Rust: What is the difference between mut and const?

mut — mutable variable (value can change). const — compile-time constant (value fixed).

mut:

• Declares a mutable binding — value can be reassigned.
• Default immutability — must opt in with mut.
• Value is computed at runtime.
• Type can be inferred or explicit.
• Mutable references (&mut T) allow modifying borrowed data.

1let mut count = 0;
2count += 1;  // OK — value changes
3println!("{}", count);  // 1
4
5let mut s = String::from("hello");
6s.push_str(" world");  // Modifying the String

const:

• Declares a constant — value is fixed and known at compile time.
• Always immutable — no const mut in standard Rust.
• Requires explicit type annotation.
• Inlined at compile time — zero runtime cost.
• Can only use const-evaluable expressions.

1const MAX_SIZE: usize = 1024;
2const PI: f64 = 3.14159265358979;
3const APP_NAME: &str = "MyApp";
4
5// const fn — can be evaluated at compile time
6const fn add(a: i32, b: i32) -> i32 {
7    a + b
8}
9const RESULT: i32 = add(1, 2);  // 3

When to use:

• Use const for magic numbers, configuration values, buffer sizes.
• Use mut when you need to reassign or build up a value.
• Use static for global values that need a fixed address.

Common mistakes:

• Forgetting type annotation on const — required.
• Trying to assign a runtime value to const — compile error.
• Using let mut for values that never change — use const instead.