Rust - Constant


Constants represent values that cannot be changed. If you declare a constant then there is no way its value changes. The keyword for using constants is const. Constants must be explicitly typed. Following is the syntax to declare a constant.

const VARIABLE_NAME:dataType = value;

Rust Constant Naming Convention

The naming convention for Constants are similar to that of variables. All characters in a constant name are usually in uppercase. Unlike declaring variables, the let keyword is not used to declare a constant.

We have used constants in Rust in the example below −

fn main() {
   const USER_LIMIT:i32 = 100;    // Declare a integer constant
   const PI:f32 = 3.14;           //Declare a float constant

   println!("user limit is {}",USER_LIMIT);  //Display value of the constant
   println!("pi value is {}",PI);            //Display value of the constant

Constants v/s Variables

In this section, we will learn about the differentiating factors between constants and variables.

  • Constants are declared using the const keyword while variables are declared using the let keyword.

  • A variable declaration can optionally have a data type whereas a constant declaration must specify the data type. This means const USER_LIMIT=100 will result in an error.

  • A variable declared using the let keyword is by default immutable. However, you have an option to mutate it using the mut keyword. Constants are immutable.

  • Constants can be set only to a constant expression and not to the result of a function call or any other value that will be computed at runtime.

  • Constants can be declared in any scope, including the global scope, which makes them useful for values that many parts of the code need to know about.

Shadowing of Variables and Constants

Rust allows programmers to declare variables with the same name. In such a case, the new variable overrides the previous variable.

Let us understand this with an example.

fn main() {
   let salary = 100.00;
   let salary = 1.50 ; 
   // reads first salary
   println!("The value of salary is :{}",salary);

The above code declares two variables by the name salary. The first declaration is assigned a 100.00 while the second declaration is assigned value 1.50. The second variable shadows or hides the first variable while displaying output.


The value of salary is :1.50

Rust supports variables with different data types while shadowing.

Consider the following example.

The code declares two variables by the name uname. The first declaration is assigned a string value, whereas the second declaration is assigned an integer. The len function returns the total number of characters in a string value.

fn main() {
   let uname = "Mohtashim";
   let uname = uname.len();
   println!("name changed to integer : {}",uname);


name changed to integer: 9

Unlike variables, constants cannot be shadowed. If variables in the above program are replaced with constants, the compiler will throw an error.

fn main() {
   const NAME:&str = "Mohtashim";
   const NAME:usize = NAME.len(); 
   //Error : `NAME` already defined
   println!("name changed to integer : {}",NAME);