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 — variables are immutable by default.
• Value can be reassigned after initial binding.
• Computed at runtime.
• Enables mutable references (&mut T) for borrowing.
• Useful for building up values incrementally.

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

const:

• True constant — value fixed at compile time.
• Always immutable (no const mut in stable Rust).
• Requires explicit type annotation.
• Can only hold const-evaluable expressions.
• Inlined by the compiler — zero runtime cost.

1const MAX_BUFFER: usize = 4096;
2const API_URL: &str = "https://api.example.com";
3const VERSION: (u8, u8, u8) = (1, 0, 0);
4
5const fn add(a: i32, b: i32) -> i32 { a + b }
6const RESULT: i32 = add(10, 20);  // 30

When to use:

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

Common mistakes:

• Forgetting type annotation on const — required by compiler.
• Assigning runtime values to const — compile error.
• Using let mut when const is more appropriate.