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Compile-time string formatting.

This crate provides types and macros for formatting strings at compile-time.

Rust versions

There are some features that require Rust 1.46.0, and others that require Rust nightly, the sections below describe the features that are available.

Rust 1.46.0

These macros are the only things available in Rust 1.46.0:

  • concatcp: Concatenates integers, bool, and &str constants into a &'static str constant.

  • formatcp: format-like formatting which takes integers, bool, and &str constants, and emits a &'static str constant.

Rust nightly

By enabling the "fmt" feature, you can use a std::fmt-like API.

This requires the nightly compiler because it uses mutable references in const fn, which have not been stabilized as of writing these docs.

All the other features of this crate are implemented on top of the const_format::fmt API:

  • concatc: Concatenates many standard library and user defined types into a &'static str constant.

  • formatc: format-like macro that can format many standard library and user defined types into a &'static str constant.

  • writec: write-like macro that can format many standard library and user defined types into a type that implements WriteMarker.

The "derive" feature enables the ConstDebug macro, and the "fmt" feature.
ConstDebug derives the FormatMarker trait, and implements an inherent const_debug_fmt method for compile-time debug formatting.

The "assert" feature enables the assertc, assertc_eq, assertc_ne macros, and the "fmt" feature.
These macros are like the standard library assert macros, but evaluated at compile-time.


Concatenation of primitive types

This example works in Rust 1.46.0.

use const_format::concatcp;

const NAME: &str = "Bob";
const FOO: &str = concatcp!(NAME, ", age ", 21u8,"!");

assert_eq!(FOO, "Bob, age 21!");

Formatting primitive types

This example works in Rust 1.46.0.

use const_format::formatcp;

const NAME: &str = "John";

const FOO: &str = formatcp!("{NAME}, age {}!", compute_age(NAME));

assert_eq!(FOO, "John, age 24!");

const fn compute_age(s: &str) -> usize { s.len() * 6 }

Formatting custom types

This example demonstrates how you can use the ConstDebug derive macro, and then format the type into a &'static str constant.

This example requires Rust nightly, and the "derive" feature.


use const_format::{ConstDebug, formatc};

struct Message{
    ip: [Octet; 4],
    value: &'static str,

struct Octet(u8);

const MSG: Message = Message{
    ip: [Octet(127), Octet(0), Octet(0), Octet(1)],
    value: "Hello, World!",

const FOO: &str = formatc!("{:?}", MSG);

    "Message { ip: [Octet(127), Octet(0), Octet(0), Octet(1)], value: \"Hello, World!\" }"

Formatted const panics

This example demonstrates how you can use the assertc_ne macro to do compile-time inequality assertions with formatted error messages.

This requires the "assert" feature,because as of writing these docs (2020-09-XX), panicking at compile-time requires a nightly feature.


use const_format::{StrWriter, assertc_ne, strwriter_as_str, writec};
use const_format::utils::str_eq;

macro_rules! check_valid_pizza{
    ($user:expr, $topping:expr) => {
            "You can't put pineapple on pizza, {}",

check_valid_pizza!("John", "salami");
check_valid_pizza!("Dave", "sausage");
check_valid_pizza!("Bob", "pineapple");

# fn main(){}

This is the compiler output, the first compilation error is there to have an indicator of what assertion failed, and the second is the assertion failure:

error: any use of this value will cause an error
  --> src/lib.rs:140:1
22 | check_valid_pizza!("Bob", "pineapple");
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ exceeded interpreter step limit (see `#[const_eval_limit]`)
   = note: `#[deny(const_err)]` on by default
   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0080]: could not evaluate constant
  --> /const_format/src/panicking.rs:32:5
32 |     .
   |     ^ the evaluated program panicked at '
module_path: rust_out
line: 22

assertion failed: LEFT != RIGHT

 left: "pineapple"
right: "pineapple"

You can't put pineapple on pizza, Bob
', /const_format/src/panicking.rs:31:1
   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to 2 previous errors


All of the macros from const_format have these limitations:

  • The formatting macros that expand to &'static strs can only use constants from concrete types, so while a Type::<u8>::FOO argument would be fine, Type::<T>::FOO would not be (T being a type parameter).

  • Integer arguments must have a type inferrable from context, more details in the Integer arguments section.

  • They cannot be used places that take string literals. So #[doc = "foobar"] cannot be replaced with #[doc = concatcp!("foo", "bar") ].

Integer arguments

Integer arguments must have a type inferrable from context. so if you only pass an integer literal it must have a suffix.

Example of what does compile:

const N: u32 = 1;
assert_eq!(const_format::concatcp!(N + 1, 2 + N), "23");

assert_eq!(const_format::concatcp!(2u32, 2 + 1u8, 3u8 + 1), "234");

Example of what does not compile:

assert_eq!(const_format::concatcp!(1 + 1, 2 + 1), "23");


None right now.

Renaming crate

All function-like macros from const_format can be used when the crate is renamed.

The ConstDebug derive macro has the #[cdeb(crate = "foo::bar")] attribute to tell it where to find the const_format crate.

Example of renaming the const_format crate in the Cargo.toml file:

cfmt = {version = "0.*", package = "const_format"}

Cargo features

  • "fmt": Enables the std::fmt-like API, requires Rust nightly because it uses mutable references in const fn.
    This feature includes the formatc/writec formatting macros.

  • "derive": implies the "fmt" feature, provides the ConstDebug derive macro to format user-defined types at compile-time.
    This implicitly uses the syn crate, so clean compiles take a bit longer than without the feature.

  • "assert": implies the "fmt" feature, enables the assertion macros.
    This is a separate cargo feature because:

    • It uses nightly Rust features that are less stable than the "fmt" feature does.
    • It requires the std crate, because core::panic requires a string literal argument.
  • "constant_time_as_str": implies the "fmt" feature. An optimization that requires a few additional nightly features, allowing the as_bytes_alt methods and slice_up_to_len_alt methods to run in constant time, rather than linear time proportional to the truncated part of the slice.

No-std support

const_format is #![no_std], it can be used anywhere Rust can be used.

Caveat: The opt-in "assert" feature uses the std::panic macro to panic, as of 2020-09-06 core::panic requires the argument to be a literal.

Minimum Supported Rust Version

const_format requires Rust 1.46.0, because it uses looping an branching in const contexts.

Features that require newer versions of Rust, or the nightly compiler, need to be explicitly enabled with cargo features.