values/types, a # Warning section containing warnings for unsafe behaviours or panics that could happen. For publicly accessible types/constants/objects/functions/methods, you SHOULD also include an # Examples section with runnable doctests.

You MUST document your module with module docstring comments, i.e. //! at the beginning of each line.

Testing

All code MUST be unittested and integration tested.

Public functions/objects exported from a crate SHOULD include doctests describing how the function/object is expected to be used.

Integration tests SHOULD go into a tests/ directory inside your crate. Unittests SHOULD go into their own module inside the module they are testing, e.g. in .../src/rust/tor_addition/addition.rs you should put:

#[cfg(test)]
mod test {
    use super::*;

    #[test]
    fn addition_with_zero() {
        let sum: i32 = get_sum(5i32, 0i32);
        assert_eq!(sum, 5);
    }
}

Benchmarking

If you wish to benchmark some of your Rust code, you MUST put the following in the [features] section of your crate's Cargo.toml:

[features]
bench = []

Next, in your crate's lib.rs you MUST put:

#[cfg(all(test, feature = "bench"))]
extern crate test;

This ensures that the external crate test, which contains utilities for basic benchmarks, is only used when running benchmarks via cargo bench --features bench. (This is due to the test module requiring nightly Rust, and since we may want to switch to a more stable Rust compiler eventually we don't want to break builds for stable compilers by always requiring the test crate.)

Finally, to write your benchmark code, in .../src/rust/tor_addition/addition.rs you SHOULD put:

#[cfg(all(test, features = "bench"))]
mod bench {
    use test::Bencher;
    use super::*;

    #[bench]
    fn addition_small_integers(b: &mut Bencher) {
        b.iter(| | get_sum(5i32, 0i32));
    }
}

Safety

You SHOULD read the nomicon before writing Rust FFI code. It is highly advised that you read and write normal Rust code before attempting to write FFI or any other unsafe code.

Here are some additional bits of advice and rules:

1. unwrap()

   If you call unwrap(), anywhere, even in a test, you MUST include an inline comment stating how the unwrap will either 1) never fail, or 2) should fail (i.e. in a unittest).

2. unsafe

   If you use unsafe, you MUST describe a contract in your documentation which describes how and when the unsafe code may fail, and what expectations are made w.r.t. the interfaces to unsafe code. This is also REQUIRED for major pieces of FFI between C and Rust.

   When creating an FFI in Rust for C code to call, it is NOT REQUIRED to declare the entire function unsafe. For example, rather than doing:

    #[no_mangle]
    pub unsafe extern "C" fn increment_and_combine_numbers(mut numbers: [u8; 4]) -> u32 {
        for index in 0..numbers.len() {
            numbers[index] += 1;
        }
        std::mem::transmute::<[u8; 4], u32>(numbers)
    }
   

   You SHOULD instead do:

    #[no_mangle]
    pub extern "C" fn increment_and_combine_numbers(mut numbers: [u8; 4]) -> u32 {
        for index in 0..numbers.len() {
            numbers[index] += 1;
        }
        unsafe {
            std::mem::transmute::<[u8; 4], u32>(numbers)
        }
    }
   

3. Pass only integer types and bytes over the boundary

   The only non-integer type which may cross the FFI boundary is bytes, e.g. &[u8]. This SHOULD be done on the Rust side by passing a pointer (*mut libc::c_char) and a length (libc::size_t).

   One might be tempted to do this via doing CString::new("blah").unwrap().into_raw(). This has several problems:

   a) If you do CString::new("bl\x00ah") then the unwrap() will fail due to the additional NULL terminator, causing a dangling pointer to be returned (as well as a potential use-after-free).

   b) Returning the raw pointer will cause the CString to run its deallocator, which causes any C code which tries to access the contents to dereference a NULL pointer.

   c) If we were to do as_raw() this would result in a potential double-free since the Rust deallocator would run and possibly Tor's deallocator.

   d) Calling into_raw() without later using the same pointer in Rust to call from_raw() and then deallocate in Rust can result in a memory leak.

   It was determined that this is safe to do if you use the same allocator in C and Rust and also specify the memory alignment for CString (except that there is no way to specify the alignment for CString). It is believed that the alignment is always 1, which would mean it's safe to dealloc the resulting *mut c_char in Tor's C code. However, the Rust developers are not willing to guarantee the stability of, or a contract for, this behaviour, citing concerns that this is potentially extremely and subtly unsafe.

4. Perform an allocation on the other side of the boundary

   After crossing the boundary, the other side MUST perform an allocation to copy the data and is therefore responsible for freeing that memory later.

5. No touching other language's enums

   Rust enums should never be touched from C (nor can they be safely #[repr(C)]) nor vice versa:

   "The chosen size is the default enum size for the target platform's C ABI. Note that enum representation in C is implementation defined, so this is really a "best guess". In particular, this may be incorrect when the C code of interest is compiled with certain flags."

   (from https://gankro.github.io/nomicon/other-reprs.html)

6. Type safety

   Wherever possible and sensical, you SHOULD create new types, either as tuple structs (e.g. struct MyInteger(pub u32)) or as type aliases (e.g. pub type MyInteger = u32).

Whitespace & Formatting

You MUST run rustfmt (https://github.com/rust-lang-nursery/rustfmt) on your code before your code will be merged. You can install rustfmt by doing cargo install rustfmt-nightly and then run it with cargo fmt.