rtformat

Runtime string formatting for Rust, with std::fmt-compatible placeholder syntax.

Rust's built-in format! macro requires the format string to be a compile-time literal. rtformat lets you take a template string at runtime — from a config file, a database, or user input — and format it with the same placeholder syntax you already know.

Features

• {} implicit positional arguments; {0} / {1} explicit indices (reusable)
• Types: {:?} {:#?} {:x} {:X} {:o} {:b} {:e} {:E}
• Alignment and fill: {:<10} {:>10} {:^10} {:_>10}
• Sign / radix prefix / zero padding: {:+} {:#x} {:010}
• Width and precision: {:.3} {:1$} {:.1$} (n$ takes the width/precision from argument n)
• {{ / }} escapes
• Custom argument types via #[derive(FormatArg)]
• Fallible formatting (try_format) and an incremental builder API

Not supported: named arguments {name}, pointers {:p}, hexadecimal debug {x?} / {X?}.

Usage

Add the dependency:

[dependencies]
rtformat = "0.1"

The rformat! macro

use rtformat::rformat;

assert_eq!(rformat!("Hello {}!", "world"), "Hello world!");
assert_eq!(rformat!("{0} + {1} = {2}", 1, 2, 3), "1 + 2 = 3");
assert_eq!(rformat!("{:#010x}", 255), "0x000000ff");
assert_eq!(rformat!("{:.2}", 3.14159), "3.14");

The Format trait

Format is implemented for str (and available on String, Box<str>, etc. through deref):

use rtformat::Format;

assert_eq!("{} + {} = {2}".format(&(1, 2, 3)), "1 + 2 = 3");

// Fallible variants, for templates that may be invalid:
let result = "{}".try_format(&("hello",));
assert_eq!(result.as_deref(), Ok("hello"));

Builder API

Add arguments one at a time:

use rtformat::Format;

let s = "{} + {} = {2}".builder().arg(&1).arg(&2).arg(&3).build();
assert_eq!(s, "1 + 2 = 3");

Custom argument types

Types implementing Display and/or Debug can derive FormatArg. The derive adapts: {} prefers Display and falls back to Debug, while {:?} / {:#?} require Debug:

use core::fmt;
use rtformat::{rformat, Format, FormatArg};

#[derive(Debug, FormatArg)]
struct Color(u8, u8, u8);

impl fmt::Display for Color {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "#{:02x}{:02x}{:02x}", self.0, self.1, self.2)
    }
}

assert_eq!(rformat!("{}", Color(255, 128, 0)), "#ff8000");

For full control over each format type, implement FormatArg manually instead.

Prelude

All commonly used items are available through the prelude:

use rtformat::prelude::*;

assert_eq!(rformat!("Hello {}!", "world"), "Hello world!");

Workspace layout

License

Licensed under either of

• Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.