Macro format::lazy_format

lazy_format!() { /* proc-macro */ }

Lazy format macro

[lazy_format!] is syntax sugar of Display.

The first form of [lazy_format!] receives closure as the only one argument.

lazy_format!(|f| ...);

it expands to:

Display(move |f| ...);

The second form of [lazy_format!] has a syntax identical to the syntax of format!. See fmt for more information.

lazy_format!("...", arg0, arg1, ...);

it expands to:

Display(move |f| {
    write!(f, "...", arg0, arg1, ...)
});

Examples

Nested debug help struct

#![feature(proc_macro_hygiene)]

use core::fmt::{Debug, Formatter, Result};
use format::lazy_format;

struct Foo {
    bar: [u32; 10],
}

impl Debug for Foo {
    fn fmt(&self, f: &mut Formatter) -> Result {
        let bar = lazy_format!(|f| f.debug_list().entries(&self.bar).finish());
        f.debug_struct("Foo")
            .field("bar", &format_args!("{}", bar))
            .finish()
    }
}

assert_eq!(
    "Foo { bar: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] }",
    format!("{:?}", Foo { bar: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] })
);

Control flow

#![feature(proc_macro_hygiene)]

use core::fmt::{Debug, Formatter, Result};
use format::lazy_format;

struct Foo(usize);

impl Debug for Foo {
    fn fmt(&self, f: &mut Formatter) -> Result {
        let alternate = f.alternate();
        let bar = lazy_format!(|f| if alternate {
            write!(f, "{:#x}", self.0)
        } else {
            write!(f, "{:x}", self.0)
        });
        f.debug_tuple("Foo")
            .field(&format_args!("{}", bar))
            .finish()
    }
}

assert_eq!("Foo(75bcd15)", format!("{:?}", Foo(0123456789)));
assert_eq!("Foo(\n    0x75bcd15,\n)", format!("{:#?}", Foo(0123456789)));