Rust by Example

Each Rust source file in tests directory is compiled as a separate crate. One way of sharing some code between integration tests is making module with public functions, importing and using it within tests.

File tests/common.rs:

pub fn setup() {

// some setup code, like creating required files/directories, starting

// servers, etc.

}

File with test: tests/integration_test.rs

// importing common module.

mod common;

#[test]

fn test_add() {

// using common code.

common::setup();

assert_eq!(adder::add(3, 2), 5);

}

Modules with common code follow the ordinary modules rules, so it's ok to create common module as tests/common/mod.rs.

Development dependencies

Sometimes there is a need to have dependencies for tests (or examples, or benchmarks) only. Such dependencies are added to Cargo.toml in the [dev-dependencies] section. These dependencies are not propagated to other packages which depend on this package.

One such example is using a crate that extends standard assert! macros. File Cargo.toml:

# standard crate data is left out

[dev-dependencies]

pretty_assertions = "0.4.0"

File src/lib.rs:

// externing crate for test-only use

#[cfg(test)]

#[macro_use]

extern crate pretty_assertions;

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

a + b

}

#[cfg(test)]

mod tests {

use super::*;

#[test]

fn test_add() {

assert_eq!(add(2, 3), 5);

}

}

See Also

Cargo docs on specifying dependencies.

Unsafe Operations

As an introduction to this section, to borrow from the official docs, "one should try to minimize the amount of unsafe code in a code base." With that in mind, let's get started! Unsafe annotations in Rust are used to bypass protections put in place by the compiler; specifically, there are four primary things that unsafe is used for:

   • dereferencing raw pointers

   • calling functions or methods which are unsafe (including calling a function over FFI, see a previous chapter of the book)

   • accessing or modifying static mutable variables

   • implementing unsafe traits

Raw Pointers

Raw pointers * and references &T function similarly, but references are always safe because they are guaranteed to point to valid data due to the borrow checker. Dereferencing a raw pointer can only be done through an unsafe block.

fn main() {

let raw_p: *const u32 = &10;

unsafe {

assert!(*raw_p == 10);

}

}

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Calling Unsafe Functions

Some functions can be declared as unsafe, meaning it is the programmer's responsibility to ensure correctness instead of the compiler's. One example of this is std::slice::from_raw_parts which will create a slice given a pointer to the first element and a length.

use std::slice;

fn main() {

let some_vector = vec![1, 2, 3, 4];

let pointer = some_vector.as_ptr();

let length = some_vector.len();

unsafe {

let my_slice: &[u32] = slice::from_raw_parts(pointer, length);

assert_eq!(some_vector.as_slice(), my_slice);

}

}