Variables - The Rust Reference

# Variables

A variable is a component of a stack frame, either a named function parameter, an anonymous temporary, or a named local variable.

A local variable (or stack-local allocation) holds a value directly, allocated within the stack's memory. The value is a part of the stack frame.

Local variables are immutable unless declared otherwise. For example: `let mut x = ...`.

Function parameters are immutable unless declared with `mut`. The `mut` keyword applies only to the following parameter. For example: `|mut x, y|` and `fn f(mut x: Box<i32>, y: Box<i32>)` declare one mutable variable `x` and one immutable variable `y`.

Local variables are not initialized when allocated. Instead, the entire frame worth of local variables are allocated, on frame-entry, in an uninitialized state. Subsequent statements within a function may or may not initialize the local variables. Local variables can be used only after they have been initialized through all reachable control flow paths.

In this next example, `init_after_if` is initialized after the `if` expression while `uninit_after_if` is not because it is not initialized in the `else` case.

    # #![allow(unused_variables)]
    #fn main() {
    # fn random_bool() -> bool { true }
    fn initialization_example() {
        let init_after_if: ();
        let uninit_after_if: ();

        if random_bool() {
            init_after_if = ();
            uninit_after_if = ();
        } else {
            init_after_if = ();
        }

        init_after_if; // ok
        // uninit_after_if; // err: use of possibly uninitialized `uninit_after_if`
    }
    #}