Crate const_format

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 a variety of stable Rust versions and others that 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 “rust_1_51” 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.

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 1.64.0

The "rust_1_64" feature enables these macros:

str_split: splits a string 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 “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.

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 “rust_1_51” feature. Enables the assertcp, assertcp_eq, and assertcp_ne assertion macros.
“rust_1_51”: 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.
“rust_1_64”: Enables the str_split macro. Allows 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.

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_caserust_1_51
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_replacerust_1_51
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_splitrust_1_64
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

Caserust_1_51
The casing style of a string.

Derive Macros

ConstDebugderive
Derives const debug formatting for a type.