Source code style guide

Identifier names

Long versus abbreviated

We use unabbreviated identifier names to avoid ambiguity. Short (even single letter) variable names are allowed in lambdas (closures), in one-liners, and when the context allows it.

The shorter the scope the shorter the variable names, and the longer the function […] names. And vice versa.

— Uncle Bob Martin
Source: Twitter

The usage of well-known acronyms like CPU, TLS or OIDC are allowed.

const ONE_H_IN_SECS: usize = 3600;

let param = Some("foo");
let buf = &[];

fn func(elems: Vec<String>) {}

const ONE_HOUR_IN_SECONDS: usize = 3600;

let parameter = Some("foo");
let buffer = &[];

fn function(elements: Vec<String>) {}

for i in 0..5 {}

Closures and one-liners

It should be noted that the second example is meant to illustrate the use of single letter variable names in closures. It does not reflect production-level Rust code. The snippet would be simplified in the real world.

let length = parameter.map(|p| p.len());

let sum: usize = vec![Some(2), None, Some(4), Some(3), None]
    .iter()
    .filter(|o| o.is_some())
    .map(|n| n.unwrap())
    .map(|n| n * 2)
    .sum()

Well-known acronyms

const K8S_LABEL_KEY: &str = "app.kubernetes.io";

let oidc_provider = OidcProvider {};
let tls_settings = TlsSettings {};

Optional function parameters and variables

Optional function parameters and variables containing Option must not use any prefixes or suffixes to indicate the value is of type Option. This rule does not apply to function names like Client::get_opt().

let tls_settings_or_none: Option<TlsSettings> = None;
let maybe_tls_settings: Option<TlsSettings> = None;
let opt_tls_settings: Option<TlsSettings> = None;

let tls_settings: Option<TlsSettings> = None;

Structs and enums

Naming convention

Structs can use singular and plural names. Enums must use singular names, because only one variant is valid, e.g. Error::NotFound and not Errors::NotFound.

enum Errors {
  NotFound,
  Timeout,
}

enum Colors {
  Red,
  Green,
  Blue,
}

enum Error {
  NotFound,
  Timeout,
}

enum Color {
  Red,
  Green,
  Blue,
}

Formatting of struct fields and enum variants

We add newlines to struct fields and enum variants when they include additional information like documentation comments or attributes, because the variants can become difficult to read. This is especially the case when fields include doc comments, attributes like #[snafu()], and in case of enum variants, various embedded types.

Enum variants and struct fields don’t need to be separated when no additional information is attached to any of the variants or fields.

enum Color {
    Red,

    Green,

    Blue,
}

struct Foo {
    /// My doc comment for bar
    bar: usize,
    /// My doc comment for baz
    baz: usize,
}

enum Error {
    /// Indicates that we failed to foo.
    #[snafu(display("failed to foo"))]
    Foo,
    /// Indicates that we failed to bar.
    #[snafu(display("failed to bar"))]
    Bar,
    Baz,
}

enum Color {
    Red,
    Green,
    Blue,
}

struct Foo {
    /// My doc comment for bar
    bar: usize,

    /// My doc comment for baz
    baz: usize,
}

enum Error {
    /// Indicates that we failed to foo.
    #[snafu(display("failed to foo"))]
    Foo,

    /// Indicates that we failed to bar.
    #[snafu(display("failed to bar"))]
    Bar,
    Baz,
}

Any single uncommented variants or fields in an otherwise-commented enum or struct is considered to be a smell. If any of the items is commented, all items should be. It should however also be noted that there is no requirement to comment fields or variants. Comments should only be added if they provide additional information not available from context.

Error handling

Choice of error crate and usage

We use snafu for all error handling in library and application code because we want to provide as much context to the user as possible. Further, snafu allows us to use the same source error in multiple error variants. This feature can be used for cases were we need / require more fine-grained error variants. This behaviour is not possible when using thiserror, as it uses the From trait to convert the source error into an error variant.

Additionally, we restrict the usage of the #[snafu(context(false))] atrribute on error variants. This ensures that fallible functions need to call .context() to pass the error along.

The usage of thiserror is considered invalid.

#[derive(thiserror::Error)]
enum Error {
  #[error("failed to read config file")]
  FileRead(#[from] std::io::Error)
}

fn config_file(user: User) -> Result<(), Error> {
  std::fs::read_to_string(user.file_path)?;
}

#[derive(Snafu)]
enum Error {
  #[snafu(context(false))]
  FileRead { source: std::io::Error }
}

fn config_file(user: User) -> Result<(), Error> {
  std::fs::read_to_string(user.file_path)?;
}

#[derive(Snafu)]
enum Error {
  #[snafu(display("failed to read config file of user {user_name}"))]
  FileRead {
    source: std::io::Error,
    user_name: String,
  }
}

fn config_file(user: User) -> Result<(), Error> {
  std::fs::read_to_string(user.file_path).context(FileReadSnafu {
    user_name: user.name,
  });
}

Error messages

All our error messages must start with a lowercase letter and must not end with a dot. It is recommended to start the error messages with "failed to…" or "unable to …".

#[derive(Snafu)]
enum Error {
  #[snafu(display("Foo happened."))]
  Foo,

  #[snafu(display("Bar encountered"))]
  Bar,

  #[snafu(display("arghh baz."))]
  Baz,
}

#[derive(Snafu)]
enum Error {
  #[snafu(display("failed to foo"))]
  Foo,

  #[snafu(display("unable to bar"))]
  Bar,
}

Examples for "failed to …" error messages

failed to parse config file to indicate the parsing of the config file failed, usually because the file doesn’t conform to the configuration language.
failed to construct http client to indicate we wanted to construct a HTTP client to retrieve remote content.

Exampled for "unable to …" error messages

unable to read config file from … to indicate we could load the file (for example because the file doesn’t exist).
unable to parse value … to indicate we failed to parse a user provided value which didn’t conform to the expected syntax.

String formatting

Named versus unnamed format string identifiers

For simple string formatting (up to two substitutions), we allow unnamed (and thus also uncaptured) identifiers.

For more complex formatting (more than two substitutions), we require named identifiers to avoid ambiguity, and to decouple argument order from the text (which can lead to incorrect text when the wording is changed and {} are reordered while the arguments aren’t). This rule needs to strike a balance between explicitness and concise format!() invocations. Long format!() expressions can lead to rustfmt breakage. It might be better to split up long formatting strings into multiple smaller ones.

Mix-and-matching of named versus unnamed identifiers must be avoided. See the next section about captured versus uncaptured identifiers.

format!(
    "My {} {} string with {} substitutions is {}!",
    "super",
    "long",
    4,
    "crazy",
);

format!(
    "My {quantifier} {} string with {count} substitutions is {}!",
    quantifier = "super",
    "long",
    count = 4,
    "crazy",
);

format!(
    "My {quantifier} {adjective} string with {count} substitutions is {description}!",
    quantifier = "super",
    adjective = "long",
    count = 4,
    description = "crazy",
);

Captured versus uncaptured format string identifiers

We place no restriction on named format string identifiers. All options below are considered valid.

let greetee = "world";

format!("Hello, {greetee}!");
format!("Hello, {greetee}!", greetee = "universe");
format!("Hello {name}, hello again {name}", name = greetee);

Specifying resources measured in bytes and CPU fractions

We follow the Kubernetes convention described here.

Resources measured in bytes

// Biggest matching unit
let memory: MemoryQuantity = "100Mi".parse();
let memory: MemoryQuantity = "1Gi".parse();
let memory: MemoryQuantity = "1.5Gi".parse();
let memory: MemoryQuantity = "10Gi".parse();

// Always Mi
let memory: MemoryQuantity = "100Mi".parse();
let memory: MemoryQuantity = "1024Mi".parse();
let memory: MemoryQuantity = "1536Mi".parse();
let memory: MemoryQuantity = "10240Mi".parse();

// No unit at all
let memory: MemoryQuantity = "12345678".parse();

let memory: MemoryQuantity = "100Mi".parse();
let memory: MemoryQuantity = "1Gi".parse();
let memory: MemoryQuantity = "1536Mi".parse();
let memory: MemoryQuantity = "10Gi".parse();

Resources measured in CPU fractions

// Always m
let memory: CpuQuantity = "100m".parse();
let memory: CpuQuantity = "500m".parse();
let memory: CpuQuantity = "1000m".parse();
let memory: CpuQuantity = "2000m".parse();

// Floating points
let memory: CpuQuantity = "0.1".parse();
let memory: CpuQuantity = "0.5".parse();
let memory: CpuQuantity = "1".parse();
let memory: CpuQuantity = "2".parse();

let memory: CpuQuantity = "100m".parse();
let memory: CpuQuantity = "500m".parse();
let memory: CpuQuantity = "1".parse();
let memory: CpuQuantity = "2".parse();