path: root/src/doc/style-guide/src/statements.md

## Statements

### Let statements

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

```rust
// 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.

```rust
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](https://github.com/rust-lang-nursery/fmt-rfcs/issues/61) (WIP).


```rust
let pattern:
    Type =
    expr;
```

e.g,

```rust
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:

```rust
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](#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

```rust
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.

```rust
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`.

```rust
    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:

```rust
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:

```rust
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 `;`.

```rust
// 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.,

```rust
{
    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.,

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

fn bar() {
    foo();
}
```