Crate const_format

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Expand description

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, some that require Rust 1.51.0, and others that require Rust nightly, the sections below describe the features that are available for each version.

Rust 1.46.0

These macros are the only things available in Rust 1.46.0:

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

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

  • str_get: Indexes a &'static str constant, returning None when the index is out of bounds.

  • str_index: Indexes a &'static str constant.

  • str_repeat: Creates a &'static str by repeating a &'static str constant times times.

  • str_splice: Replaces a substring in a &'static str constant.

Rust 1.51.0

By enabling the “const_generics” feature, you can use these macros:

  • map_ascii_case: Converts a &'static str constant to a different casing style, determined by a Case argument.

  • str_replace: Replaces all the instances of a pattern in a &'static str constant with another &'static str constant.

Rust 1.57.0

The “assertcp” feature enables the assertcp, assertcp_eq, and assertcp_ne macros. These macros are like the standard library assert macros, but evaluated at compile-time, with the limitation that they can only have primitive types as arguments (just like concatcp and formatcp).

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 “assertc” 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.

Examples

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!");

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.

#![feature(const_mut_refs)]

use const_format::{ConstDebug, formatc};

#[derive(ConstDebug)]
struct Message{
    ip: [Octet; 4],
    value: &'static str,
}

#[derive(ConstDebug)]
struct Octet(u8);

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

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

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

Formatted const assertions

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

This requires the “assertcp” feature, because using the panic macro at compile-time requires Rust 1.57.0.

ⓘ
#![feature(const_mut_refs)]

use const_format::assertcp_ne;

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

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

This is the compiler output:

error[E0080]: evaluation of constant value failed
  --> src/lib.rs:178:27
   |
20 | check_valid_pizza!("Bob", "pineapple");
   |                           ^^^^^^^^^^^ the evaluated program panicked at '
assertion failed: `(left != right)`
 left: `"pineapple"`
right: `"pineapple"`
You can't put pineapple on pizza, Bob
', src/lib.rs:20:27

Limitations

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");

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.

  • “assertc”: implies the “fmt” feature, enables the assertc, assertc_eq, and assertc_ne assertion macros.
    This feature was previously named “assert”, but it was renamed to avoid confusion with the “assertcp” feature.

  • “assertcp”: Requires Rust 1.57.0, implies the “const_generics” feature. Enables the assertcp, assertcp_eq, and assertcp_ne assertion macros.

  • “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.

  • “const_generics”: Requires Rust 1.51.0. Enables the macros listed in the Rust 1.51.0 section. Also changes the the implementation of the concatcp and formatcp macros to use const generics.

  • “more_str_macros”: Requires Rust nightly, implies the “const_generics” feature. Enables the str_split macro.

No-std support

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

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.

Re-exports

pub use crate::fmt::Error;
pub use crate::fmt::Formatter;
pub use crate::fmt::FormattingFlags;
pub use crate::fmt::Result;
pub use crate::fmt::StrWriter;
pub use crate::fmt::StrWriterMut;

Modules

fmtfmt

std::fmt-like api that can be used at compile-time.

for_examplesfmt

Types for the documentation examples.

marker_traitsfmt

Marker traits for types that can be formatted and/or be written to.

utilsfmt

Miscelaneous functions.

wrapper_typesfmt

Some wrapper types.

Macros

ascii_strfmt

Constructs an AsciiStr constant from an ascii string,

assertcassertc

Compile-time assertions with formatting.

assertc_eqassertc

Compile-time equality assertion with formatting.

assertc_neassertc

Compile-time inequality assertion with formatting.

assertcpassertcp

Compile-time assertion with formatting.

assertcp_eqassertcp

Compile-time equality assertion with formatting.

assertcp_neassertcp

Compile-time inequality assertion with formatting.

call_debug_fmtfmt

For debug formatting of some specific generic std types, and other types.

coerce_to_fmtfmt

Coerces a reference to a type that has a const_*_fmt method.

concatcfmt

Concatenates constants of standard library and/or user-defined types into a &'static str.

concatcp

Concatenates constants of primitive types into a &'static str.

formatcfmt

Formats constants of standard library and/or user-defined types into a &'static str.

formatcp

Formats constants of primitive types into a &'static str

impl_fmtfmt

For implementing debug or display formatting “manually”.

map_ascii_caseconst_generics

Converts the casing style of a &'static str constant, ignoring non-ascii unicode characters.

str_get

Indexes a &'static str constant, returning None when the index is not on a character boundary.

str_index

Indexes a &'static str constant.

str_repeat

Creates a &'static str by repeating a &'static str constant times times

str_replaceconst_generics

Replaces all the instances of $pattern in $input (a &'static str constant) with $replace_with (a &'static str constant).

str_splice

Replaces a substring in a &'static str constant. Returns both the new resulting &'static str, and the replaced substring.

str_splitmore_str_macros

Splits $string (a &'static str constant) with $splitter, returning an array of &'static strs.

strwriter_as_strDeprecatedfmt

Converts a &'static StrWriter to a &'static str, in a const/static initializer.

try_fmt

For returning early on an error, otherwise evaluating to ().

unwrapfmt

Equivalent to Result::unwrap, for use with const_format::Error errors.

unwrap_or_elsefmt

Equivalent to Result::unwrap_or_else but allows returning from the enclosing function.

writecfmt

Writes some formatted standard library and/or user-defined types into a buffer.

Structs

AsciiStrfmt

An ascii string slice.

PWrapperfmt

Wrapper for many std types, which implements the const_debug_fmt and/or const_display_fmt methods for them.

Slicedfmt

Wrapper for writing a range of a string slice.

SplicedStr

The return value of str_splice

Enums

Case

The casing style of a string.

Derive Macros

ConstDebugderive

Derives const debug formatting for a type.