Variable bindings, constants and statics

⭐️ In Rust variable are immutable by default, so we call them Variable bindings. To make them mutable, mut keyword is used.

⭐️ Rust is a statically typed language; It checks data type at compile time. But it doesn’t require you to actually type it when declare variable bindings. On that case, the compiler checks the usage and sets a better data type for it. But for constants and statics you must annotate the type. Types come after a colon(:)

• Variable bindings

let a = true;
let b: bool = true;

let (x, y) = (1, 2);

let mut z = 5;
z = 6;

• Constants

const N: i32 = 5;

• Statics

static N: i32 = 5;

let keyword is used in binding expressions. We can bind a name to a value or a function. Also because the left-hand side of a let expression is a ‘pattern’, you can bind multiple names to set of values or function values.

const keyword is used to define constants. It lives for the entire lifetime of a program but have no fixed address in memory. static keyword is used to define ‘global variable’ type facility. There is only one instance for each value, and it’s at a fixed location in memory.

💡 Always use const, instead of static. It’s pretty rare that you actually want a memory location associated with your constant, and using a const allows for optimizations like constant propagation not only in your crate but also in downstream crates.

💡 Usually statics are placed at top of the code file, outside the functions.