Expressions

Blocks

A block expression must have a newline after the initial { and before the terminal }, unless it qualifies to be written as a single line based on another style rule.

A keyword before the block (such as unsafe or async) must be on the same line as the opening brace, with a single space between the keyword and the opening brace. Indent the contents of the block.

#![allow(unused)]
fn main() {
fn block_as_stmt() {
    a_call();

    {
        a_call_inside_a_block();

        // a comment in a block
        the_value
    }
}

fn block_as_expr() {
    let foo = {
        a_call_inside_a_block();

        // a comment in a block
        the_value
    };
}

fn unsafe_block_as_stmt() {
    a_call();

    unsafe {
        a_call_inside_a_block();

        // a comment in a block
        the_value
    }
}
}

If a block has an attribute, put it on its own line before the block:

#![allow(unused)]
fn main() {
fn block_as_stmt() {
    #[an_attribute]
    {
        #![an_inner_attribute]

        // a comment in a block
        the_value
    }
}
}

Avoid writing comments on the same lines as either of the braces.

Write an empty block as {}.

Write a block on a single line if:

it is either used in expression position (not statement position) or is an unsafe block in statement position,
it contains a single-line expression and no statements, and
it contains no comments

For a single-line block, put spaces after the opening brace and before the closing brace.

Examples:

fn main() {
    // Single line
    let _ = { a_call() };
    let _ = unsafe { a_call() };

    // Not allowed on one line
    // Statement position.
    {
        a_call()
    }

    // Contains a statement
    let _ = {
        a_call();
    };
    unsafe {
        a_call();
    }

    // Contains a comment
    let _ = {
        // A comment
    };
    let _ = {
        // A comment
        a_call()
    };

    // Multiple lines
    let _ = {
        a_call();
        another_call()
    };
    let _ = {
        a_call(
            an_argument,
            another_arg,
        )
    };
}

Closures

Don't put any extra spaces before the first | (unless the closure is prefixed by a keyword such as move); put a space between the second | and the expression of the closure. Between the |s, use function definition syntax, but elide types where possible.

Use closures without the enclosing {}, if possible. Add the {} when you have a return type, when there are statements, when there are comments inside the closure, or when the body expression is a control-flow expression that spans multiple lines. If using braces, follow the rules above for blocks. Examples:

#![allow(unused)]
fn main() {
|arg1, arg2| expr

move |arg1: i32, arg2: i32| -> i32 {
    expr1;
    expr2
}

|| Foo {
    field1,
    field2: 0,
}

|| {
    if true {
        blah
    } else {
        boo
    }
}

|x| unsafe {
    expr
}
}

Struct literals

If a struct literal is small, format it on a single line, and do not use a trailing comma. If not, split it across multiple lines, with each field on its own block-indented line, and use a trailing comma.

For each field: value entry, put a space after the colon only.

Put a space before the opening brace. In the single-line form, put spaces after the opening brace and before the closing brace.

#![allow(unused)]
fn main() {
Foo { field1, field2: 0 }
let f = Foo {
    field1,
    field2: an_expr,
};
}

Functional record update syntax is treated like a field, but it must never have a trailing comma. Do not put a space after ...

#![allow(unused)]
fn main() {
let f = Foo {
    field1,
    ..an_expr
};
}

Tuple literals

Use a single-line form where possible. Do not put spaces between the opening parenthesis and the first element, or between the last element and the closing parenthesis. Separate elements with a comma followed by a space.

Where a single-line form is not possible, write the tuple across multiple lines, with each element of the tuple on its own block-indented line, and use a trailing comma.

#![allow(unused)]
fn main() {
(a, b, c)

let x = (
    a_long_expr,
    another_very_long_expr,
);
}

Tuple struct literals

Do not put space between the identifier and the opening parenthesis. Otherwise, follow the rules for tuple literals:

#![allow(unused)]
fn main() {
Foo(a, b, c)

let x = Foo(
    a_long_expr,
    another_very_long_expr,
);
}

Enum literals

Follow the formatting rules for the various struct literals. Prefer using the name of the enum as a qualifying name, unless the enum is in the prelude:

#![allow(unused)]
fn main() {
Foo::Bar(a, b)
Foo::Baz {
    field1,
    field2: 1001,
}
Ok(an_expr)
}

Array literals

Write small array literals on a single line. Do not put spaces between the opening square bracket and the first element, or between the last element and the closing square bracket. Separate elements with a comma followed by a space.

If using the repeating initializer, put a space after the semicolon only.

Apply the same rules if using vec! or similar array-like macros; always use square brackets with such macros. Examples:

fn main() {
    let x = [1, 2, 3];
    let y = vec![a, b, c, d];
    let a = [42; 10];
}

For arrays that have to be broken across lines, if using the repeating initializer, break after the ;, not before. Otherwise, follow the rules below for function calls. In any case, block-indent the contents of the initializer, and put line breaks after the opening square bracket and before the closing square bracket:

fn main() {
    [
        a_long_expression();
        1234567890
    ]
    let x = [
        an_expression,
        another_expression,
        a_third_expression,
    ];
}

Array accesses, indexing, and slicing

Don't put spaces around the square brackets. Avoid breaking lines if possible. Never break a line between the target expression and the opening square bracket. If the indexing expression must be broken onto a subsequent line, or spans multiple lines itself, then block-indent the indexing expression, and put newlines after the opening square bracket and before the closing square bracket:

Examples:

fn main() {
    foo[42];
    &foo[..10];
    bar[0..100];
    foo[4 + 5 / bar];
    a_long_target[
        a_long_indexing_expression
    ];
}

Unary operations

Do not include a space between a unary op and its operand (i.e., !x, not ! x). However, there must be a space after &mut. Avoid line-breaking between a unary operator and its operand.

Binary operations

Do include spaces around binary ops (i.e., x + 1, not x+1) (including = and other assignment operators such as += or *=).

For comparison operators, because for T op U, &T op &U is also implemented: if you have t: &T, and u: U, prefer *t op u to t op &u. In general, within expressions, prefer dereferencing to taking references, unless necessary (e.g. to avoid an unnecessarily expensive operation).

Use parentheses liberally; do not necessarily elide them due to precedence. Tools should not automatically insert or remove parentheses. Do not use spaces to indicate precedence.

If line-breaking, block-indent each subsequent line. For assignment operators, break after the operator; for all other operators, put the operator on the subsequent line. Put each sub-expression on its own line:

#![allow(unused)]
fn main() {
foo_bar
    + bar
    + baz
    + qux
    + whatever
}

Prefer line-breaking at an assignment operator (either = or +=, etc.) rather than at other binary operators.

Casts (`as`)

Format as casts like a binary operator. In particular, always include spaces around as, and if line-breaking, break before the as (never after) and block-indent the subsequent line. Format the type on the right-hand side using the rules for types.

However, unlike with other binary operators, if chaining a series of as casts that require line-breaking, and line-breaking before the first as suffices to make the remainder fit on the next line, don't break before any subsequent as; instead, leave the series of types all on the same line:

#![allow(unused)]
fn main() {
let cstr = very_long_expression()
    as *const str as *const [u8] as *const std::os::raw::c_char;
}

If the subsequent line still requires line-breaking, break and block-indent before each as as with other binary operators.

Control flow

Do not include extraneous parentheses for if and while expressions.

#![allow(unused)]
fn main() {
if true {
}
}

is better than

#![allow(unused)]
fn main() {
if (true) {
}
}

Do include extraneous parentheses if it makes an arithmetic or logic expression easier to understand ((x * 15) + (y * 20) is fine)

Function calls

Do not put a space between the function name, and the opening parenthesis.

Do not put a space between an argument, and the comma which follows.

Do put a space between an argument, and the comma which precedes it.

Prefer not to break a line in the callee expression.

Single-line calls

Do not put a space between the function name and open paren, between the open paren and the first argument, or between the last argument and the close paren.

Do not put a comma after the last argument.

#![allow(unused)]
fn main() {
foo(x, y, z)
}

Multi-line calls

If the function call is not small, it would otherwise over-run the max width, or any argument or the callee is multi-line, then format the call across multiple lines. In this case, put each argument on its own block-indented line, break after the opening parenthesis and before the closing parenthesis, and use a trailing comma:

#![allow(unused)]
fn main() {
a_function_call(
    arg1,
    a_nested_call(a, b),
)
}

Method calls

Follow the function rules for calling.

Do not put any spaces around the ..

#![allow(unused)]
fn main() {
x.foo().bar().baz(x, y, z);
}

Macro uses

If a macro can be parsed like other constructs, format it like those constructs. For example, a macro use foo!(a, b, c) can be parsed like a function call (ignoring the !), so format it using the rules for function calls.

The style guide defines specific formatting for particular macros in the language or standard library. The style guide does not define formatting for any third-party macros, even if similar to those in the language or standard library.

Format string macros

For macros which take a format string, if all other arguments are small, format the arguments before the format string on a single line if they fit, and format the arguments after the format string on a single line if they fit, with the format string on its own line. If the arguments are not small or do not fit, put each on its own line as with a function. For example:

#![allow(unused)]
fn main() {
println!(
    "Hello {} and {}",
    name1, name2,
);

assert_eq!(
    x, y,
    "x and y were not equal, see {}",
    reason,
);
}

Chains of fields and method calls

A chain is a sequence of field accesses, method calls, and/or uses of the try operator ?. E.g., a.b.c().d or foo?.bar().baz?.

Format the chain on one line if it is "small" and otherwise possible to do so. If formatting on multiple lines, put each field access or method call in the chain on its own line, with the line-break before the . and after any ?. Block-indent each subsequent line:

#![allow(unused)]
fn main() {
let foo = bar
    .baz?
    .qux();
}

If the length of the last line of the first element plus its indentation is less than or equal to the indentation of the second line, then combine the first and second lines if they fit. Apply this rule recursively.

#![allow(unused)]
fn main() {
x.baz?
    .qux()

x.y.z
    .qux()

let foo = x
    .baz?
    .qux();

foo(
    expr1,
    expr2,
).baz?
    .qux();
}

Multi-line elements

If any element in a chain is formatted across multiple lines, put that element and any later elements on their own lines.

#![allow(unused)]
fn main() {
a.b.c()?
    .foo(
        an_expr,
        another_expr,
    )
    .bar
    .baz
}

Note there is block indent due to the chain and the function call in the above example.

Prefer formatting the whole chain in multi-line style and each element on one line, rather than putting some elements on multiple lines and some on a single line, e.g.,

#![allow(unused)]
fn main() {
// Better
self.pre_comment
    .as_ref()
    .map_or(false, |comment| comment.starts_with("//"))

// Worse
self.pre_comment.as_ref().map_or(
    false,
    |comment| comment.starts_with("//"),
)
}

Control flow expressions

This section covers if, if let, loop, while, while let, and for expressions.

Put the keyword, any initial clauses, and the opening brace of the block all on a single line, if they fit. Apply the usual rules for block formatting to the block.

If there is an else component, then put the closing brace, else, any following clause, and the opening brace all on the same line, with a single space before and after the else keyword:

#![allow(unused)]
fn main() {
if ... {
    ...
} else {
    ...
}

if let ... {
    ...
} else if ... {
    ...
} else {
    ...
}
}

If the control line needs to be broken, prefer to break before the = in * let expressions and before in in a for expression; block-indent the following line. If the control line is broken for any reason, put the opening brace on its own line, not indented. Examples:

#![allow(unused)]
fn main() {
while let Some(foo)
    = a_long_expression
{
    ...
}

for foo
    in a_long_expression
{
    ...
}

if a_long_expression
    && another_long_expression
    || a_third_long_expression
{
    ...
}
}

Where the initial clause spans multiple lines and ends with one or more closing parentheses, square brackets, or braces, and there is nothing else on that line, and that line is not indented beyond the indent on the first line of the control flow expression, then put the opening brace of the block on the same line with a preceding space. For example:

#![allow(unused)]
fn main() {
if !self.config.file_lines().intersects(
    &self.codemap.lookup_line_range(
        stmt.span,
    ),
) {  // Opening brace on same line as initial clause.
    ...
}
}

Single line `if else`

Put an if else or if let else on a single line if it occurs in expression context (i.e., is not a standalone statement), it contains a single else clause, and is small:

#![allow(unused)]
fn main() {
let y = if x { 0 } else { 1 };

// Examples that must be multi-line.
let y = if something_very_long {
    not_small
} else {
    also_not_small
};

if x {
    0
} else {
    1
}
}

Match

Prefer not to line-break inside the discriminant expression. Always break after the opening brace and before the closing brace. Block-indent the match arms once:

#![allow(unused)]
fn main() {
match foo {
    // arms
}

let x = match foo.bar.baz() {
    // arms
};
}

Use a trailing comma for a match arm if and only if not using a block.

Never start a match arm pattern with |:

#![allow(unused)]
fn main() {
match foo {
    // Don't do this.
    | foo => bar,
    // Or this.
    | a_very_long_pattern
    | another_pattern
    | yet_another_pattern
    | a_fourth_pattern => {
        ...
    }
}
}

Prefer:

#![allow(unused)]
fn main() {
match foo {
    foo => bar,
    a_very_long_pattern
    | another_pattern
    | yet_another_pattern
    | a_fourth_pattern => {
        ...
    }
}
}

Avoid splitting the left-hand side (before the =>) of a match arm where possible. If the right-hand side of the match arm is kept on the same line, never use a block (unless the block is empty).

If the right-hand side consists of multiple statements, or has line comments, or the start of the line does not fit on the same line as the left-hand side, use a block. Do not flatten a right-hand side block containing a single macro call because its expanded form could contain a trailing semicolon.

Block-indent the body of a block arm.

Examples:

#![allow(unused)]
fn main() {
match foo {
    foo => bar,
    a_very_long_pattern | another_pattern if an_expression() => {
        no_room_for_this_expression()
    }
    foo => {
        // A comment.
        an_expression()
    }
    foo => {
        let a = statement();
        an_expression()
    }
    bar => {}
    // Trailing comma on last item.
    foo => bar,
    baz => qux!(),
    lorem => {
        ipsum!()
    }
}
}

If the body is a single expression with no line comments and not a control flow expression, start it on the same line as the left-hand side. If not, then it must be in a block. Example:

#![allow(unused)]
fn main() {
match foo {
    // A combinable expression.
    foo => a_function_call(another_call(
        argument1,
        argument2,
    )),
    // A non-combinable expression
    bar => {
        a_function_call(
            another_call(
                argument1,
                argument2,
            ),
            another_argument,
        )
    }
}
}

Line-breaking

If using a block form on the right-hand side of a match arm makes it possible to avoid breaking on the left-hand side, do that:

#![allow(unused)]
fn main() {
    // Assuming the following line does not fit in the max width
    a_very_long_pattern | another_pattern => ALongStructName {
        ...
    },
    // Prefer this
    a_very_long_pattern | another_pattern => {
        ALongStructName {
            ...
        }
    }
    // To splitting the pattern.
}

Never break after => without using the block form of the body.

If the left-hand side must be split and there is an if clause, break before the if and block indent. In this case, always use a block body and start the body on a new line:

#![allow(unused)]
fn main() {
    a_very_long_pattern | another_pattern
        if expr =>
    {
        ...
    }
}

If required to break the pattern, put each clause of the pattern on its own line with no additional indent, breaking before the |. If there is an if clause, use the above form:

#![allow(unused)]
fn main() {
    a_very_long_pattern
    | another_pattern
    | yet_another_pattern
    | a_forth_pattern => {
        ...
    }
    a_very_long_pattern
    | another_pattern
    | yet_another_pattern
    | a_forth_pattern
        if expr =>
    {
        ...
    }
}

If the pattern is multi-line, and the last line is less wide than the indent, do not put the if clause on a new line. E.g.,

#![allow(unused)]
fn main() {
    Token::Dimension {
         value,
         ref unit,
         ..
    } if num_context.is_ok(context.parsing_mode, value) => {
        ...
    }
}

If every clause in a pattern is small, but the whole pattern does not fit on one line, then format the pattern across multiple lines with as many clauses per line as possible. Again, break before a |:

#![allow(unused)]
fn main() {
    foo | bar | baz
    | qux => {
        ...
    }
}

We define a pattern clause to be small if it fits on a single line and matches "small" in the following grammar:

small:
    - small_no_tuple
    - unary tuple constructor: `(` small_no_tuple `,` `)`
    - `&` small

small_no_tuple:
    - single token
    - `&` small_no_tuple

E.g., &&Some(foo) matches, Foo(4, Bar) does not.

Combinable expressions

Where a function call has a single argument, and that argument is formatted across multiple-lines, format the outer call as if it were a single-line call, if the result fits. Apply the same combining behaviour to any similar expressions which have multi-line, block-indented lists of sub-expressions delimited by parentheses (e.g., macros or tuple struct literals). E.g.,

#![allow(unused)]
fn main() {
foo(bar(
    an_expr,
    another_expr,
))

let x = foo(Bar {
    field: whatever,
});

foo(|param| {
    action();
    foo(param)
})

let x = combinable([
    an_expr,
    another_expr,
]);

let arr = [combinable(
    an_expr,
    another_expr,
)];
}

Apply this behavior recursively.

For a function with multiple arguments, if the last argument is a multi-line closure with an explicit block, there are no other closure arguments, and all the arguments and the first line of the closure fit on the first line, use the same combining behavior:

#![allow(unused)]
fn main() {
foo(first_arg, x, |param| {
    action();
    foo(param)
})
}

Ranges

Do not put spaces in ranges, e.g., 0..10, x..=y, ..x.len(), foo...

When writing a range with both upper and lower bounds, if the line must be broken within the range, break before the range operator and block indent the second line:

#![allow(unused)]
fn main() {
a_long_expression
    ..another_long_expression
}

For the sake of indicating precedence, if either bound is a compound expression, use parentheses around it, e.g., ..(x + 1), (x.f)..(x.f.len()), or 0..(x - 10).

Hexadecimal literals

Hexadecimal literals may use upper- or lower-case letters, but they must not be mixed within the same literal. Projects should use the same case for all literals, but we do not make a recommendation for either lower- or upper-case.

Patterns

Format patterns like their corresponding expressions. See the section on match for additional formatting for patterns in match arms.

The Rust Style Guide