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Style Guide for Rust in Polkadot

  • Indent using tabs
  • Lines should be longer than 80 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.
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:
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 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.
// 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) {
   ...
}
{
    // 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:
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.
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.
match meal {
    Meal::Breakfast => "eggs",
    Meal::Lunch => { check_diet(); recipe() },
//  Meal::Dinner => { return Err("Fasting") }   // WRONG
    Meal::Dinner => return Err("Fasting"),
}