Understanding Rust Error E0063

Introduction to Rust Error E0063

Rust Error E0063 occurs when a struct or a struct-like enum variant is missing one or more fields while instantiating the structure. In Rust, all fields in a struct must be provided during its creation.

Example of Erroneous Code

struct Foo {
    x: i32,
    y: i32,
}

fn main() {
    let x = Foo { x: 0 }; // error: missing field: `y`
}

In the example above, the `Foo` struct has two fields, `x` and `y`. During the instantiation of the struct, only the `x` field is provided, and the `y` field is left missing. This will result in Rust Error E0063.

Correcting the Code

To fix Rust Error E0063, you must provide all the fields of the struct or struct-like enum variant in the instantiation. The corrected code is shown below:

struct Foo {
    x: i32,
    y: i32,
}

fn main() {
    let x = Foo { x: 0, y: 0 }; // ok!
}

In this corrected code, both the `x` and `y` fields are provided, fulfilling the requirement that all fields must be specified when instantiating the struct.

Further Considerations

There may be cases where you would like to instantiate a struct with some default values for certain fields. In such situations, you can utilize the `Default` trait to implement default values for your struct. This can make it easier to instantiate structs without specifying every field individually.

Implementing Defaults for the Struct

struct Foo {
    x: i32,
    y: i32,
}

impl Default for Foo {
    fn default() -> Self {
        Foo { x: 0, y: 0 }
    }
}

fn main() {
    let x = Foo::default(); // ok! x: 0, y: 0
}