Rust Coding Standards

You MUST follow the standards laid out in .../doc/HACKING/, 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 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/ and the public interface to it should be exported in .../src/rust/yourcrate/

If your code is to be called from Tor C code, you MUST define a safe See the “Safety” section further down for more details.

For example, in a hypothetical tor_addition Rust module:

In .../src/rust/tor_addition/

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

In .../src/rust/tor_addition/

pub use addition::*;

In .../src/rust/tor_addition/

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.

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 and NOT in any other module.

Dependencies and versions

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.

If a crate only contains traits meant for compatibility between Rust crates, such as the digest crate or the failure crate, it is very likely permissible to add it as a dependency. However, a brief review should be conducted as to the usefulness of implementing external traits (i.e. how widespread is the usage, how many other crates either implement the traits or have trait bounds based upon them), as well as the stability of the traits (i.e. if the trait is going to change, we’ll potentially have to re-do all our implementations of it).

For large external libraries, especially which implement features which would be labour-intensive to reproduce/maintain ourselves, such as cryptographic or mathematical/statistics libraries, only crates which have stabilised to 1.0.0 should be considered, however, again, we may make exceptions on a case-by-case basis.

Currently, Tor requires that you use the latest stable Rust version. At some point in the future, we will freeze on a given stable Rust version, to ensure backward compatibility with stable distributions that ship it.

Updating/Adding Dependencies

To add/remove/update dependencies, first add your dependencies, exactly specifying their versions, into the appropriate crate-level Cargo.toml in src/rust/ (i.e. not /src/rust/Cargo.toml, but instead the one for your crate). Also, investigate whether your dependency has any optional dependencies which are unnecessary but are enabled by default. If so, you’ll likely be able to enable/disable them via some feature, e.g.:

foo = { version = "1.0.0", default-features = false }

Next, run /scripts/maint/ Then, go into src/ext/rust and commit the changes to the tor-rust-dependencies repo.


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

For function/method comments, you SHOULD include a one-sentence, “first person” description of function behaviour (see requirements for documentation as described in .../src/HACKING/, 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.


You SHOULD consider breaking up large literal numbers with _ when it makes it more human readable to do so, e.g. let x: u64 = 100_000_000_000.


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/ you should put:

mod test {
    use super::*;

    fn addition_with_zero() {
        let sum: i32 = get_sum(5i32, 0i32);
        assert_eq!(sum, 5);


The external test crate can be used for most benchmarking. However, using this crate requires nightly Rust. Since we may want to switch to a more stable Rust compiler eventually, we shouldn’t do things which will automatically break builds for stable compilers. Therefore, you MUST feature-gate your benchmarks in the following manner.

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:

bench = []

Next, in your crate’s 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.

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

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

    fn addition_small_integers(b: &mut Bencher) {
        b.iter(| | get_sum(5i32, 0i32));


If you wish to fuzz parts of your code, please see the cargo fuzz crate, which uses libfuzzer-sys.

Whitespace & Formatting

You MUST run 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.


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. Any behaviours which Rust considers to be undefined are forbidden


    Behavior considered undefined

    The following is a list of behavior which is forbidden in all Rust code, including within unsafe blocks and unsafe functions. Type checking provides the guarantee that these issues are never caused by safe code.

    • Data races
    • Dereferencing a null/dangling raw pointer
    • Reads of undef (uninitialized) memory
    • Breaking the pointer aliasing rules with raw pointers (a subset of the rules used by C)
    • &mut T and &T follow LLVM’s scoped noalias model, except if the &T contains an UnsafeCell<U>. Unsafe code must not violate these aliasing guarantees.
    • Mutating non-mutable data (that is, data reached through a shared reference or data owned by a let binding), unless that data is contained within an UnsafeCell<U>.
    • Invoking undefined behavior via compiler intrinsics:
      • Indexing outside of the bounds of an object with std::ptr::offset (offset intrinsic), with the exception of one byte past the end which is permitted.
      • Using std::ptr::copy_nonoverlapping_memory (memcpy32/memcpy64 intrinsics) on overlapping buffers
    • Invalid values in primitive types, even in private fields/locals:
      • Dangling/null references or boxes
      • A value other than false (0) or true (1) in a bool
      • A discriminant in an enum not included in the type definition
      • A value in a char which is a surrogate or above char::MAX
      • Non-UTF-8 byte sequences in a str
    • Unwinding into Rust from foreign code or unwinding from Rust into foreign code. Rust’s failure system is not compatible with exception handling in other languages. Unwinding must be caught and handled at FFI boundaries.
  2. 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).

    You SHOULD NOT use unwrap() anywhere in which it is possible to handle the potential error with the eel operator, ? or another non panicking way. For example, consider a function which parses a string into an integer:

     fn parse_port_number(config_string: &str) -> u16 {
         u16::from_str_radix(config_string, 10).unwrap()

    There are numerous ways this can fail, and the unwrap() will cause the whole program to byte the dust! Instead, either you SHOULD use ok() (or another equivalent function which will return an Option or a Result) and change the return type to be compatible:

     fn parse_port_number(config_string: &str) -> Option<u16> {
         u16::from_str_radix(config_string, 10).ok()

    or you SHOULD use or() (or another similar method):

     fn parse_port_number(config_string: &str) -> Option<u16> {
         u16::from_str_radix(config_string, 10).or(Err("Couldn't parse port into a u16")

    Using methods like or() can be particularly handy when you must do something afterwards with the data, for example, if we wanted to guarantee that the port is high. Combining these methods with the eel operator (?) makes this even easier:

     fn parse_port_number(config_string: &str) -> Result<u16, Err> {
         let port = u16::from_str_radix(config_string, 10).or(Err("Couldn't parse port into a u16"))?;
         if port > 1024 {
             return Ok(port);
         } else {
             return Err("Low ports not allowed");
  3. 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:

     pub unsafe extern "C" fn increment_and_combine_numbers(mut numbers: [u8; 4]) -> u32 {
         for number in &mut numbers {
             *number += 1;
         std::mem::transmute::<[u8; 4], u32>(numbers)

    You SHOULD instead do:

     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)
  4. Pass only C-compatible primitive types and bytes over the boundary

    Rust’s C-compatible primitive types are integers and floats. These types are declared in the libc crate. Most Rust objects have different representations in C and Rust, so they can’t be passed using FFI.

    Tor currently uses the following Rust primitive types from libc for FFI:

    • defined-size integers: uint32_t
    • native-sized integers: c_int
    • native-sized floats: c_double
    • native-sized raw pointers: * c_void, * c_char, ** c_char

    TODO: C smartlist to Stringlist conversion using FFI

    The only non-primitive 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). The length can be passed explicitly (libc::size_t), or the string can be NUL-byte terminated C string.

    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.

  5. 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.

  6. 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.”


  7. Type safety

    Wherever possible and sensical, you SHOULD create new types in a manner which prevents type confusion or misuse. For example, rather than using an untyped mapping between strings and integers like so:

     use std::collections::HashMap;
     pub fn get_elements_with_over_9000_points(map: &HashMap<String, usize>) -> Vec<String> {

    It would be safer to define a new type, such that some other usage of HashMap<String, usize> cannot be confused for this type:

     pub struct DragonBallZPowers(pub HashMap<String, usize>);
     impl DragonBallZPowers {
         pub fn over_nine_thousand<'a>(&'a self) -> Vec<&'a String> {
             let mut powerful_enough: Vec<&'a String> = Vec::with_capacity(5);
             for (character, power) in &self.0 {
                 if *power > 9000 {

    Note the following code, which uses Rust’s type aliasing, is valid but it does NOT meet the desired type safety goals:

     pub type Power = usize;
     pub fn over_nine_thousand(power: &Power) -> bool {
         if *power > 9000 {
             return true;
     // We can still do the following:
     let his_power: usize = 9001;
  8. Unsafe mucking around with lifetimes

    Because lifetimes are technically, in type theory terms, a kind, i.e. a family of types, individual lifetimes can be treated as types. For example, one can arbitrarily extend and shorten lifetime using std::mem::transmute:

     struct R<'a>(&'a i32);
     unsafe fn extend_lifetime<'b>(r: R<'b>) -> R<'static> {
         std::mem::transmute::<R<'b>, R<'static>>(r)
     unsafe fn shorten_invariant_lifetime<'b, 'c>(r: &'b mut R<'static>) -> &'b mut R<'c> {
         std::mem::transmute::<&'b mut R<'static>, &'b mut R<'c>>(r)

    Calling extend_lifetime() would cause an R passed into it to live forever for the life of the program (the 'static lifetime). Similarly, shorten_invariant_lifetime() could be used to take something meant to live forever, and cause it to disappear! This is incredibly unsafe. If you’re going to be mucking around with lifetimes like this, first, you better have an extremely good reason, and second, you may as be honest and explicit about it, and for ferris’ sake just use a raw pointer.

    In short, just because lifetimes can be treated like types doesn’t mean you should do it.

  9. Doing excessively unsafe things when there’s a safer alternative

    Similarly to #7, often there are excessively unsafe ways to do a task and a simpler, safer way. You MUST choose the safer option where possible.

    For example, std::mem::transmute can be abused in ways where casting with as would be both simpler and safer:

     // Don't do this
     let ptr = &0;
     let ptr_num_transmute = unsafe { std::mem::transmute::<&i32, usize>(ptr)};
     // Use an `as` cast instead
     let ptr_num_cast = ptr as *const i32 as usize;

    In fact, using std::mem::transmute for any reason is a code smell and as such SHOULD be avoided.

  10. Casting integers with as

    This is generally fine to do, but it has some behaviours which you should be aware of. Casting down chops off the high bits, e.g.:

     let x: u32 = 4294967295;
     println!("{}", x as u16); // prints 65535

    Some cases which you MUST NOT do include:

    • Casting an u128 down to an f32 or vice versa (e.g. u128::MAX as f32 but this isn’t only a problem with overflowing as it is also undefined behaviour for 42.0f32 as u128),

    • Casting between integers and floats when the thing being cast cannot fit into the type it is being casted into, e.g.:

      println!("{}", 42949.0f32 as u8); // prints 197 in debug mode and 0 in release
      println!("{}", 1.04E+17 as u8);   // prints 0 in both modes
      println!("{}", (0.0/0.0) as i64); // prints whatever the heck LLVM wants

      Because this behaviour is undefined, it can even produce segfaults in safe Rust code. For example, the following program built in release mode segfaults:

      pub fn trigger_ub(sl: &[u8; 666]) -> &[u8] {
          // Note that the float is out of the range of `usize`, invoking UB when casting.
          let idx = 1e99999f64 as usize;
          &sl[idx..] // The bound check is elided due to `idx` being of an undefined value.
      fn main() {
          println!("{}", trigger_ub(&[1; 666])[999999]); // ~ out of bound

      And in debug mode panics with:

      thread 'main' panicked at 'slice index starts at 140721821254240 but ends at 666', /checkout/src/libcore/slice/