enso/doc/rust-style-guide.md

Rust Style Guide

Like many style guides, this Rust style guide exists for two primary reasons. The first is to provide guidelines that result in a consistent code style across all of the Enso codebases, while the second is to guide people towards a style that is expressive while still easy to read and understand.

In general, it aims to create a set of 'zero-thought' rules in order to ease the programmer burden; there is usually only one way to lay out code correctly.

• Code Formatting
  • Line Width
  • Imports
  • Sections
  • Vertical Spacing
  • Multi-Line Expressions
  • Vertical Alignment
  • Spacing
  • Impl Definitions
  • Getters and Setters
• Naming
• Package Structure and Naming
  • The Public API
• Build Tooling
• Commenting
  • Documentation Comments
  • Source Notes
  • TODO Comments
  • Other Comment Usage
• Program Design
  • Code Complexity
  • Safety
  • Testing and Benchmarking
  • Warnings, and Lints

Code Formatting

This section explains the rules for visually laying out your code. They provide a robust set of guidelines for creating a consistent visual to the code.

Code style is far more than just the visual formatting of the code, especially as formatting can often be automated. According to the documentation of rustfmt, "formatting code is a mostly mechanical task which takes both time and mental effort." While, in many cases, the programmer can be relieved of this burden through use of an automated formatter, it is sometimes the case that such a tool imposes more cognitive load in programmers. With rustfmt, programmers tend to have to refactor long lines to use variables, and move code to specific modules or sections lest rustfmt produce code that is hard to read and write. Thus, it is very important to write code in such a way that we can be proud of its quality.

Due to the fact that rustfmt doesn't support multiple of our requirements, we have created a guide for how to format Rust code for this project. Please read it carefully.

We hope that, in the future, rustfmt will come to support many of the things described below, but even so, many portions of this guide will need to be handled manually.

Line Width

Each line in the source file should be of a maximum of 80 characters of text. This includes comments.

Imports

The imports section at the top of a file should be separated into four groups. These groups should be sorted in alphabetical order and are divided as follows:

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

Please look at the following by way of example:

pub mod display_object;

use crate::prelude::*;

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

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

Sections

Rust source files should be divided into sections, with a header placed before the definition of each new concept in a file.

By the term "concept," we are referring primarily to a structure with a set of related implementations. However if the related implementations rely on some simple helper structs, these may also be defined in the same section. A section should have a header as follows.

// ===================
// === SectionName ===
// ===================

Additionally, the code in each section should further be divided into sub-sections that group relevant functionality within the section. The header for a sub-section is as follows.

// === SubSectionName ===

At least one section should be defined in every file.

An Example of Using Sections

Here is a large-scale example of how sections should be used in source files.

// =================
// === 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 {
    ScaleRotateTranslate,
    ScaleTranslateRotate,
    RotateScaleTranslate,
    RotateTranslateScale,
    TranslateRotateScale,
    TranslateScaleRotate
}

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> {
        &self.transform.position
    }

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

    ...
}

// === Setters ===

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

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

    ...
}

Vertical Spacing

We use the following rules for the amount of vertical space separating various constructs in the source:

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

Multi-Line Expressions

In an ideal world, all expressions in the code should be a single line. This is because multi-line expressions are usually hard to read, and because they can introduce lots of noise in the code. In the vast majority of cases, the presence of a multi-line expression indicates that the code needs refactoring.

Please try to refactor portions of multi-line expressions to well-named variables, and divide them up to a set of single-line expressions.

Multi-Line Expression Examples

The following is an example of poorly formatted code:

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

The following is an example of the same code properly formatted:

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

In order to create a visual flow to our code that aids readability, the following constructs should be aligned vertically where possible:

• Assignment operators (=)
• Type operators (:)
• Match arrows (=>)
• Similar parameters or types

A Vertical Alignment Example

The following is an example of a function that correctly uses the vertical alignment rules above:

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),
        }
    }
}

Spacing

The following spacing rules are also employed in order to create a visual flow to our code to aid readability:

• The type operator is not spaced: fn test(foo:String, bar:Int) { ... }
• Commas between complex expressions (including the argument 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;

Spacing Examples as Function Definitions

The following function definitions are all good examples of correct use of spacing.

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>
      L:IsLogger
      ... {
    ...
}

Impl Definitions

In order to aid in fast discovery of the header of an impl definition, we use the following style. In all cases, the where block should be placed after a line break.

// No constraints
impl<T> Printer for Option<T> {
    ...
}

// Some constraints
impl<T:Printer>
Printer for Option<T> {
    ...
}

// Constraints in where block
impl<T> Printer for Option<T>
where T: Printer {
    ...
}

Getters and Setters

We have the following rules for getters and setters in our codebase.

• Getters do not have the get_ prefix, while setters do have the set_ prefix.
• If a setter is provided, a mut accessor should be provided as well.

Correct examples for the definition of getters and setters can be found below:

fn field(&self) -> &Type {
    &self.field
}

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

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

Naming

Enso has some fairly simple general naming conventions, though the sections below may provide more rules for use in specific cases.

• Types are written using UpperCamelCase.
• Variables and function names are written using snake_case.
• If a name contains an initialism or acronym, all parts of that initialism should be lower-case: make_http_request, not make_HTTP_request.
• Short variable names such as a and b should only be used in the following contexts:
  • Where there is no other appropriate name.
  • Named lifetimes. They should never be used to refer to temporary data in a function, as all temporaries should be given descriptive names.
• Names should be descriptive, even if this makes them longer.
• Any function that performs an unsafe operation that is not documented in its type (e.g. fn head<T>(ts: Vec<T>) -> T, which fails if the list is empty), must be named using the word 'unsafe' (e.g. unsafeHead). For more information on unsafe function usage, see the section on safety.

Package Structure and Naming

Enso follows the standard rust convention for structuring crates, as provided by cargo new. This is discussed more in depth here.

In order to match up with the project naming convention we use for Scala and Java projects, any rust code must be in a directory named using UpperCamelCase in the root of the project (e.g. enso/BaseGL).

The Public API

Whereas Rust defaults to making module members private by default, this is not the philosophy used by the Enso codebases. We tend to want our codebase to be flexible for consumers, so we tend to avoid making things private. Instead, we use the concept of an internal module to separate public from private.

If you are writing code in a module foo.bar.baz and would like to signal that a particular construct (e.g. a function) is for internal use in that package, you should create a foo.bar.baz.internal package. You can then write the relevant language construct in that package instead of the source package.

Using Access Modifiers

Given Rust's performance guarantees, making things pub has no impact on the performance of the compiled code. As a result, the only circumstance under which things are allowed to not be pub is when doing so would allow consumers of an API to break internal guarantees provided by that API (e.g. building an immutable collection on top of a mutable buffer).

Build Tooling

All Rust projects are built and managed using cargo.

Commenting

Comments in code are a tricky area to get right as we have found that comments often expire quickly, and in absence of a way to validate them, remain incorrect for long periods of time. In order to best deal with this problem, we make the keeping of comments up-to-date into an integral part of our programming practice while also limiting the types and kinds of comments we allow.

Comments across the Enso codebases fall into three main types:

• Documentation Comments: API documentation for all appropriate language constructs.
• Source Notes: Detailed explorations of design reasoning that avoid cluttering the code itself.
• Tasks: Things that need doing or fixing in the codebase.

When we write comments, we try to follow one general guideline. A comment should explain what and why, without mentioning how. The how should be self-explanatory from reading the code, and if you find that it is not, that is a sign that the code in question needs refactoring.

Code should be written in such a way that it guides you over what it does, and comments should not be used as a crutch for badly-designed code.

Documentation Comments

One of the primary forms of comment that we allow across the Enso codebases is the doc comment. We use these comments to document the public API of a module, as defined in The Public API. For constructs that are part of the public API, the following should be documented:

1. Top-Level Type Definitions: All top-level type definitions must have a doc comment.
2. Functions: Function documentation should provide at-a-glance intuition for how to use that function.

Documentation comments are intended for consumption by the users of the API, and are written using the standard rustdoc syntax. Doc comments should contain:

1. Summary: A one-line summary of the construct's behaviour or purpose.
2. Description (Optional): Any useful information that would be necessary for a consumer of the API to know (that is not encoded in the types). This should be written in grammatically correct English.

Convention in rust is to not document function or return parameters, and so rustdoc does not provide a way to do so.

An example of a valid set of comments for some rust code is as follows:

/// A representation of tree structures containing elements of type `T`.
pub trait Tree<T> {
  /// Provides a sequence representation of the tree.
  ///
  /// The function provides configurable behaviour for the order in which the
  /// tree is walked. See [WalkStrategy](org.enso.WalkStrategy.html) for
  /// the provided options.
  pub fn walk_to_sequence(self: &Self, order: WalkStrategy<T>) -> Vec<T> {
    // ...
  }

  fn getBuffer(self: &Self) -> Vec<T> {
    // ...
  }
}

Documentation comments should not reference internal implementation details, or be used to explain choices made in the implementation. For this kind of info, you should use Source Notes as described below.

You may document more than what is specified here, but this is the minimum required for acceptance at code-review time.

Source Notes

Source Notes is a mechanism for moving detailed design information about a piece of code out of the code itself. In doing so, it retains the key information about the design while not impeding the flow of the code. They are used in the following circumstances:

• Design Information: Documentation about why something was written in a particular fashion, as well as information on the process that led to it being done this way.
• Explaining Complexity: If an implementation uses complex constructs or any elements that are non-obvious, these should be explained as part of a source note.
• Knowledge Provenance: Explaining where some knowledge (e.g. a mathematical formula or an algorithm) was obtained from. It is also useful to accompany these by some commentary on why the choice was made.
• Safety: Any unsafe usage of a function must be accompanied by a source note that explains what makes this particular usage safe.

Source notes are detailed comments that, like all comments, explain both the what and the why of the code being described. In very rare cases, it may include some how, but only to refer to why a particular method was chosen to achieve the goals in question.

A source note comment is broken into two parts:

1. Referrer: This is a small comment left at the point where the explanation is relevant. It takes the following form: // Note [Note Name], where Note Name is a unique identifier across the codebase. These names should be descriptive, and make sure you search for it before using it, in case it is already in use.
2. Source Note: This is the comment itself, which is a large block comment placed after the first function in which it is referred to in the module. The first line names the note using the same referrer as above: // Note [Note Name]. The name(s) in the note are underlined using a string of the = (equals) character.

A source note may contain sections within it where necessary. These are titled using the following syntax: == Note [Note Name (Section Name)], and can be referred to from a referrer much as the main source note can be.

Sometimes it is necessary to reference a source note in another module, but this should never be done in-line. Instead, a piece of code should reference a source note in the same module that references the other note while providing additional context to that reference.

An example can be seen below:

/// A representation of tree structures containing elements of type `T`.
pub trait Tree<T> {
  /// Provides a sequence representation of the tree.
  ///
  /// The function provides configurable behaviour for the order in which the
  /// tree is walked. See [WalkStrategy](org.enso.WalkStrategy.html) for
  /// the provided options.
  pub fn walk_to_sequence(self: &Self, order: WalkStrategy<T>) -> Vec<T> {
    let mut output_vec = Vec.new(self.getBuffer().len()); // Note [Buffer Size]
    // ...
  }

  // Note [Buffer Size]
  // ==================
  // When working with the buffer for the tree walk, it is important that you
  // ensure....

  fn getBuffer(self: &Self) -> Vec<T> {
    // ...
  }
}

TODO Comments

We follow a simple convention for TODO comments in our codebases:

• The line starts with TODO or FIXME.
• It is then followed by the author's initials [ARA], or for multiple people [ARA, MK], in square brackets.
• It is then followed by an explanation of what needs to be done.

For example:

// TODO [ARA] This is a bit of a kludge. Instead of X it should to Y, accounting
// for the fact that Z.

Other Comment Usage

There are, of course, a few other situations where commenting is very useful:

• Commenting Out: You may comment out code while developing it, but if you commit any commented out code, it should be accompanied by an explanation of why said code can't just be deleted.
• Bugs: You can use comments to indicate bugs in our code, as well as third-party bugs. In both cases, the comment should link to the issue tracker where the bug has been reported.

Program Design

Any good style guide goes beyond purely stylistic rules, and also talks about design styles to use in code.

Code Complexity

While we often have to write complex functionality, we want to ensure that the code itself is kept as simple and easy to read as possible. To do this, please use the following rules:

• Write single-line expressions wherever possible, rather than writing one complex chunk of code.
• Separate intermediate results out to their own variables with appropriate names. Even if they are temporaries, giving them a name is a great aid to code comprehension.

Safety

Whereas most languages don't have a concept of safety, rust comes with a built in notion of unsafe. When working with unsafe functions and code blocks, you must account for the following:

• As unsafe functions are explicitly declared with the keyword unsafe, we do not need any special naming convention for them.
• Usage of unsafety should be confined to the smallest possible block.
• Usage of unsafety should be accompanied by a source note that explains why it is necessary, and any constraints on its usage.
• Unsafe function usage must be accompanied by a source note explaining how this usage of it is made safe.

Furthermore, we do not allow for code containing pattern matches that can fail.

Testing and Benchmarking

New code should always be accompanied by tests. These can be unit, integration, or some combination of the two, and they should always aim to test the new code in a rigorous fashion.

• Testing should be performed as described in the Rust book and should use the functionality for testing built into the language.
• Tests should cover as much code as possible, and may be a combination of unit and integration tests.

Any performance-critical code should also be accompanied by a set of benchmarks. These are intended to allow us to catch performance regressions as the code evolves, but also ensure that we have some idea of the code's performance in general.

• We use nightly rust in order to access the built-in benchmarking functionality.
• We measure time, CPU, and memory usage where possible.
• Where relevant, benchmarks may set thresholds which, when surpassed, cause the benchmark to fail. These thresholds should be set for a release build, and not for a development build.

Do not benchmark a development build as the data you get will often be entirely useless.

Warnings, and Lints

In general, we aim for a codebase that is free of warnings and lints, and we do this using the following ideas:

Warnings

New code should introduce no new warnings onto master. You may build with warnings on your own branch, but the code that is submitted as part of a PR should not introduce new warnings. You should also endeavour to fix any warnings that you come across during development.

Sometimes it is impossible to fix a warning (often in situations involving the use of macros). In such cases, you are allowed to suppress the warning locally, but this must be accompanied by a source note explaining why you are doing so.