---
title: Style Guide for Rust in Substrate
---

Where possible these styles are enforced by settings in `rustfmt.toml` so if you run `cargo fmt` 
then you will adhere to most of these style guidelines automatically.

# Formatting

-   Indent using tabs.
-   Lines should be longer than 100 characters long only in exceptional circumstances and certainly
    no longer than 120. For this purpose, tabs are considered 4 characters wide.
-   Indent levels should be greater than 5 only in exceptional circumstances and certainly no
    greater than 8. If they are greater than 5, then consider using `let` or auxiliary functions in
    order to strip out complex inline expressions.
-   Never have spaces on a line prior to a non-whitespace character
-   Follow-on lines are only ever a single indent from the original line.

```rust
fn calculation(some_long_variable_a: i8, some_long_variable_b: i8) -> bool {
	let x = some_long_variable_a * some_long_variable_b
		- some_long_variable_b / some_long_variable_a
		+ sqrt(some_long_variable_a) - sqrt(some_long_variable_b);
	x > 10
}
```

-   Indent level should follow open parens/brackets, but should be collapsed to the smallest number
    of levels actually used:

```rust
fn calculate(
	some_long_variable_a: f32,
	some_long_variable_b: f32,
	some_long_variable_c: f32,
) -> f32 {
	(-some_long_variable_b + sqrt(
		// two parens open, but since we open & close them both on the
		// same line, only one indent level is used
		some_long_variable_b * some_long_variable_b
		- 4 * some_long_variable_a * some_long_variable_c
	// both closed here at beginning of line, so back to the original indent
	// level
	)) / (2 * some_long_variable_a)
}
```

-   `where` is indented, and its items are indented one further.
-   Argument lists or function invocations that are too long to fit on one line are indented
    similarly to code blocks, and once one param is indented in such a way, all others should be,
    too. Run-on parameter lists are also acceptable for single-line run-ons of basic function calls.

```rust
// OK
fn foo(
	really_long_parameter_name_1: SomeLongTypeName,
	really_long_parameter_name_2: SomeLongTypeName,
	shrt_nm_1: u8,
	shrt_nm_2: u8,
) {
   ...
}

// NOT OK
fn foo(really_long_parameter_name_1: SomeLongTypeName, really_long_parameter_name_2: SomeLongTypeName,
	shrt_nm_1: u8, shrt_nm_2: u8) {
	...
}
```

```rust
{
	// Complex line (not just a function call, also a let statement). Full
	// structure.
	let (a, b) = bar(
		really_long_parameter_name_1,
		really_long_parameter_name_2,
		shrt_nm_1,
		shrt_nm_2,
	);

	// Long, simple function call.
	waz(
		really_long_parameter_name_1,
		really_long_parameter_name_2,
		shrt_nm_1,
		shrt_nm_2,
	);

	// Short function call. Inline.
	baz(a, b);
}
```

-   Always end last item of a multi-line comma-delimited set with `,` when legal:

```rust
struct Point<T> {
	x: T,
	y: T,    // <-- Multiline comma-delimited lists end with a trailing ,
}

// Single line comma-delimited items do not have a trailing `,`
enum Meal { Breakfast, Lunch, Dinner };
```

-   Avoid trailing `;`s where unneeded.

```rust
if condition {
	return 1    // <-- no ; here
}
```

-   `match` arms may be either blocks or have a trailing `,` but not both.
-   Blocks should not be used unnecessarily.

```rust
match meal {
	Meal::Breakfast => "eggs",
	Meal::Lunch => { check_diet(); recipe() },
//	Meal::Dinner => { return Err("Fasting") }   // WRONG
	Meal::Dinner => return Err("Fasting"),
}
```

# Style

-   Panickers require explicit proofs they don't trigger. Calling `unwrap` is discouraged. The
    exception to this rule is test code. Avoiding panickers by restructuring code is preferred if
    feasible.

```rust
let mut target_path =
	self.path().expect(
		"self is instance of DiskDirectory;\
		DiskDirectory always returns path;\
		qed"
	);
```

-   Unsafe code requires explicit proofs just as panickers do. When introducing unsafe code,
    consider trade-offs between efficiency on one hand and reliability, maintenance costs, and
    security on the other. Here is a list of questions that may help evaluating the trade-off while
    preparing or reviewing a PR:
    -   how much more performant or compact the resulting code will be using unsafe code,
    -   how likely is it that invariants could be violated,
    -   are issues stemming from the use of unsafe code caught by existing tests/tooling,
    -   what are the consequences if the problems slip into production.
