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`.

Methods that take either `self` or `Box<Self>` can optionally place them in a mutable variable by prefixing them with `mut` (similar to regular arguments). For example:

    # #![allow(unused_variables)]
    #fn main() {
    trait Changer: Sized {
        fn change(mut self) {}
        fn modify(mut self: Box<Self>) {}
    }

    #}

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; this is enforced by the compiler.