Statements

Let statements

There should be spaces after the : and on both sides of the = (if they are present). No space before the semicolon.

#![allow(unused)]
fn main() {
// A comment.
let pattern: Type = expr;

let pattern;
let pattern: Type;
let pattern = expr;
}

If possible the declaration should be formatted on a single line. If this is not possible, then try splitting after the =, if the declaration can fit on two lines. The expression should be block indented.

#![allow(unused)]
fn main() {
let pattern: Type =
    expr;
}

If the first line does not fit on a single line, then split after the colon, using block indentation. If the type covers multiple lines, even after line- breaking after the :, then the first line may be placed on the same line as the :, subject to the combining rules (WIP).

#![allow(unused)]
fn main() {
let pattern:
    Type =
    expr;
}

e.g,

#![allow(unused)]
fn main() {
let Foo {
    f: abcd,
    g: qwer,
}: Foo<Bar> =
    Foo { f, g };

let (abcd,
    defg):
    Baz =
{ ... }
}

If the expression covers multiple lines, if the first line of the expression fits in the remaining space, it stays on the same line as the =, the rest of the expression is not indented. If the first line does not fit, then it should start on the next lines, and should be block indented. If the expression is a block and the type or pattern cover multiple lines, then the opening brace should be on a new line and not indented (this provides separation for the interior of the block from the type), otherwise the opening brace follows the =.

Examples:

#![allow(unused)]
fn main() {
let foo = Foo {
    f: abcd,
    g: qwer,
};

let foo =
    ALongName {
        f: abcd,
        g: qwer,
    };

let foo: Type = {
    an_expression();
    ...
};

let foo:
    ALongType =
{
    an_expression();
    ...
};

let Foo {
    f: abcd,
    g: qwer,
}: Foo<Bar> = Foo {
    f: blimblimblim,
    g: blamblamblam,
};

let Foo {
    f: abcd,
    g: qwer,
}: Foo<Bar> = foo(
    blimblimblim,
    blamblamblam,
);
}

else blocks (let-else statements)

A let statement can contain an else component, making it a let-else statement. In this case, always apply the same formatting rules to the components preceding the else block (i.e. the let pattern: Type = initializer_expr portion) as described for other let statements.

The entire let-else statement may be formatted on a single line if all the following are true:

the entire statement is short
the else block contains only a single-line expression and no statements
the else block contains no comments
the let statement components preceding the else block can be formatted on a single line

#![allow(unused)]
fn main() {
let Some(1) = opt else { return };
}

Formatters may allow users to configure the value of the threshold used to determine whether a let-else statement is short.

Otherwise, the let-else statement requires some line breaks.

If breaking a let-else statement across multiple lines, never break between the else and the {, and always break before the }.

If the let statement components preceding the else can be formatted on a single line, but the let-else does not qualify to be placed entirely on a single line, put the else { on the same line as the initializer expression, with a space between them, then break the line after the {. Indent the closing } to match the let, and indent the contained block one step further.

#![allow(unused)]
fn main() {
let Some(1) = opt else {
    return;
};

let Some(1) = opt else {
    // nope
    return
};
}

If the let statement components preceding the else can be formatted on a single line, but the else { does not fit on the same line, break the line before the else.

#![allow(unused)]
fn main() {
    let Some(x) = some_really_really_really_really_really_really_really_really_really_long_name
    else {
        return;
    };
}

If the initializer expression is multi-line, the else keyword and opening brace of the block (i.e. else {) should be put on the same line as the end of the initializer expression, with a space between them, if and only if all the following are true:

The initializer expression ends with one or more closing parentheses, square brackets, and/or braces
There is nothing else on that line
That line has the same indentation level as the initial let keyword.

For example:

#![allow(unused)]
fn main() {
let Some(x) = y.foo(
    "abc",
    fairly_long_identifier,
    "def",
    "123456",
    "string",
    "cheese",
) else {
    bar()
}
}

Otherwise, the else keyword and opening brace should be placed on the next line after the end of the initializer expression, and the else keyword should have the same indentation level as the let keyword.

For example:

fn main() {
    let Some(x) = abcdef()
        .foo(
            "abc",
            some_really_really_really_long_ident,
            "ident",
            "123456",
        )
        .bar()
        .baz()
        .qux("fffffffffffffffff")
    else {
        return
    };

    let Some(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa) =
        bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
    else {
        return;
    };

    let LongStructName(AnotherStruct {
        multi,
        line,
        pattern,
    }) = slice.as_ref()
    else {
        return;
    };

    let LongStructName(AnotherStruct {
        multi,
        line,
        pattern,
    }) = multi_line_function_call(
        arg1,
        arg2,
        arg3,
        arg4,
    ) else {
        return;
    };
}

Macros in statement position

A macro use in statement position should use parentheses or square brackets as delimiters and should be terminated with a semicolon. There should be no spaces between the name, !, the delimiters, or the ;.

#![allow(unused)]
fn main() {
// A comment.
a_macro!(...);
}

Expressions in statement position

There should be no space between the expression and the semicolon.

<expr>;

All expressions in statement position should be terminated with a semicolon, unless they end with a block or are used as the value for a block.

E.g.,

#![allow(unused)]
fn main() {
{
    an_expression();
    expr_as_value()
}

return foo();

loop {
    break;
}
}

Use a semicolon where an expression has void type, even if it could be propagated. E.g.,

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

fn bar() {
    foo();
}
}