Rust: What is the difference between let and const?

let — variable binding (can be mutable). const — compile-time constant (always immutable).

let:

• Binds a value to a name.
• Can be let mut for mutability.
• Value computed at runtime.
• Supports destructuring.
• Type inferred unless annotated.

1let x = 5;
2let mut y = 10;
3y += 5;
4
5let (name, age) = ("Alice", 30);
6let Point { x, y } = point;

const:

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

1const MAX: u32 = 100_000;
2const PI: f64 = 3.14159265358979;
3const COLORS: [u8; 3] = [255, 128, 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 values.
• Use let for general variables.
• Use static for global values.

Common mistakes:

• Forgetting type on const — required.
• Trying to compute const at runtime.
• Confusing const and static.