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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 a variety of Rust versions,
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 1.83.0

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

This requires Rust 1.83.0, because it uses mutable references in const fn.

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

```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 1.83.0, and the `"derive"` feature.

```rust
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
   |
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 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") ]`.

### 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 and `"rust_1_83"` feature,
requires Rust 1.83.0 because it uses mutable references in const fn.
This feature includes the [`formatc`]/[`writec`] formatting macros.

- `"derive"`: requires Rust 1.83.0, 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"`: requires Rust 1.83.0, 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"`:
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).

- `"rust_1_83"`: Enables the `"rust_1_64"` feature
and makes macros that evaluate to a value compatible with [inline const patterns].

# 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

[inline const patterns]: https://doc.rust-lang.org/1.83.0/unstable-book/language-features/inline-const-pat.html
