🦀 Rust – Constants: Declare Fixed Values Safely and Globally

🧲 Introduction – Why Learn Constants in Rust?

Constants in Rust give you a way to define unchangeable values that remain fixed across your program. Unlike regular variables, constants are always immutable, must have an explicit type, and are ideal for defining global configuration values or application-wide settings.

🎯 In this guide, you’ll learn:

• How to declare constants using const
• Differences between const, let, and static
• Rules, best practices, and naming conventions
• Common mistakes and how to avoid them

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🔐 Declaring Constants in Rust

const MAX_SCORE: u32 = 100;

🔍 Explanation:

• const keyword defines a compile-time constant
• Must include explicit type
• Cannot use mut (always immutable)
• Typically written in UPPER_SNAKE_CASE

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🧠 Constants vs. Variables vs. Static

Feature	const	let	static
Mutability	Always immutable	Immutable by default	Mutable (with unsafe)
Requires Type	✅ Yes	❌ Optional (inferred)	✅ Yes
Memory Location	Inlined at compile time	Stack or heap	Fixed memory address (static)
Scope	Global or module-level only	Any block or function	Global (often used with unsafe)

🧠 Use const for lightweight, safe, and fast compile-time constants. Use static only when working with global mutable state, and prefer let inside functions for dynamic values.

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✅ Example: Using Constants

const PI: f64 = 3.14159;
const GREETING: &str = "Welcome to Rust!";

fn main() {
    let radius = 5.0;
    let area = PI * radius * radius;

    println!("{} The area is {:.2}", GREETING, area);
}

📤 Output:

Welcome to Rust! The area is 78.54

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🚫 Invalid Constant Usage – Common Mistakes

Mistake	Error Message / Behavior	Solution
Missing type for constant	“missing type for const item”	Always include explicit type
Using const inside a function	Valid, but use let unless value is reused	Prefer const at module level
Trying to mutate a const	Compiler error: cannot assign to const	const is always read-only

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📏 Best Practices for Using Constants

• ✅ Use uppercase with underscores for clarity: MAX_USERS, PI, WELCOME_MSG
• ✅ Declare constants at the top of modules for visibility
• ✅ Prefer const for values that are reused and never change
• ✅ Use let for runtime-calculated values
• ⚠️ Avoid using static mut unless absolutely necessary (unsafe)

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📌 Summary – Recap & Next Steps

Constants in Rust provide a reliable way to define compile-time, type-safe, unchangeable values. Mastering const helps make your code more readable, efficient, and safe.

🔍 Key Takeaways:

• Use const for fixed values with explicit types
• Constants are inlined at compile time for performance
• Naming convention: UPPER_SNAKE_CASE
• Do not confuse const with let or static

⚙️ Next, explore Rust – Data Types to understand how constants and variables interact with Rust’s rich type system.

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❓FAQs

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❓ Can I declare constants inside functions in Rust?
✅ Yes, but only if the value is compile-time evaluable. Still, constants are typically declared at the module or global level.

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❓ What’s the difference between const and static?
✅ const is inlined at compile time. static has a fixed memory address and is suitable for global values, especially when working with FFI or mutable global state.

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❓ Why must constants have explicit types?
✅ Rust enforces this for compile-time safety and to eliminate ambiguity in constant evaluation.

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❓ Can I use mut with const?
❌ No. Constants are inherently immutable. Use let mut for mutable values or static mut for global mutable state (requires unsafe).

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