W3cubDocs

/Rust

Statements

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 and 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 declarations

An item declaration statement has a syntactic form identical to an item declaration within a module. Declaring an item — a function, enumeration, struct, type, static, trait, implementation or module — locally within a statement block is simply a way of restricting its scope to a narrow region containing all of its uses; it is otherwise identical in meaning to declaring the item outside the statement block.

Note: there is no implicit capture of the function's dynamic environment when declaring a function-local item.

let statements

A let statement introduces a new set of variables, given by a pattern. The pattern may be followed by a type annotation, and/or an initializer expression. 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.

Expression statements

An expression statement is one that evaluates an expression and ignores its result. The type of an expression statement e; is always (), regardless of the type of e. 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 or control flow expression, that doesn't end a block and evaluates to () can also be used as an expression statement by omitting the trailing semicolon.

# #![allow(unused_variables)]
#fn main() {
# 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.

#}

© 2010 The Rust Project Developers
Licensed under the Apache License, Version 2.0 or the MIT license, at your option.
https://doc.rust-lang.org/reference/statements.html