Rust: What is the difference between let and const?

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

let:

• Binds a value to a name — basic variable declaration.
• Can be let mut for mutable variables.
• Value computed at runtime.
• Type inference works automatically.
• Scoped to the block where declared.

1let x = 5;           // Immutable
2let mut y = 10;      // Mutable
3y = 20;              // OK
4
5// Destructuring
6let (a, b) = (1, 2);
7let Point { x, y } = point;

const:

• Declares a true constant — fixed at compile time.
• Always immutable (no const mut).
• Requires explicit type annotation.
• Can only hold const-evaluable expressions.
• Inlined by the compiler — no runtime overhead.

1const MAX_POINTS: u32 = 100_000;
2const GREETING: &str = "Hello, World!";
3const ARRAY: [i32; 3] = [1, 2, 3];
4
5// const can use const fn
6const fn square(x: i32) -> i32 { x * x }
7const RESULT: i32 = square(5);  // 25

When to use:

• Use const for configuration values, sizes, magic numbers.
• Use let for general-purpose variables and computations.
• Use let mut when values need to change.

Common mistakes:

• Forgetting type annotation on const — it's required by the compiler.
• Trying to compute const at runtime — it must be a constant expression.
• Using static thinking it's like const — they have different semantics.