Mutability
A constant in Rust is immutable. You neither can reassign nor modify it:
struct Foo(u32);
const FOO: Foo = Foo(5);
const mut FOO: Foo = Foo(5); // illegal
fn main() {
FOO = Foo(1); //illegal
FOO.0 = 2; //illegal
}
A static variable can be mutable and therefore can either be modified or reassigned. Note that writing/modifying a global static variable is unsafe and therefore needs an unsafe block:
struct Foo(u32);
static FOO: Foo = Foo(5);
static mut FOO_MUT: Foo = Foo(3);
fn main() {
unsafe {
FOO = Foo(1); //illegal
FOO.0 = 2; //illegal
FOO_MUT = Foo(1);
FOO_MUT.0 = 2;
}
}
Occurrences
When you compile a binary, all const "occurrences" (where you use that const in your source code) will be replaced by that value directly.
statics will have a dedicated section in your binary where they will be placed (the BSS section, see Where are static variables stored in C and C++? for further information).
All in all, stick to a const whenever possible. When not possible, because you need to initialize a variable later in the program of with non-const methods, use lazy_static!.
Interior mutability
While both const and static can use interior mutability you should never ever do it with a const. Here's an example
use std::sync::atomic::{AtomicU32, Ordering};
static STATIC: AtomicU32 = AtomicU32::new(0);
const CONST: AtomicU32 = AtomicU32::new(0);
fn print() {
println!("static: {}", STATIC.load(Ordering::Relaxed));
println!("const: {}", CONST.load(Ordering::Relaxed));
}
fn main() {
STATIC.store(3, Ordering::Relaxed);
CONST.store(3, Ordering::Relaxed);
print();
}
This compiles fine without any warnings, but leads to unwanted behavoir. Output:
static: 3
const: 0
When using clippy, it will show the two following warnings:
warning: a `const` item should never be interior mutable
--> src/main.rs:4:1
|
4 | const CONST: AtomicU32 = AtomicU32::new(0);
| -----^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| |
| make this a static item (maybe with lazy_static)
|
= note: `#[warn(clippy::declare_interior_mutable_const)]` on by default
= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#declare_interior_mutable_const
warning: a `const` item with interior mutability should not be borrowed
--> src/main.rs:8:27
|
8 | println!("const: {}", CONST.load(Ordering::Relaxed));
| ^^^^^
|
= note: `#[warn(clippy::borrow_interior_mutable_const)]` on by default
= help: assign this const to a local or static variable, and use the variable here
= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#borrow_interior_mutable_const
warning: a `const` item with interior mutability should not be borrowed
--> src/main.rs:13:5
|
13 | CONST.store(3, Ordering::Relaxed);
| ^^^^^
|
= help: assign this const to a local or static variable, and use the variable here
= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#borrow_interior_mutable_const
