What is the difference between immutable and const variables in Rust?

rust constants immutability
7_R3X picture 7_R3X · Jun 17, 2016 · Viewed 10k times · Source

I learned that if a variable is not explicitly declared mutable using mut, it becomes immutable (it cannot be changed after declaration). Then why do we have the const keyword in Rust? Aren't they same? If not, how do they differ?

Answer

Matthieu M. picture Matthieu M. · Jun 17, 2016

const, in Rust, is short for constant and is related to compile-time evaluation. It shows up:

  • when declaring constants: const FOO: usize = 3;
  • when declaring compile-time evaluable functions: const fn foo() -> &'static str

These kinds of values can be used as generic parameters: [u8; FOO]. For now this is limited to array size, but there is talk, plans, and hope to extend it further in the future.

By contrast, a let binding is about a run-time computed value.

Note that despite mut being used because the concept of mutability is well-known, Rust actually lies here. &T and &mut T are about aliasing, not mutability:

  • &T: shared reference
  • &mut T: unique reference

Most notably, some types feature interior mutability and can be mutated via &T (shared references): Cell, RefCell, Mutex, etc.

Note: there is an alternative use of mut and const with raw pointers (*mut T and *const T) which is not discussed here.

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