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[](https://crates.io/crates/const_format)
[](https://docs.rs/const_format/*)
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 require Rust nightly,
the sections below describe the features that are available for each version.
### Rust 1.57.0
These macros are available in Rust 1.57.0:
- [`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.
- [`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.<br>
[`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.<br>
These macros are like the standard library assert macros, but evaluated at compile-time.
# Examples
### Concatenation of primitive types
```rust
use const_format::concatcp;
const NAME: &str = "Bob";
const FOO: &str = concatcp!(NAME, ", age ", 21u8,"!");
assert_eq!(FOO, "Bob, age 21!");
```
### Formatting primitive types
```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 }
```
### 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.
```rust
#![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.
```rust, compile_fail
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:
```text
error[E0080]: evaluation of constant value failed
--> src/lib.rs:178:27
|
assertion failed: `(left != right)`
left: `"pineapple"`
right: `"pineapple"`
You can't put pineapple on pizza, Bob
', src/lib.rs:20:27
```
<div id="macro-limitations"></div>
# Limitations
All of the macros from `const_format` have these limitations:
- The formatting macros that expand to
`&'static str`s 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](#integer-args).
- They cannot be used places that take string literals.
So `#[doc = "foobar"]` cannot be replaced with `#[doc = concatcp!("foo", "bar") ]`.
<span id="integer-args"></span>
### 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:
```rust
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:
```rust,compile_fail
assert_eq!(const_format::concatcp!(1 + 1, 2 + 1), "23");
```
# Plans
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:
```toml
[dependencies]
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.<br>
This feature includes the [`formatc`]/[`writec`] formatting macros.
- `"derive"`: requires Rust nightly, implies the `"fmt"` feature,
provides the [`ConstDebug`] derive macro to format user-defined types at compile-time.<br>
This implicitly uses the `syn` crate, so clean compiles take a bit longer than without the feature.
- `"assertc"`: requires Rust nightly, implies the `"fmt"` feature,
enables the [`assertc`], [`assertc_eq`], and [`assertc_ne`] assertion macros.<br>
This feature was previously named `"assert"`,
but it was renamed to avoid confusion with the `"assertcp"` feature.
- `"assertcp"`:
Enables the [`assertcp`], [`assertcp_eq`], and [`assertcp_ne`] assertion macros.
- `"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).
# 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 1.57.0.
Features that require newer versions of Rust, or the nightly compiler,
need to be explicitly enabled with cargo features.
[`assertc`]: https://docs.rs/const_format/0.2.*/const_format/macro.assertc.html
[`assertc_eq`]: https://docs.rs/const_format/0.2.*/const_format/macro.assertc_eq.html
[`assertc_ne`]: https://docs.rs/const_format/0.2.*/const_format/macro.assertc_ne.html
[`assertcp`]: https://docs.rs/const_format/0.2.*/const_format/macro.assertcp.html
[`assertcp_eq`]: https://docs.rs/const_format/0.2.*/const_format/macro.assertcp_eq.html
[`assertcp_ne`]: https://docs.rs/const_format/0.2.*/const_format/macro.assertcp_ne.html
[`concatcp`]: https://docs.rs/const_format/0.2.*/const_format/macro.concatcp.html
[`formatcp`]: https://docs.rs/const_format/0.2.*/const_format/macro.formatcp.html
[`format`]: https://doc.rust-lang.org/std/macro.format.html
[`std::fmt`]: https://doc.rust-lang.org/std/fmt/index.html
[`const_format::fmt`]: https://docs.rs/const_format/0.2.*/const_format/fmt/index.html
[`concatc`]: https://docs.rs/const_format/0.2.*/const_format/macro.concatc.html
[`formatc`]: https://docs.rs/const_format/0.2.*/const_format/macro.formatc.html
[`writec`]: https://docs.rs/const_format/0.2.*/const_format/macro.writec.html
[`write`]: https://doc.rust-lang.org/std/macro.write.html
[`Formatter`]: https://docs.rs/const_format/0.2.*/const_format/fmt/struct.Formatter.html
[`StrWriter`]: https://docs.rs/const_format/0.2.*/const_format/fmt/struct.StrWriter.html
[`ConstDebug`]: https://docs.rs/const_format/0.2.*/const_format/derive.ConstDebug.html
[`FormatMarker`]: https://docs.rs/const_format/0.2.*/const_format/marker_traits/trait.FormatMarker.html
[`WriteMarker`]: https://docs.rs/const_format/0.2.*/const_format/marker_traits/trait.WriteMarker.html
[`map_ascii_case`]: https://docs.rs/const_format/0.2.*/const_format/macro.map_ascii_case.html
[`Case`]: https://docs.rs/const_format/0.2.*/const_format/enum.Case.html
[`str_get`]: https://docs.rs/const_format/0.2.*/const_format/macro.str_get.html
[`str_index`]: https://docs.rs/const_format/0.2.*/const_format/macro.str_index.html
[`str_repeat`]: https://docs.rs/const_format/0.2.*/const_format/macro.str_repeat.html
[`str_splice`]: https://docs.rs/const_format/0.2.*/const_format/macro.str_splice.html
[`str_replace`]: https://docs.rs/const_format/0.2.*/const_format/macro.str_replace.html
[`str_split`]: https://docs.rs/const_format/0.2.*/const_format/macro.str_split.html
[`str::replace`]: https://doc.rust-lang.org/std/primitive.str.html#method.replace