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 binding.
• Computed at runtime.
• Enables mutable references for borrowing.
• Useful for building values incrementally.

1let mut count = 0;
2count += 1;  // OK
3println!("{}", count);  // 1
4
5let mut items = Vec::new();
6items.push("item1");
7items.push("item2");

const:

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

1const MAX_SIZE: usize = 1024;
2const PI: f64 = 3.14159265358979;
3const EMPTY_STR: &str = "";
4
5const fn compute(x: i32) -> i32 { x * 2 }
6const RESULT: i32 = compute(5);  // 10

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.
• Assigning runtime values to const — compile error.
• Using let mut when const is more appropriate.