dfmt - dynamic format!

dfmt provides core::fmt-like formatting for dynamic templates and is a fully featured dynamic drop in replacment for the macros: format!, print!, println!, eprint!, eprintln!, write!, writeln!.

// Check out the documentation for a complete overview.
use dfmt::*;

let str_template = "Hello, {0} {{{world}}} {} {day:y<width$}!";
let precompiled_template = Template::parse(str_template).unwrap();

// Parsing the str template on the fly
dprintln!(str_template, "what a nice", world = "world", day = "day", width=20);

// Using a precompiled template
dprintln!(precompiled_template, "what a nice", world = "world", day = "day", width=20);

// Uses println! under the hood
dprintln!("Hello, {0} {{{world}}} {} {day:y<width$}!", "what a nice", 
    world = "world", day = "day", width=20);

// Other APIs
let using_dformat = dformat!(precompiled_template, "what a nice", 
    world = "world", day = "day", width=20).unwrap();
println!("{}", using_dformat);

let using_manual_builder_api = precompiled_template
    .arguments()
    .builder()
    .display(0, &"what a nice")
    .display("world", &"world")
    .display("day", &"day")
    .width_or_precision_amount("width", &20)
    .format()
    .unwrap();
println!("{}", using_manual_builder_api);

let using_str_extension = "Hello, {0} {{{world}}} {} {day:y<width$}!"
    .format(vec![
        (&0, ArgumentValue::Display(&"what a nice")),
        (&"world", ArgumentValue::Display(&"world")),
        (&"day", ArgumentValue::Display(&"day")),
        (&"width", ArgumentValue::WidthOrPrecisionAmount(&20)),
    ])
    .unwrap();
println!("{}", using_str_extension);

let using_manual_template_builder = Template::new()
    .literal("Hello, ")
    .specified_argument(0, Specifier::default()
        .alignment(Alignment::Center)
        .width(Width::Fixed(20)))
    .literal("!")
    .arguments()
    .builder()
    .display(0, &"World")
    .format()
    .unwrap();
println!("{}", using_manual_template_builder);

Features

✅ Support dynamic templates
✅ All formatting specifiers
✅ Indexed and named arguments
✅ Easy to use API and macros
✅ With safety in mind
✅ Blazingly fast
✅ No-std support (Using a global allocator, and only dformat! and write!)

Formatting features

Name	Feature
Fill/Alignment	<, ^, >
Sign	+, -
Alternate	#
Zero-padding	0
Width	{:20}, {:width$}
Precision	{:.5}, {:.precision$}, {:*}
Type	?, x, X, o, b, e, E, p
Argument keys	{}, {0}, {arg}

How it works

• If the template is a literal, then the format! macro is used under the hood.
• Uses the core::fmt machinery under the hood. Therefore, you can expect the same formatting behaviour.
• It uses black magic to provide a comfortable macro.

Safety

There are multiple runtime checks to prevent you from creating an invalid format string.

• Check if the required argument value exists and implements the right formatter.
• Check for duplicate arguments
• Validate the template

Performance

In the best case dfmt is as fast as format!. In the worst case, its up to 60% - 100% slower.

However, I believe with further optimization this gap could be closed. In fact, with the formatting_options feature we are even faster in some cases.

Considerations

• While the template parsing is fast, you can just create it once and then reuse it for multiple arguments.
• There is a unchecked version, which skips safety checks.
• If the template is a literal, it will fall back to format! internally if you use the macro.

Overhead

• When creating the Arguments structure, a vector is allocated for the arguments. This is barely noticeable for many arguments.
• Right now padding a string with a fill character will cost some overhead.
• If a pattern reuses an argument multiple times, it will push a typed version of this value multiple times right now. This allocates more memory, but is required to provide a convinient API.
• The macros for convinience, incour some overhead due to auto-deref specialization. Its about 5-10 ns per value.

Nightly

If you are on nightly, you can opt in to the nightly_formatting_options feature to further improve the performance, especially for the fill character case and to reduce compilation complexity.

Benchmarks

These benchmarks compare dfmt with format! with dynamic arguments only. Obviously, if format! makes use of const folding, it will be much faster.

Without formatting_options feature

Benchmark	simple - 1 arg	simple - 7 args	complex
Template::parse	67 ns	249 ns	684 ns
format!	30 ns	174 ns	515 ns
Template unchecked	46 ns	173 ns	845 ns
Template checked	49 ns	250 ns	911 ns
dformat! unchecked	51 ns	235 ns	952 ns
dformat! checked	51 ns	260 ns	1040 ns

With formatting_options feature

Benchmark	simple - 1 arg	simple - 7 args	complex
Template::parse	67 ns	249 ns	684 ns
format!	30 ns	174 ns	515 ns
Template unchecked	46 ns	169 ns	464 ns
Template checked	49 ns	238 ns	527 ns
dformat! unchecked	51 ns	232 ns	576 ns
dformat! checked	51 ns	257 ns	658 ns

Minimal rustc version

Right now it compiles until 1.81, this is when std::error went into core::Error. You can opt out of error-impl by disabling the feature error. Then you can go down until 1.56.

License

This project is dual licensed under the Apache 2.0 license and the MIT license.