Custom Format Strings

Instead of using a built-in format, you can provide a custom format string. Field names are interpolated directly using {field_name} syntax.

Basic Interpolation

Reference fields by name inside curly braces:

use moxy::Display;

#[derive(Display)]
#[moxy(display("hi! my name is {name} and my email is {email}"))]
struct User {
    name: String,
    email: String,
}

let user = User {
    name: "John".into(),
    email: "john@example.com".into(),
};

assert_eq!(
    format!("{user}"),
    "hi! my name is John and my email is john@example.com"
);

Expression Arguments

Use std::fmt-style positional arguments with arbitrary Rust expressions. The format string comes first, followed by comma-separated expressions:

#[moxy(display("{}", expr))]

Self Field Access

Access fields through self:

use moxy::Display;

#[derive(Display)]
#[moxy(display("{}", self.name))]
struct User {
    name: String,
}

// John

Self Method Calls

Call methods on self:

use moxy::Display;

#[derive(Display)]
#[moxy(display("{}", self.greeting()))]
struct User {
    name: String,
}

impl User {
    fn greeting(&self) -> String {
        format!("Hello, {}!", self.name)
    }
}

// Hello, John!

Arithmetic and Other Expressions

Any valid Rust expression works:

use moxy::Display;

#[derive(Display)]
#[moxy(display("double: {}", count * 2))]
struct Counter {
    count: i32,
}

let counter = Counter { count: 5 };
assert_eq!(format!("{counter}"), "double: 10");