xesite/blog/TLDR-rust-2020-09-19.markdown

---
title: "TL;DR Rust"
date: 2020-09-19
series: rust-for-gophers
tags:
  - rust
  - go
  - golang
---

# TL;DR Rust

Recently I've been starting to use Rust more and more for larger and larger
projects. As things have come up, I realized that I am missing a good reference
for common things in Rust as compared to Go. This post contains a quick
high-level overview of patterns in Rust and how they compare to patterns
in Go. This will focus on code samples. This is no replacement for the [Rust
book](https://doc.rust-lang.org/book/), but should help you get spun up on the
various patterns used in Rust code.

Also I'm happy to introduce Mara to the blog!

[Hey, happy to be here! I'm Mara, a shark hacker from Christine's imagination. 
I'll interject with side information, challenge assertions and more! Thanks for
inviting me!](conversation://Mara/hacker)

Let's start somewhere simple: functions.

## Making Functions

Functions are defined using `fn` instead of `func`:

```go
func foo() {}
```

```rust
fn foo() {}
```

### Arguments

Arguments can be passed by separating the name from the type with a colon:

```go
func foo(bar int) {}
```

```rust
fn foo(bar: i32) {}
```

### Returns

Values can be returned by adding `-> Type` to the function declaration:

```go
func foo() int {
  return 2
}
```

```rust
fn foo() -> i32 {
  return 2;
}
```

In Rust values can also be returned on the last statement without the `return`
keyword or a terminating semicolon:

```rust
fn foo() -> i32 {
  2
}
```

[Hmm, what if I try to do something like this. Will this
work?](conversation://Mara/hmm)

```rust
fn foo() -> i32 {
    if some_cond {
        2
    }
    
    4
}
```

Let's find out! The compiler spits back an error:

```
error[E0308]: mismatched types
 --> src/lib.rs:3:9
  |
2 | /     if some_cond {
3 | |         2
  | |         ^ expected `()`, found integer
4 | |     }
  | |     -- help: consider using a semicolon here
  | |_____|
  |       expected this to be `()`
```

This happens because most basic statements in Rust can return values. The best
way to fix this would be to move the `4` return into an `else` block:

```rust
fn foo() -> i32 {
    if some_cond {
        2
    } else {
        4
    }
}
```

Otherwise, the compiler will think you are trying to use that `if` as a
statement, such as like this:

```rust
let val = if some_cond { 2 } else { 4 };
```

### Functions that can fail

The [Result](https://doc.rust-lang.org/std/result/) type represents things that
can fail with specific errors. The [eyre Result
type](https://docs.rs/eyre) represents things that can fail
with any error. For readability, this post will use the eyre Result type. 

[The angle brackets in the `Result` type are arguments to the type, this allows
the Result type to work across any type you could imagine.](conversation://Mara/hacker)

```go
import "errors"

func divide(x, y int) (int, err) {
  if y == 0 {
    return 0, errors.New("cannot divide by zero")
  }
  
  return x / y, nil
}
```

```rust
use eyre::{eyre, Result};

fn divide(x: i32, y: i32) -> Result<i32> {
  match y {
    0 => Err(eyre!("cannot divide by zero")),
    _ => Ok(x / y),
  }
}
```

[Huh? I thought Rust had the <a
href="https://doc.rust-lang.org/std/error/trait.Error.html">Error trait</a>,
shouldn't you be able to use that instead of a third party package like
eyre?](conversation://Mara/wat)

Let's try that, however we will need to make our own error type because the
[`eyre!`](https://docs.rs/eyre/0.6.0/eyre/macro.eyre.html) macro creates its own
transient error type on the fly.

First we need to make our own simple error type for a DivideByZero error:

```rust
use std::error::Error;
use std::fmt;

#[derive(Debug)]
struct DivideByZero;

impl fmt::Display for DivideByZero {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "cannot divide by zero")
    }
}

impl Error for DivideByZero {}
```

So now let's use it:

```rust
fn divide(x: i32, y: i32) -> Result<i32, DivideByZero> {
  match y {
    0 => Err(DivideByZero{}),
    _ => Ok(x / y),
  }
}
```

However there is still one thing left: the function returns a DivideByZero
error, not _any_ error like the [error interface in
Go](https://godoc.org/builtin#error). In order to represent that we need to
return something that implements the Error trait:

```rust
fn divide(x: i32, y: i32) -> Result<i32, impl Error> {
    // ...
}
```

And for the simple case, this will work. However as things get more complicated
this simple facade will not work due to reality and its complexities. This is
why I am shipping as much as I can out to other packages like eyre or
[anyhow](https://docs.rs/anyhow). Check out this code in the [Rust
Playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=946057d8eb02f388cb3f03bae226d10d)
to mess with this code interactively.

[Pro tip: eyre (via <a href="https://docs.rs/color-eyre">color-eyre</a>) also
has support for adding <a href="https://docs.rs/color-eyre/0.5.4/color_eyre/#custom-sections-for-error-reports-via-help-trait">custom
sections and context</a> to errors similar to Go's <a href="https://godoc.org/fmt#Errorf">`fmt.Errorf` `%w`
format argument</a>, which will help in real world
applications. When you do need to actually make your own errors, you may want to look into
crates like <a href="https://docs.rs/thiserror">thiserror</a> to help with
automatically generating your error implementation.](conversation://Mara/hacker)

### The `?` Operator

In Rust, the `?` operator checks for an error in a function call and if there is
one, it automatically returns the error and gives you the result of the function
if there was no error. This only works in functions that return either an Option
or a Result.

[The <a href="https://doc.rust-lang.org/std/option/index.html">Option</a> type
isn't shown in very much detail here, but it acts like a "this thing might not exist and it's your
responsibility to check" container for any value. The closest analogue in Go is
making a pointer to a value or possibly putting a value in an `interface{}`
(which can be annoying to deal with in practice).](conversation://Mara/hacker)

```go
func doThing() (int, error) {
  result, err := divide(3, 4)
  if err != nil {
    return 0, err
  }
  
  return result, nil
}
```

```rust
use eyre::Result;

fn do_thing() -> Result<i32> {
  let result = divide(3, 4)?;
  Ok(result)
}
```

If the second argument of divide is changed to `0`, then `do_thing` will return
an error.

[And how does that work with eyre?](conversation://Mara/hmm)

It works with eyre because eyre has its own error wrapper type called
[`Report`](https://docs.rs/eyre/0.6.0/eyre/struct.Report.html), which can
represent anything that implements the Error trait.

## Macros

Rust macros are function calls with `!` after their name:

```rust
println!("hello, world");
```

## Variables

Variables are created using `let`:

```go
var foo int
var foo = 3
foo := 3
```

```rust
let foo: i32;
let foo = 3;
```

### Mutability

In Rust, every variable is immutable (unchangeable) by default. If we try to
change those variables above we get a compiler error:

```rust
fn main() {
    let foo: i32;
    let foo = 3;
    foo = 4;
}
```

This makes the compiler return this error:

```
error[E0384]: cannot assign twice to immutable variable `foo`
 --> src/main.rs:4:5
  |
3 |     let foo = 3;
  |         ---
  |         |
  |         first assignment to `foo`
  |         help: make this binding mutable: `mut foo`
4 |     foo = 4;
  |     ^^^^^^^ cannot assign twice to immutable variable
```

As the compiler suggests, you can create a mutable variable by adding the `mut`
keyword after the `let` keyword. There is no analog to this in Go.

```rust
let mut foo: i32 = 0;
foo = 4;
```

[This is slightly a lie. There's more advanced cases involving interior
mutability and other fun stuff like that, however this is a more advanced topic
that isn't covered here.](conversation://Mara/hacker)

### Lifetimes

Rust does garbage collection at compile time. It also passes ownership of memory
to functions as soon as possible. Lifetimes are how Rust calculates how "long" a
given bit of data should exist in the program. Rust will then tell the compiled
code to destroy the data from memory as soon as possible.

[This is slightly inaccurate in order to make this simpler to explain and
understand. It's probably more accurate to say that Rust calculates _when_ to
collect garbage at compile time, but the difference doesn't really matter for
most cases](conversation://Mara/hacker)

For example, this code will fail to compile because `quo` was moved into the
second divide call:

```rust
let quo = divide(4, 8)?;
let other_quo = divide(quo, 5)?;

// Fails compile because ownership of quo was given to divide to create other_quo
let yet_another_quo = divide(quo, 4)?;
```

To work around this you can pass a reference to the divide function:

```rust
let other_quo = divide(&quo, 5);
let yet_another_quo = divide(&quo, 4)?;
```

Or even create a clone of it:

```rust
let other_quo = divide(quo.clone(), 5);
let yet_another_quo = divide(quo, 4)?;
```

[You can also get more fancy with <a
href="https://doc.rust-lang.org/rust-by-example/scope/lifetime/explicit.html">explicit
lifetime annotations</a>, however as of Rust's 2018 edition they aren't usually
required unless you are doing something weird. This is something that is also
covered in more detail in <a
href="https://doc.rust-lang.org/stable/book/ch04-00-understanding-ownership.html">The
Rust Book</a>.](conversation://Mara/hacker)

### Passing Mutability

Sometimes functions need mutable variables. To pass a mutable reference, add
`&mut` before the name of the variable:

```rust
let something = do_something_to_quo(&mut quo)?;
```

## Project Setup

### Imports

External dependencies are declared using the [Cargo.toml
file](https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html):

```toml
# Cargo.toml

[dependencies]
eyre = "0.6"
```

This depends on the crate [eyre](https://crates.io/crates/eyre) at version
0.6.x.

[You can do much more with version requirements with cargo, see more <a
href="https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html">here</a>.](conversation://Mara/hacker)

Dependencies can also have optional features:

```toml
# Cargo.toml

[dependencies]
reqwest = { version = "0.10", features = ["json"] }
```

This depends on the crate [reqwest](https://crates.io/reqwest) at version 0.10.x
with the `json` feature enabled (in this case it enables reqwest being able to
automagically convert things to/from json using Serde).

External dependencies can be used with the `use` statement:

```go
// go

import "github.com/foo/bar"
```

```rust
use foo; //      -> foo now has the members of crate foo behind the :: operator
use foo::Bar; // -> Bar is now exposed as a type in this file

use eyre::{eyre, Result}; // exposes the eyre! and Result members of eyre
```

[This doesn't cover how the <a
href="http://www.sheshbabu.com/posts/rust-module-system/">module system</a>
works, however the post I linked there covers this better than I
can.](conversation://Mara/hacker)

## Async/Await

Async functions may be interrupted to let other things execute as needed. This
program uses [tokio](https://tokio.rs/) to handle async tasks. To run an async
task and wait for its result, do this:

```
let printer_fact = reqwest::get("https://printerfacts.cetacean.club/fact")
  .await?
  .text()
  .await?;
println!("your printer fact is: {}", printer_fact);
```

This will populate `response` with an amusing fact about everyone's favorite
household pet, the [printer](https://printerfacts.cetacean.club).

To make an async function, add the `async` keyword before the `fn` keyword:

```rust
async fn get_text(url: String) -> Result<String> {
  reqwest::get(&url)
    .await?
    .text()
    .await?
}
```

This can then be called like this:

```rust
let printer_fact = get_text("https://printerfacts.cetacean.club/fact").await?;
```

## Public/Private Types and Functions

Rust has three privacy levels for functions:

- Only visible to the current file (no keyword, lowercase in Go)
- Visible to anything in the current crate (`pub(crate)`, internal packages in
  go)
- Visible to everyone (`pub`, upper case in Go)

[You can't get a perfect analog to `pub(crate)` in Go, but <a
href="https://docs.google.com/document/d/1e8kOo3r51b2BWtTs_1uADIA5djfXhPT36s6eHVRIvaU/edit">internal
packages</a> can get close to this behavior. Additionally you can have a lot
more control over access levels than this, see <a
href="https://doc.rust-lang.org/nightly/reference/visibility-and-privacy.html">here</a>
for more information.](conversation://Mara/hacker)

## Structures

Rust structures are created using the `struct` keyword:

```go
type Client struct {
  Token string
}
```

```rust
pub struct Client {
  pub token: String,
}
```

If the `pub` keyword is not specified before a member name, it will not be
usable outside the Rust source code file it is defined in:

```go
type Client struct {
  token string
}
```

```rust
pub(crate) struct Client {
  token: String,
}
```

### Encoding structs to JSON

[serde](https://serde.rs) is used to convert structures to json. The Rust
compiler's
[derive](https://doc.rust-lang.org/stable/rust-by-example/trait/derive.html)
feature is used to automatically implement the conversion logic.

```go
type Response struct {
  Name        string  `json:"name"`
  Description *string `json:"description,omitempty"`
}
```

```rust
use serde::{Serialize, Deserialize};

#[derive(Serialize, Deserialize, Debug)]
pub(crate) struct Response {
  pub name: String,
  pub description: Option<String>,
}
```

## Strings

Rust has a few string types that do different things. You can read more about
this [here](https://fasterthanli.me/blog/2020/working-with-strings-in-rust/),
but at a high level most projects only uses a few of them:

- `&str`, a slice reference to a String owned by someone else
- String, an owned UTF-8 string
- PathBuf, a filepath string (encoded in whatever encoding the OS running this
  code uses for filesystems)
  
The strings are different types for safety reasons. See the linked blogpost for
more detail about this.

## Enumerations / Tagged Unions

Enumerations, also known as tagged unions, are a way to specify a superposition
of one of a few different kinds of values in one type. A neat way to show them
off (along with some other fancy features like the derivation system) is with the
[structopt](https://docs.rs/structopt/0.3.14/structopt/) crate. There is no easy
analog for this in Go.

[We've actually been dealing with enumerations ever since we touched the Result
type earlier. <a
href="https://doc.rust-lang.org/std/result/enum.Result.html">Result</a> and <a
href="https://doc.rust-lang.org/std/option/enum.Option.html">Option</a> are
implemented with enumerations.](conversation://Mara/hacker)

```rust
#[derive(StructOpt, Debug)]
#[structopt(about = "A simple release management tool")]
pub(crate) enum Cmd {
    /// Creates a new release for a git repo
    Cut {
        #[structopt(flatten)]
        common: Common,
        /// Changelog location
        #[structopt(long, short, default_value="./CHANGELOG.md")]
        changelog: PathBuf,
    },

    /// Runs releases as triggered by GitHub Actions
    GitHubAction {
        #[structopt(flatten)]
        gha: GitHubAction,
    },
}
```

Enum variants can be matched using the `match` keyword:

```rust
match cmd {
    Cmd::Cut { common, changelog } => {
        cmd::cut::run(common, changelog).await
    }
    Cmd::GitHubAction { gha } => {
        cmd::github_action::run(gha).await
    }
}
```

All variants of an enum must be matched in order for the code to compile.

[This code was borrowed from <a
href="https://github.com/lightspeed/palisade">palisade</a> in order to
demonstrate this better. If you want to see these patterns in action, check this
repository out!](conversation://Mara/hacker)

## Testing

Test functions need to be marked with the `#[test]` annotation, then they will
be run alongside `cargo test`:

```rust
mod tests { // not required but it is good practice
  #[test]
  fn math_works() {
    assert_eq!(2 + 2, 4);
  }
  
  #[tokio::test] // needs tokio as a dependency
  async fn http_works() {
    let _ = get_html("https://within.website").await.unwrap();
  }
}
```

Avoid the use of `unwrap()` outside of tests. In the wrong cases, using
`unwrap()` in production code can cause the server to crash and can incur data
loss.

[Alternatively, you can also use the <a href="https://learning-rust.github.io/docs/e4.unwrap_and_expect.html#expect">`.expect()`</a> method instead
of `.unwrap()`. This lets you attach a message that will be shown when the
result isn't Ok.](conversation://Mara/hacker)

---

This is by no means comprehensive, see the rust book or [Learn X in Y Minutes
Where X = Rust](https://learnxinyminutes.com/docs/rust/) for more information.
This code is written to be as boring and obvious as possible. If things don't
make sense, please reach out and don't be afraid to ask questions.