Module std::const_keyword

Compile-time constants, compile-time evaluable functions, and raw pointers.

Compile-time constants

Sometimes a certain value is used many times throughout a program, and it can become inconvenient to copy it over and over. What’s more, it’s not always possible or desirable to make it a variable that gets carried around to each function that needs it. In these cases, the const keyword provides a convenient alternative to code duplication:

const THING: u32 = 0xABAD1DEA;

let foo = 123 + THING;

Constants must be explicitly typed; unlike with let, you can’t ignore their type and let the compiler figure it out. Any constant value can be defined in a const, which in practice happens to be most things that would be reasonable to have in a constant (barring const fns). For example, you can’t have a File as a const.

The only lifetime allowed in a constant is 'static, which is the lifetime that encompasses all others in a Rust program. For example, if you wanted to define a constant string, it would look like this:

const WORDS: &'static str = "hello rust!";

Thanks to static lifetime elision, you usually don’t have to explicitly use 'static:

const WORDS: &str = "hello convenience!";

const items looks remarkably similar to static items, which introduces some confusion as to which one should be used at which times. To put it simply, constants are inlined wherever they’re used, making using them identical to simply replacing the name of the const with its value. Static variables, on the other hand, point to a single location in memory, which all accesses share. This means that, unlike with constants, they can’t have destructors, and act as a single value across the entire codebase.

Constants, like statics, should always be in SCREAMING_SNAKE_CASE.

For more detail on const, see the Rust Book or the Reference.

Compile-time evaluable functions

The other main use of the const keyword is in const fn. This marks a function as being callable in the body of a const or static item and in array initializers (commonly called “const contexts”). const fn are restricted in the set of operations they can perform, to ensure that they can be evaluated at compile-time. See the Reference for more detail.

Turning a fn into a const fn has no effect on run-time uses of that function.

Other uses of const

The const keyword is also used in raw pointers in combination with mut, as seen in *const T and *mut T. More about const as used in raw pointers can be read at the Rust docs for the pointer primitive.