Rust Style Guide

Motivation - why not rely on a formatting tool for code style?

The code style is way more than just formatting. In many cases formatting can be automated. According to rustfmt docs: “formatting code is a mostly mechanical task which takes both time and mental effort. By using an automatic formatting tool, a programmer is relieved of this task and can concentrate on more important things.”. While in many cases it is true, if the code author does not take extra effort to make his code pretty by refactoring long lines to variables, moving code to specific modules, or sections, the formatting tool will result in code that is hard to read and hard to write. Thus, it is important to take time to write code in such way that we can be proud of its quality.

Because rustfmt does not support many of our requirements, we have created a guide to describe how to format Rust code in this project. Please read it carefully. We hope that in the future, some of the things described below will be possible while using rustfmt (and we encourage you to contribute there!), however, even if it happens, many parts of this guide will still be valid and will need to be handled manually.

Styling rules

Code width

Each line in a source file should have max 80 chars of text (including comments).


Imports should be divided into 4 groups separated by blank lines. Items in the groups should be sorted alphabetically.

// Group 1: sub-module definitions.
// Group 2: prelude-like imports.
// Group 3: local-crate imports.
// Group 4: external imports.

For example:

pub mod display_object;

use crate::prelude::*;

use crate::closure;
use crate::data::opt_vec::OptVec;
use crate::data::dirty;
use crate::system::web::group;

use nalgebra::Matrix4;
use nalgebra::Vector3;


Source files should be divided into sections. Section headers should be placed before each new “concept” defined in a file. By “concept” we normally mean a structure with related implementations. In case related implementations use some helper structs with very small implementations, these helper structs may be defined in the same section. Moreover, the code in each section should be divided into sub-sections, grouping related definitions. At least one section should be defined in a file (if there is at least one struct definition as well). For example:

// =================
// === AxisOrder ===
// =================

/// Defines the order in which particular axis coordinates are processed. Used
/// for example to define the rotation order in `DisplayObject`.
pub enum AxisOrder {XYZ,XZY,YXZ,YZX,ZXY,ZYX}

impl Default for AxisOrder {
    fn default() -> Self {Self::XYZ}

// =================
// === Transform ===
// =================

/// Defines the order in which transformations (scale, rotate, translate) are
/// applied to a particular object.
pub enum TransformOrder {

impl Default for TransformOrder {
    fn default() -> Self { Self::ScaleRotateTranslate }

// =============================
// === HierarchicalTransform ===
// =============================

pub struct HierarchicalTransform<OnChange> {
    transform        : Transform,
    transform_matrix : Matrix4<f32>,
    origin           : Matrix4<f32>,
    matrix           : Matrix4<f32>,
    pub dirty        : dirty::SharedBool<OnChange>,
    pub logger       : Logger,

impl<OnChange> HierarchicalTransform<OnChange> {
    pub fn new(logger:Logger, on_change:OnChange) -> Self {
        let logger_dirty     = logger.sub("dirty");
        let transform        = default();
        let transform_matrix = Matrix4::identity();
        let origin           = Matrix4::identity();
        let matrix           = Matrix4::identity();
        let dirty            = dirty::SharedBool::new(logger_dirty,on_change);
        Self {transform,transform_matrix,origin,matrix,dirty,logger}

// === Getters ===

impl<OnChange> HierarchicalTransform<OnChange> {
    pub fn position(&self) -> &Vector3<f32> {

    pub fn rotation(&self) -> &Vector3<f32> {


// === Setters ===

impl<OnChange:Callback0> HierarchicalTransform<OnChange> {
    pub fn position_mut(&mut self) -> &mut Vector3<f32> {
        &mut self.transform.position

    pub fn rotation_mut(&mut self) -> &mut Vector3<f32> {
        &mut self.transform.rotation


Vertical spacing

We use the following amount of vertical spacing:

  • 3 blank lines after imports.
  • 3 blank lines before each section.
  • 2 blank lines before each sub-section.
  • 1 blank line after each section / sub-section.
  • 1 blank line before functions / structures / impls.
  • 1 blank line at the end of the file.

Please note that vertical spacing overlaps. For example, if there is a section after imports, the total number of blank lines is 3, not 6.

Multiline Expressions

Most (preferably all) expressions should be single line. Multiline expressions are hard to read and introduce noise in the code. Often, it is also an indicator of code that is not properly refactored. Try to refactor parts of multiline expressions to well-named variables, and divide them to several single-line expressions.

Example of poorly formatted code:

pub fn new() -> Self {
    let shape_dirty = ShapeDirty::new(logger.sub("shape_dirty"),
    let dirty_flag = MeshRegistryDirty::new(logger.sub("mesh_registry_dirty"),
    Self { dirty_flag, dirty_flag }

Example of properly formatted code:

pub fn new() -> Self {
    let sub_logger  = logger.sub("shape_dirty");
    let shape_dirty = ShapeDirty::new(sub_logger,on_dirty.clone());
    let sub_logger  = logger.sub("mesh_registry_dirty");
    let dirty_flag  = MeshRegistryDirty::new(sub_logger,on_dirty);
    Self {shape_dirty,dirty_flag}

Vertical alignment

The following elements should be aligned vertically in subsequent lines:

  • assignment operators (=),
  • type operators (:),
  • match arrows (=>),
  • similar parameters or types.


impl Printer for GlobalVarStorage {
    fn print(&self, builder:&mut Builder) {
        match self {
            Self::ConstStorage      => build!(builder,"const"),
            Self::UniformStorage    => build!(builder,"uniform"),
            Self::InStorage  (qual) => build!(builder,"in" ,qual),
            Self::OutStorage (qual) => build!(builder,"out",qual),


  • Type operator is not spaced: fn test(foo:String, bar:Int) { ... }.
  • Commas between complex expressions (including arg list) are spaced.
  • Commas between simple elements are not spaced: Result<Self,Error>.
  • Arguments to functions are not spaced: build(builder,"out",qual).
  • Operators are always spaced: let foo = a + b * c;.

Function definitions

The following examples show proper function styles:

pub fn new<Dom:Str>(dom:Dom, logger:Logger) -> Result<Self,Error> {
pub fn new<Dom:Str>(dom:Dom, logger:Logger) -> Result<Self,Error> {
pub fn new<Dom:Str>
(dom:Dom, logger:Logger, on_dirty:OnDirty) -> Result<Self,Error> {
pub fn new<Dom:Str>
(dom:Dom, logger:Logger, on_dirty:OnDirty, on_remove:OnRemove) 
-> Result<Self,Error> {
pub fn new<Dom:Str>
( dom        : Dom
, logger     : Logger
, on_dirty   : OnDirty
, on_remove  : OnRemove
, on_replace : OnReplace
) -> Result<Self,Error> {

Long where clauses are formatted this way:

pub fn new<D,L>(dom:D, logger:L) -> Result<Self,Error> 
where D:AsRef<str>, L:IsLogger {

Or, in case they are really long, this way:

pub fn new<D,L>(dom:D, logger:L) -> Result<Self,Error> 
where D:AsRef<str>
      ... {

Impl definitions

Sometimes when browsing code it is hard to understand where the header of an impl declaration is. Thus, the following style allows to find it easier. The where block should be placed after a linebreak. All of the following are correct:

// No constraints
impl<T> Printer for Option<T> {
// Some constraints
Printer for Option<T> {
// Constraints in where block
impl<T> Printer for Option<T> 
where T: Printer {

Getters and Setters

Getters do not have the get_ prefix, while setters do. If a setter is provided (method with the set_ prefix), a mut accessor should be provided as well. The correct way of defining getters and setters is presented below:

fn field(&self) -> &Type { 

fn field_mut(&mut self) -> &mut Type { 
    &mut self.field 

fn set_field(&mut self, val:Type) {
    *self.field_mut = val;

Trait exporting

All names should be designed to be used in a qualified fashion. However, this makes one situation tricky. In order to use methods defined in a trait, it has to be in scope. Consider a trait display::Object. We want to use it as function bound like fn test<T:display::Object>(t:T) {...}, and we also want to use methods defined in this trait (so it has to be in scope). In such a case, Clippy warns that display::Object is unnecessary qualification and could be replaced simply by Object, which is not what we want. Thus, in order to export traits, please always rename them using the following convention:

/// Common traits.
pub mod traits {
    // Read the Rust Style Guide to learn more about the used naming.
    pub use super::Object    as TRAIT_Object;
    pub use super::ObjectOps as TRAIT_ObjectOps;

Having such a definition, we can import traits to scope using use display::object::traits::*, and we would not have any warning about unnecessary qualification anymore.