Rust: What is the difference between mut and const?

mut — mutable variable (value can be reassigned). const — compile-time constant (value is fixed).

mut:

• Opt-in mutability — default is immutable.
• Value can be reassigned after binding.
• Computed at runtime.
• Enables mutable references.
• Useful for incremental computation.

1let mut sum = 0;
2for i in 1..=10 {
3    sum += i;
4}
5println!("{}", sum);  // 55
6
7let mut buf = String::new();
8buf.push_str("hello");

const:

• True constant — fixed at compile time.
• Always immutable.
• Requires explicit type.
• Must be a constant expression.
• Zero runtime cost.

1const MAX_SIZE: usize = 4096;
2const PI: f64 = 3.14159265358979;
3const EMPTY: &str = "";
4
5const fn square(x: i32) -> i32 { x * x }
6const RESULT: i32 = square(5);  // 25

When to use:

• Use const for configuration, buffer sizes, constants.
• Use mut when values need to change.
• Prefer const over magic numbers.

Common mistakes:

• Forgetting type on const — required.
• Assigning runtime values to const — compile error.
• Using let mut when const is better.