Rust: What is the difference between struct and enum?

Struct — custom type with fields (single shape). Enum — type with multiple variants.

Struct:

• Defines a type where all fields exist together.
• Three forms: named fields, tuple structs, unit structs.
• Methods defined in impl blocks.
• Represents "this thing has these properties."

1// Named fields
2struct Config {
3    host: String,
4    port: u16,
5    debug: bool,
6}
7
8// Tuple struct
9struct Meters(f64);
10
11// Unit struct
12struct Marker;
13
14impl Config {
15    fn new(host: String, port: u16) -> Self {
16        Self { host, port, debug: false }
17    }
18}

Enum:

• Defines a type that can be one of several variants.
• Variants can hold different data (algebraic data types).
• Pattern matching with match ensures all cases are handled.
• Represents "this thing is one of these."

1enum Command {
2    Quit,
3    Echo(String),
4    Move { x: i32, y: i32 },
5}
6
7fn execute(cmd: Command) {
8    match cmd {
9        Command::Quit => println!("Quitting"),
10        Command::Echo(msg) => println!("{}", msg),
11        Command::Move { x, y } => println!("Move to ({}, {})", x, y),
12    }
13}

When to use:

• Structs for data containers with fixed fields.
• Enums when a value can be one of several kinds.
• Enums for state machines, commands, and domain modeling.

Common mistakes:

• Forgetting a variant in match — compiler enforces exhaustiveness.
• Using enums when all variants have the same fields — structs are simpler.
• Not deriving useful traits like Debug, Clone, PartialEq.