Constant items - The Rust Reference

# Constant items

> Syntax
> ConstantItem :
> `const` IDENTIFIER `:` Type `=` Expression `;`

A constant item is a named constant value which is not associated with a specific memory location in the program. Constants are essentially inlined wherever they are used, meaning that they are copied directly into the relevant context when used. References to the same constant are not necessarily guaranteed to refer to the same memory address.

Constants must be explicitly typed. The type must have a `'static` lifetime: any references it contains must have `'static` lifetimes.

Constants may refer to the address of other constants, in which case the address will have elided lifetimes where applicable, otherwise – in most cases – defaulting to the `static` lifetime. (See static lifetime elision.) The compiler is, however, still at liberty to translate the constant many times, so the address referred to may not be stable.

    # #![allow(unused_variables)]
    #fn main() {
    const BIT1: u32 = 1 << 0;
    const BIT2: u32 = 1 << 1;

    const BITS: [u32; 2] = [BIT1, BIT2];
    const STRING: &'static str = "bitstring";

    struct BitsNStrings<'a> {
        mybits: [u32; 2],
        mystring: &'a str,
    }

    const BITS_N_STRINGS: BitsNStrings<'static> = BitsNStrings {
        mybits: BITS,
        mystring: STRING,
    };
    #}

## Constants with Destructors

Constants can contain destructors. Destructors are run when the value goes out of scope.

    # #![allow(unused_variables)]
    #fn main() {
    struct TypeWithDestructor(i32);

    impl Drop for TypeWithDestructor {
        fn drop(&mut self) {
            println!("Dropped. Held {}.", self.0);
        }
    }

    const ZERO_WITH_DESTRUCTOR: TypeWithDestructor = TypeWithDestructor(0);

    fn create_and_drop_zero_with_destructor() {
        let x = ZERO_WITH_DESTRUCTOR;
        // x gets dropped at end of function, calling drop.
        // prints "Dropped. Held 0.".
    }
    #}