teloxide/CODE_STYLE.md

# Code style

This is a description of a coding style that every contributor must follow.
Please, read the whole document before you start pushing code.

## Generics

All trait bounds should be written in `where`

```rust
// GOOD
pub fn new<N, T, P, E>(user_id: i32, name: N, title: T, png_sticker: P, emojis: E) -> Self
where
    N: Into<String>,
    T: Into<String>,
    P: Into<InputFile>,
    E: Into<String>,
{ ... }

// BAD
pub fn new<N: Into<String>,
           T: Into<String>,
           P: Into<InputFile>,
           E: Into<String>>
    (user_id: i32, name: N, title: T, png_sticker: P, emojis: E) -> Self { ... }
```
```rust
// GOOD
impl<T> Trait for Wrap<T>
where
    T: Trait
{ ... }

// BAD
impl<T: Trait> Trait for Wrap<T> { ... }
```

**Rationale:** `where` clauses are easier to read when there are a lot of bounds, uniformity.

## Documentation comments

1. Documentation must describe what your code does and mustn't describe how your code does it and bla-bla-bla.
2. Be sure that your comments follow the grammar, including punctuation, the first capital letter and so on.
   ```rust
   // GOOD
   /// This function makes a request to Telegram.
   pub fn make_request(url: &str) -> String { ... }
   
   // BAD
   /// this function make request to telegram
   pub fn make_request(url: &str) -> String { ... }
   ```
3. Do not use ending punctuation in short list items (usually containing just one phrase or sentence).
   ```md
   <!-- GOOD -->
   - Handle different kinds of Update
   - Pass dependencies to handlers
   - Disable a default Ctrl-C handling

   <!-- BAD -->
   - Handle different kinds of Update.
   - Pass dependencies to handlers.
   - Disable a default Ctrl-C handling.

   <!-- BAD -->
   - Handle different kinds of Update;
   - Pass dependencies to handlers;
   - Disable a default Ctrl-C handling;
   ```
3. Link resources in your comments when possible, for example:
   ```rust
   /// Download a file from Telegram.
   ///
   /// `path` can be obtained from the [`Bot::get_file`].
   ///
   /// To download into [`AsyncWrite`] (e.g. [`tokio::fs::File`]), see
   /// [`Bot::download_file`].
   ///
   /// [`Bot::get_file`]: crate::bot::Bot::get_file
   /// [`AsyncWrite`]: tokio::io::AsyncWrite
   /// [`tokio::fs::File`]: tokio::fs::File
   /// [`Bot::download_file`]: crate::Bot::download_file
   ```

## Use `Self` where possible

When referring to the type for which block is implemented, prefer using `Self`, rather than the name of the type.

```rust
impl ErrorKind {
    // GOOD
    fn print(&self) {
        Self::Io => println!("Io"),
        Self::Network => println!("Network"),
        Self::Json => println!("Json"),
    }

    // BAD
    fn print(&self) {
        ErrorKind::Io => println!("Io"),
        ErrorKind::Network => println!("Network"),
        ErrorKind::Json => println!("Json"),
    }
}
```
```rust
impl<'a> AnswerCallbackQuery<'a> {
    // GOOD
    fn new<C>(bot: &'a Bot, callback_query_id: C) -> Self
    where
        C: Into<String>,
    { ... }

    // BAD
    fn new<C>(bot: &'a Bot, callback_query_id: C) -> AnswerCallbackQuery<'a>
    where
        C: Into<String>,
    { ... }
}
```

**Rationale:** `Self` is generally shorter and it's easier to copy-paste code or rename the type.

## Avoid duplication in fields names

```rust
struct Message {
    // GOOD
    #[serde(rename = "message_id")]
    id: MessageId,

    // BAD
    message_id: MessageId,
}
```

**Rationale:** duplication is unnecessary 

## Conventional generic names

Use a generic parameter name `S` for streams, `Fut` for futures, `F` for functions (where possible).

**Rationale:** uniformity.

## Deriving traits

Derive `Copy`, `Clone`, `Eq`, `PartialEq`, `Hash` and `Debug` for public types when possible.

**Rationale:** these traits can be useful for users and can be implemented for most types.

Derive `Default` when there is a reasonable default value for the type.

**Rationale:** `Default` plays nicely with generic code (for example `mem::take`).

## `Into`-polymorphism

Use `T: Into<Ty>` when this can simplify user code.
I.e. when there are types that implement `Into<Ty>` that are likely to be passed to this function.

**Rationale:** conversions unnecessarily complicate caller code and can be confusing for beginners.

## `must_use`

Always mark a functions as `#[must_use]` if they don't have side-effects and the only reason to call them is to get the result.

```rust
impl User {
    // GOOD
    #[must_use]
    fn full_name(&self) -> String {
        format!("{} {}", user.first_name, user.last_name)
    }
}
```

**Rationale:** users will get warnings if they forgot to do something with the result, potentially preventing bugs.

## Creating boxed futures

Prefer `Box::pin(async { ... })` instead of `async { ... }.boxed()`.

**Rationale:** the former is generally formatted better by rustfmt.

## Full paths for logging

Always write `log::<op>!(...)` instead of importing `use log::<op>;` and invoking `<op>!(...)`.

```rust
// GOOD
log::warn!("Everything is on fire");

// BAD
use log::warn;

warn!("Everything is on fire");
```

**Rationale:** uniformity, it's clearer which log crate is used.