Initializing Structs in Rust: Quick Start and Advanced Tricks

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Introduction: Why is Struct Initialization in Rust Not Boring?

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If you've ever written in Rust, you've probably encountered a moment when you want to say: "Why do I have to write field: field over and over again? It's obvious!"

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Yes, Rust is a strict, rigorous, and incredibly expressive language. But even it has room for syntactic sugar that makes code shorter, cleaner, and more pleasing to the eye. This is especially true for struct initialization — that very routine we do dozens of times a day.

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In this article, we will not just answer the popular Stack Overflow question: "Is there a faster/shorter way to initialize variables in a Rust struct?". We will break down all the methods — from basic to advanced — and show you how to write code that is simultaneously fast, readable, and idiomatic Rust.

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Ready to speed up your work with Rust struct? Let's go!

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1. The Classic Way: Explicit Field Naming

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Let's start with the basics. Suppose we have a User struct:

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struct User {\n    username: String,\n    email: String,\n    sign_in_count: u64,\n    active: bool,\n}

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Standard initialization looks like this:

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let user1 = User {\n    email: String::from("someone@example.com"),\n    username: String::from("someusername123"),\n    active: true,\n    sign_in_count: 1,\n};

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It's honest, it's explicit. But when you have 10-15 fields and the variable names match the field names — it starts to get annoying. You write email: email and feel like a bureaucrat filling out a form.

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The good news: Rust feels this too. And it came up with a solution.

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2. Field Init Shorthand: When the Variable Name Matches the Field

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Starting with Rust 1.17, you can use a shorthand notation if the variable name matches the struct field name. This is called field init shorthand.

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let email = String::from("someone@example.com");\nlet username = String::from("someusername123");\n\nlet user1 = User {\n    email,  // instead of email: email\n    username, // instead of username: username\n    active: true,\n    sign_in_count: 1,\n};

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Nice, right? Rust itself understands that email is both a variable and a field. This technique is especially handy when you pass function parameters that are already meaningfully named.

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When to use: always, when the variable name matches the field name. This is not just "faster to write", it's idiomatic Rust.

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3. Struct Update Syntax: Copying a Struct with Changes

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Suppose we already have user1, and we want to create user2, changing only the email. Do we have to rewrite all the fields? No!

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let user2 = User {\n    email: String::from("another@example.com"),\n    ..user1  // remaining fields are taken from user1\n};

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The .. operator is the struct update syntax. It tells the compiler: "Take all the remaining fields from user1". Important: after this, user1 will be partially moved (if the fields do not implement Copy), so be careful with ownership.

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This syntax saves tons of lines when you work with Rust struct initialization based on existing data.

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4. The Default Trait: Initialization with Defaults

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What if most fields should have standard values? For example, active: true and sign_in_count: 0? You can implement the Default trait:

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#[derive(Default)]\nstruct User {\n    username: String,\n    email: String,\n    active: bool,\n    sign_in_count: u64,\n}\n\n// Now:\nlet user_default = User::default();\n// username: "", email: "", active: false, sign_in_count: 0

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But wait!