Rs
Rust Foundations
Module 1Variables & TypesLesson 4 of 1022 min

Variables, mut & Shadowing

let, mut, shadowing, and constants — immutability by default as a risk control.

Immutability is the default

rust
let price = 100.5;
// price = 101.0; // error: cannot assign twice to immutable variable
let mut volume = 1_000;
volume = 1_200; // ok

In trading systems this default is a feature: fewer accidental overwrites of spot, seed, or nav.

Shadowing (re-bind with let)

Shadowing is not mutation — it creates a new variable that reuses the name:

rust
let n = "42";           // &str
let n: i32 = n.parse().expect("int"); // i32
let n = n + 1;

Useful when transforming raw market strings into numbers.

Constants

rust
const TRADING_DAYS_US: u32 = 252;
const SECONDS_PER_DAY: u64 = 86_400;

Constants must have an explicit type and be computable at compile time.

Worked example: position size

rust
fn main() {
    const MAX_QTY: i64 = 10_000;
    let mut qty = 1_000;
    qty += 500;
    if qty > MAX_QTY {
        qty = MAX_QTY;
    }
    println!("qty={qty} (max {MAX_QTY})");
}
Output
qty=1500 (max 10000)

Exercise

Track a mutable quantity, clamp it, compute notional.

Challenge

Use shadowing to parse quantity from the string "350" then apply the same clamp logic.

Tips
  • mut allows reassignment of the same binding
  • shadowing can change types; mut cannot
  • const names are SCREAMING_SNAKE by convention

Try it live

Variables, mut & ShadowingRust 2021
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Output
Press Run to compile and execute.