Compile-time string formatting.
This crate provides types and macros for formatting strings at compile-time.
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.
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.
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.
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]).
The "rust_1_64" feature enables these macros:
str_split]: splits a string constantBy 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.
This example works in Rust 1.46.0.
```rust use const_format::concatcp;
const NAME: &str = "Bob"; const FOO: &str = concatcp!(NAME, ", age ", 21u8,"!");
assert_eq!(FOO, "Bob, age 21!"); ```
This example works in Rust 1.46.0.
```rust 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 }
```
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.
```rust
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);
fn main(){ assert_eq!( FOO, "Message { ip: [Octet(127), Octet(0), Octet(0), Octet(1)], value: \"Hello, World!\" }" ); } ```
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.
```rust, compilefail use constformat::assertcp_ne;
macrorules! checkvalidpizza{ ($user:expr, $topping:expr) => { assertcpne!( $topping, "pineapple", "You can't put pineapple on pizza, {}", $user, ); } }
checkvalidpizza!("John", "salami"); checkvalidpizza!("Dave", "sausage"); checkvalidpizza!("Bob", "pineapple"); ```
This is the compiler output:
``text
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
```
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 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:
```rust const N: u32 = 1; asserteq!(constformat::concatcp!(N + 1, 2 + N), "23");
asserteq!(constformat::concatcp!(2u32, 2 + 1u8, 3u8 + 1), "234"); ```
Example of what does not compile:
compile_fail
assert_eq!(const_format::concatcp!(1 + 1, 2 + 1), "23");
None right now.
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:
toml
cfmt = {version = "0.*", package = "const_format"}
"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.
"rust151":
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.
"rust164": 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.
const_format is #![no_std], it can be used anywhere Rust can be used.
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.