Rust Pro Tips (collection)

All variants of an enum are the same size. This can become a problem when variants have drastically different memory requirements. Use indirection (&-reference, Box, etc.) to resolve.

enum WithoutIndirection {
    Unit,
    Kilobyte([u8; 1024]),
}

enum WithIndirection<'a> {
    Unit,
    Kilobyte(&'a [u8; 1024]),
}

println!("{}", std::mem::size_of_val(&WithoutIndirection::Unit));
// => 1025

println!("{}", std::mem::size_of_val(&WithIndirection::Unit));
// => 8

30. Create a slice from a reference without copying

If you have a reference to some data and you want to pass it to a function that takes a slice, you can use std::array::from_ref to cheaply create a slice without copying.

struct Thing;

fn takes_slice(_: &[Thing]) {
    // ...
}

fn my_function(arg: &Thing) {
    takes_slice(std::array::from_ref(arg));
}

fn main() {
    my_function(&Thing);
}

29. Include README in documentation

Docs
Documentation from a crate that does this

Insert the README into a crate’s documentation by putting #![doc = include_str!("../README.md")] in lib.rs. This can help avoid duplicating information between the README and the documentation, and will also test README examples as doctests.

// lib.rs
#![doc = include_str!("../README.md")]

28. Workspace dependencies

Easily specify a uniform dependency version for all crates in a workspace with the [workspace.dependencies] table in Cargo.toml.

# Cargo.toml

[workspace]
members = ["my_crate"]

[workspace.dependencies]
serde = "1.0.183"

# my_crate/Cargo.toml

[dependencies]
serde.workspace = true # -> 1.0.183

27. Testing for compilation failure

Create tests intended to fail compilation with the compile_fail attribute on documentation tests.

/// ```compile_fail
/// my_function("hello");
/// ```
pub fn my_function(value: u8) {}

The compilation error is error[E0308]: mismatched types, which you can check for specifically:

/// ```compile_fail,E0308
/// my_function("hello");
/// ```
pub fn my_function(value: u8) {}

26. Sealed traits

If a public trait requires the implementation of a private trait, the public trait is “sealed” and can only be implemented within the crate that defines it.

// my_crate/src/lib.rs

mod private {
    pub trait Sealed {}

    impl Sealed for u8 {}
}

pub trait PublicTrait: private::Sealed {}

impl PublicTrait for u8 {}

// another_crate/src/lib.rs

use my_crate::PublicTrait;

// error: requires private::Sealed implementation
impl PublicTrait for String {}

// error: my_crate::private::Sealed is private
impl my_crate::private::Sealed for String {}

Rust API Guidelines

25. Static type size assertion

Use std::mem::transmute::<S, D> to assert that two types have the same size at compile time.

use std::mem::transmute;

struct TwoBytes(u8, u8);

let _ = transmute::<TwoBytes, u16>; // ok
let _ = transmute::<TwoBytes, u32>; // error!

The compiler will complain if the types are not the same size:

error[E0512]: cannot transmute between types of different sizes, or dependently-sized types

Warning: Memory layout, alignment, etc. is often not guaranteed, so be careful!

24. Conditional compilation

Use the cfg and cfg_attr attributes to compile different code based on the build environment. This is useful for feature-gating, platform-specific code, etc.

// Implements Serialize if the crate is built with the "serde" feature enabled
#[cfg_attr(feature = "serde", derive(serde::Serialize))]
struct Point2(f64, f64);

fn main() {
    #[cfg(target_os = "linux")]
    let person = "Linus Torvalds";
    #[cfg(target_os = "windows")]
    let person = "Bill Gates";
    #[cfg(target_os = "macos")]
    let person = "Tim Apple";

    // Approval-seeking!
    println!("Hi there, {person}!");
}

Docs
Playground

23. Declaratively create `HashMap`s from iterables

HashMaps can be built from iterators of key-value tuples:

use std::collections::HashMap;

let map: HashMap<&str, i32> = [
    ("a", 1),
    ("b", 2),
    ("c", 3),
    ("d", 4),
]
.into();

Rust Book
Rust Reference
Playground

22. `impl` vs. `dyn` in assembler

Use impl for generic monomorphization, dyn for dynamic dispatch.

fn with_impl<'a>(v: &'a impl AsRef<[u8]>) -> &'a str {
    std::str::from_utf8(v.as_ref()).unwrap()
}

fn with_dyn<'a>(v: &'a dyn AsRef<[u8]>) -> &'a str {
    std::str::from_utf8(v.as_ref()).unwrap()
}

pub fn main() {
    let array = [72, 101, 108, 108, 111];
    let vector = vec![72, 101, 108, 108, 111];
    assert_eq!("Hello", with_impl(&array));
    assert_eq!("Hello", with_dyn(&array));
    assert_eq!("Hello", with_impl(&vector));
    assert_eq!("Hello", with_dyn(&vector));
}

See the effect in the generated assembly. with_impl is generated once for each parameterization. with_dyn is generated once, but a vtable is required at runtime.

; with_impl(&[u8; 5]) implementation (monomorphized)
_ZN7example9with_impl17h38cd8c9aec305c25E:
        sub     rsp, 40
        ; constant lookup
        mov     rax, qword ptr [rip + _ZN4core5array92_$LT$impl$u20$core..convert..AsRef$LT$$u5b$T$u5d$$GT$$u20$for$u20$$u5b$T$u3b$$u20$N$u5d$$GT$6as_ref17h86d8b288e84fcf8aE@GOTPCREL]
        call    rax
        mov     rsi, rax
        ; [[snip]]

; with_impl(&Vec<u8>) implementation (monomorphized)
_ZN7example9with_impl17hd50582b2776df596E:
        sub     rsp, 40
        ; constant lookup
        mov     rax, qword ptr [rip + _ZN88_$LT$alloc..vec..Vec$LT$T$C$A$GT$$u20$as$u20$core..convert..AsRef$LT$$u5b$T$u5d$$GT$$GT$6as_ref17h56f6b2151f0bc49cE@GOTPCREL]
        call    rax
        mov     rsi, rax
        ; [[snip]]

; with_dyn(&dyn AsRef<[u8]>) fat pointer implementation
_ZN7example8with_dyn17h88123f4b1a0b56e4E:
        sub     rsp, 40
        ; get vtable entry
        mov     rax, qword ptr [rsi + 24]
        call    rax
        mov     rsi, rax
        ; [[snip]]

When invoking with_dyn, the computer must construct the vtable at runtime.

        ; [[snip]]
        ; with_dyn(&Vec<u8>) invocation
        ; load the vtable
        lea     rsi, [rip + .L__unnamed_14]
        lea     rdi, [rsp + 176]
        call    _ZN7example8with_dyn17h88123f4b1a0b56e4E
        ; [[snip]]

.L__unnamed_14:
        .quad   _ZN4core3ptr46drop_in_place$LT$alloc..vec..Vec$LT$u8$GT$$GT$17hec70dc68d599b27eE
        .asciz  "\030\000\000\000\000\000\000\000\b\000\000\000\000\000\000"
        .quad   _ZN88_$LT$alloc..vec..Vec$LT$T$C$A$GT$$u20$as$u20$core..convert..AsRef$LT$$u5b$T$u5d$$GT$$GT$6as_ref17h56f6b2151f0bc49cE

Compare this to the invocation of a monomorphized function:

        ; [[snip]]
        ; with_impl(&Vec<u8>) invocation
        lea     rdi, [rsp + 176]
        call    _ZN7example9with_impl17hd50582b2776df596E
        ; [[snip]]

No vtable required!

dyn docs
impl docs

21. Closure traits

Rule of thumb: the more a closure does with captured variables, the fewer traits it implements.

Only reads? Fn + FnMut + FnOnce.
Mutates? FnMut + FnOnce.
Moves? FnOnce.

No captures at all? Function pointer coercion, too!

fn impl_fn_once(_: &impl FnOnce() -> ()) {}
fn impl_fn_mut(_: &impl FnMut() -> ()) {}
fn impl_fn(_: &impl Fn() -> ()) {}
fn fn_pointer(_: fn() -> ()) {}

#[derive(Debug)]
struct Var(i32);

{ // A
    let mut var = Var(0);
    let f = || drop(var);
    impl_fn_once(&f);
    impl_fn_mut(&f);
    impl_fn(&f);
    fn_pointer(f);
}

{ // B
    let f = || println!("{:?}", Var(0));
    impl_fn_once(&f);
    impl_fn_mut(&f);
    impl_fn(&f);
    fn_pointer(f);
}

{ // C
    let mut var = Var(0);
    let f = || println!("{:?}", var);
    impl_fn_once(&f);
    impl_fn_mut(&f);
    impl_fn(&f);
    fn_pointer(f);
}

{ // D
    let mut var = Var(0);
    let f = || var.0 += 1;
    impl_fn_once(&f);
    impl_fn_mut(&f);
    impl_fn(&f);
    fn_pointer(f);
}

20. Write better tests with `#[should_panic]`

Require tests to panic with #[should_panic]. This is useful for testing unhappy paths. Optionally include a substring to match against the panic message.

#[test]
#[should_panic = "attempt to divide by zero"]
fn div_zero() {
    let val = 1i32.div(0);
}

19. Use `#[non_exhaustive]` to prevent breaking changes

Use the #[non_exhaustive] attribute to prevent breaking changes when adding new fields to a struct or enum variant.

#[non_exhaustive]
pub enum Error {
    Io(IoError),
    ParseInt(ParseIntError),
    ParseFloat(ParseFloatError),
}

This doesn’t make much a difference in the crate where the type is defined, but it’s useful for downstream crates (those that depend on your crate), since it prevents breaking changes when new variants are added.

So, this is fine, but only within the crate that defines the type:

match e {
    Error::Io(_) => println!("IO error"),
    Error::ParseInt(_) => println!("Parse int error"),
    Error::ParseFloat(_) => println!("Parse float error"),
}

However, a crate that depends on your crate must include a wildcard pattern to handle new variants:

match e {
    Error::Io(_) => println!("IO error"),
    Error::ParseInt(_) => println!("Parse int error"),
    Error::ParseFloat(_) => println!("Parse float error"),
    _ => println!("Other error"),
}

18. Union types

One of Rust’s lesser-known composite types is the union. While they might look like structs at first, each field is actually the same piece of memory, allowing you to reinterpret bytes as a different type. Of course, this requires unsafe code.

union Onion {
    uint: u32,
    tuple: (u16, u16),
    list: [u8; 4],
}

let mut o = Onion { uint: 0xAD0000 };

unsafe {
    o.tuple.0 = 0xBEEF;
    o.list[3] = 0xDE;
}

assert_eq!(unsafe { o.uint }, 0xDEADBEEF);

(Extra pro tip: you can do pattern matching on unions too!)

17. Struct update operator `..`

Use the struct update operator .. to easily copy a struct with a few minor modifications. This can be useful when a struct implements Default.

#[derive(Default)]
struct Post {
    title: String,
    body: String,
    image_url: Option<String>,
    view_count: u32,
    tags: Vec<String>,
}

let p = Post {
    title: "How to make $1M posting coding tips on Twitter".into(),
    body: "1. Post coding tips on Twitter.\n2. ???\n3. Profit!".into(),
    ..Default::default()
};

assert_eq!(p.image_url, None);
assert_eq!(p.view_count, 0);
assert_eq!(p.tags, Vec::<String>::new());

Playground
Documentation
Rust by Example

16. The `@` operator

Use the @ operator to bind an identifier to a value that matches a pattern.

struct User {
    age: u8,
    name: String,
}

let u = User { age: 25, name: "John".into() };

if let User { age: a @ ..=35, .. } = u {
    println!("Become POTUS in {} years", 35 - a);
}

15. Create your own iterators

Rust’s Iterator trait is super useful! Here’s how to implement it:

We’ll start with a data structure we want to iterate over:

struct Pair<T> {
    a: T,
    b: T,
}

And a function that returns the iterator:

impl<T> Pair<T> {
    pub fn iter(&self) -> PairIter<T> {
        // ...
    }
}

Here’s our iterator struct. It keeps track of:

What collection we’re iterating over.
The current location within the collection.

struct PairIter<'a, T> {
    pair: &'a Pair<T>,
    next: u8,
}

Let’s implement Iterator. Note that the type of the items emitted by the iterator is defined by an associated type.

impl<'a, T> Iterator for PairIter<'a, T> {
    type Item = &'a T;

    fn next(&mut self) -> Option<Self::Item> {
        // ...
    }
}

The next() function simply needs to update the iterator and return the current item.

match self.next {
    0 => {
        self.next += 1;
        Some(&self.pair.a)
    }
    1 => {
        self.next += 1;
        Some(&self.pair.b)
    }
    _ => None,
}

And that’s it! Now you can easily implement Iterator on your own types.

! type documentation
Standard library documentation
Infallible documentation
Nothing in Rust

14. Use longer names for lifetimes and generic parameters

Lifetimes and generic parameters don’t have to be just one character long. When you’re deep in the weeds, working your Rust magic, keep your code comprehensible.

struct View<'source, Element: Deserialize> {
    label: &'source Element,
    value: &'source [u8],
}

13. `PhantomData` 👻

PhantomData has a dead-simple definition, but fascinating use-cases. PhantomData makes it look like your type contains another type even if it really doesn’t. It’s zero-sized, so it costs you nothing to use!

PhantomData can be useful when interfacing with external resources:

struct ExternalResource<T> {
    _marker: PhantomData<T>,
}

impl<T> ExternalResource<T> {
    fn fetch(&self) -> T { /* ... */ }
    fn update(&self, v: T) { /* ... */ }
}

If you’re working with FFI or otherwise need to manually restrict lifetimes:

struct FP<'a>(usize, *const u8, PhantomData<&'a ()>);

impl<'a> From<&'a Vec<u8>> for FP<'a> {
    fn from(v: &'a Vec<u8>) -> Self {
        FP(v.len(), v.as_ptr(), PhantomData)
    }
}

Playground
More in Nomicon.

12. `String`s are collections

A String is a collection that can be built from an iterator:

let my_string = vec![1, 2, 3, 4]
    .iter()
    .map(|i| (i * 2).to_string())
    .collect::<String>();

assert_eq!(my_string, "2468");

11. Enforce Clippy lints in a workspace

Enforce a consistent set of Clippy lints across multiple crates in one workspace by adding to .cargo/config.toml at the project root.

[target.'cfg(all())']
rustflags = [
  "-Wclippy::large_digit_groups",
]

(Otherwise you’d need to have a #![warn(...)] directive in every crate.)

10. The never type `!`

The never type in Rust (denoted by !) represents a type that will never exist. Called the “bottom type” in type theory, it’s used as the type of expressions that never resolve (e.g. panics or infinite loops) or the type of a return or break statement.

The never type can be coerced to any type. For example:

let x: u32 = panic!();

9. Recursive declarative macros

Build quick-and-dirty parsers with recursive declarative macros.

macro_rules! m {
    (y) => { true };
    (n) => { false };
    ($a:tt xor $($b:tt)+) => { m!($a) != m!($($b)+) };
}

assert!(m!(y xor n xor n));

8. Rustdoc link shorthand

Link to a module, struct, or enum directly in rustdoc comments with the shorthand syntax:

/// Link to [`MyStruct`]

Works for external crates too!

/// Link to [`serde::Serialize`]

(Links to https://docs.rs/serde/latest/serde/ser/trait.Serialize.html.) Backticks are optional, they just format it nicely.

7. More precise access control

Specify item visibility with the pub(restricted) syntax:

Syntax	Visible in…
`pub(crate)`	current crate
`pub(super)`	parent module
`pub(self)`	current module
`pub(in <path>)`	ancestor module

build.rs documentation
The #[path] attribute

6. Reuse code in `build.rs` with `#[path]`

Easily reuse code from your project in the build.rs file by using the #[path] directive:

#[path = "./src/parse.rs"]
mod parse;

fn main() {
    parse::om_nom(/* ... */);
}

Feature flags work in build.rs too!

#[cfg(feature = "parser")]
let feature_gated = /* ... */;

5. Reduce unnecessary allocations with cows 🐮

The clone-on-write smart pointer std::borrow::Cow creates owned values only when necessary. It’s useful when you want to work with both owned and borrowed values, and it can help you dynamically prevent unnecessary allocations.

fn f(v: &mut Cow<str>, m: bool) {
    if m {
        v.to_mut().push_str(", world");
    }
}

let mut v: Cow<str> = "hi".into();
f(&mut v, false);
assert!(v.is_borrowed()); // note: is_borrowed() is only available on nightly
f(&mut v, true);
assert!(v.is_owned());

4. Additional trait bounds per-function

Did you know you can add additional trait bounds to a type parameter for a single function when writing traits or impl blocks?

trait MyTrait<T: Debug> {
    fn action(&self) {
        // Implemented when T is Debug
    }

    fn print(&self) where T: Display {
        // Only implemented when T is Debug + Display
    }
}

3. Use the `AsRef` trait to convert references

Vec<T>? 😢
AsRef<[T]>? 😄

fn sum_is_even(v: impl AsRef<[u32]>) -> bool {
    v.as_ref().iter().fold(true, |e, i| e == (i & 1 == 0))
}

// both work!
sum_is_even(vec![0, 2, 4]));
sum_is_even([1]);