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# Statements
> **<sup>Syntax</sup>**\
> _Statement_ :\
> `;`\
> | [_Item_]\
> | [_LetStatement_]\
> | [_ExpressionStatement_]\
> | [_MacroInvocationSemi_]
A *statement* is a component of a [block], which is in turn a component of an outer [expression] or [function].
Rust has two kinds of statement: [declaration statements](#declaration-statements) and [expression statements](#expression-statements).
## Declaration statements
A *declaration statement* is one that introduces one or more *names* into the enclosing statement block.
The declared names may denote new variables or new [items][item].
The two kinds of declaration statements are item declarations and `let` statements.
### Item declarations
An *item declaration statement* has a syntactic form identical to an [item declaration][item] within a [module].
Declaring an item within a statement block restricts its scope to the block containing the statement.
The item is not given a [canonical path] nor are any sub-items it may declare.
The exception to this is that associated items defined by [implementations] are still accessible in outer scopes as long as the item and, if applicable, trait are accessible.
It is otherwise identical in meaning to declaring the item inside a module.
There is no implicit capture of the containing function's generic parameters, parameters, and local variables.
For example, `inner` may not access `outer_var`.
```rust
fn outer() {
let outer_var = true;
fn inner() { /* outer_var is not in scope here */ }
inner();
}
```
### `let` statements
> **<sup>Syntax</sup>**\
> _LetStatement_ :\
> [_OuterAttribute_]<sup>\*</sup> `let` [_PatternNoTopAlt_]
> ( `:` [_Type_] )<sup>?</sup> (`=` [_Expression_] [†](#let-else-restriction)
> ( `else` [_BlockExpression_]) <sup>?</sup> ) <sup>?</sup> `;`
>
> <span id="let-else-restriction">† When an `else` block is specified, the
> _Expression_ must not be a [_LazyBooleanExpression_], or end with a `}`.</span>
A *`let` statement* introduces a new set of [variables], given by a [pattern].
The pattern is followed optionally by a type annotation and then either ends, or is followed by an initializer expression plus an optional `else` block.
When no type annotation is given, the compiler will infer the type, or signal an error if insufficient type information is available for definite inference.
Any variables introduced by a variable declaration are visible from the point of declaration until the end of the enclosing block scope, except when they are shadowed by another variable declaration.
If an `else` block is not present, the pattern must be irrefutable.
If an `else` block is present, the pattern may be refutable.
If the pattern does not match (this requires it to be refutable), the `else` block is executed.
The `else` block must always diverge (evaluate to the [never type]).
```rust
let (mut v, w) = (vec![1, 2, 3], 42); // The bindings may be mut or const
let Some(t) = v.pop() else { // Refutable patterns require an else block
panic!(); // The else block must diverge
};
let [u, v] = [v[0], v[1]] else { // This pattern is irrefutable, so the compiler
// will lint as the else block is redundant.
panic!();
};
```
## Expression statements
> **<sup>Syntax</sup>**\
> _ExpressionStatement_ :\
> [_ExpressionWithoutBlock_][expression] `;`\
> | [_ExpressionWithBlock_][expression] `;`<sup>?</sup>
An *expression statement* is one that evaluates an [expression] and ignores its result.
As a rule, an expression statement's purpose is to trigger the effects of evaluating its expression.
An expression that consists of only a [block expression][block] or control flow expression, if used in a context where a statement is permitted, can omit the trailing semicolon.
This can cause an ambiguity between it being parsed as a standalone statement and as a part of another expression;
in this case, it is parsed as a statement.
The type of [_ExpressionWithBlock_][expression] expressions when used as statements must be the unit type.
```rust
# let mut v = vec![1, 2, 3];
v.pop(); // Ignore the element returned from pop
if v.is_empty() {
v.push(5);
} else {
v.remove(0);
} // Semicolon can be omitted.
[1]; // Separate expression statement, not an indexing expression.
```
When the trailing semicolon is omitted, the result must be type `()`.
```rust
// bad: the block's type is i32, not ()
// Error: expected `()` because of default return type
// if true {
// 1
// }
// good: the block's type is i32
if true {
1
} else {
2
};
```
## Attributes on Statements
Statements accept [outer attributes].
The attributes that have meaning on a statement are [`cfg`], and [the lint check attributes].
[block]: expressions/block-expr.md
[expression]: expressions.md
[function]: items/functions.md
[item]: items.md
[module]: items/modules.md
[never type]: types/never.md
[canonical path]: paths.md#canonical-paths
[implementations]: items/implementations.md
[variables]: variables.md
[outer attributes]: attributes.md
[`cfg`]: conditional-compilation.md
[the lint check attributes]: attributes/diagnostics.md#lint-check-attributes
[pattern]: patterns.md
[_BlockExpression_]: expressions/block-expr.md
[_ExpressionStatement_]: #expression-statements
[_Expression_]: expressions.md
[_Item_]: items.md
[_LazyBooleanExpression_]: expressions/operator-expr.md#lazy-boolean-operators
[_LetStatement_]: #let-statements
[_MacroInvocationSemi_]: macros.md#macro-invocation
[_OuterAttribute_]: attributes.md
[_PatternNoTopAlt_]: patterns.md
[_Type_]: types.md
|
