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 in the current scope.
• Can be let mut for mutable variables.
• Value computed at runtime.
• Supports destructuring and pattern matching.
• Type inferred unless annotated.

1let x = 5;              // Immutable
2let mut y = 10;         // Mutable
3y += 5;                 // OK
4
5let (name, age) = ("Alice", 30);  // Destructuring
6let [a, b, ..] = [1, 2, 3, 4];    // Slice pattern

const:

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

1const MAX_BUFFER: usize = 4096;
2const VERSION: (u8, u8, u8) = (1, 0, 0);
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 and compile-time constants.
• Use let for general variables and runtime computations.
• Use static for global values with fixed memory addresses.

Common mistakes:

• Forgetting type annotation on const — compiler requires it.
• Trying to compute const at runtime — must be a constant expression.
• Confusing const and static — they have different semantics.