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, and others that require Rust nightly, the sections below describe the features that are available.
Rust 1.46.0
These macros are the only things available in Rust 1.46.0:
-
concatcp
: Concatenatesintegers
,bool
, and&str
constants into a&'static str
constant. -
formatcp
:format
-like formatting which takesintegers
,bool
, and&str
constants, and emits a&'static str
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 implementsWriteMarker
.
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 "assert" 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 concatcp;
const NAME: &str = "Bob";
const FOO: &str = concatcp!;
assert_eq!;
Formatting primitive types
This example works in Rust 1.46.0.
use formatcp;
const NAME: &str = "John";
const FOO: &str = formatcp!;
assert_eq!;
# const
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 panics
This example demonstrates how you can use the [`assertc_ne`] macro to
do compile-time inequality assertions with formatted error messages.
This requires the "assert" feature,because as of writing these docs (2020-09-XX),
panicking at compile-time requires a nightly feature.
#![feature(const_mut_refs)]
use const_format::{StrWriter, assertc_ne, strwriter_as_str, writec};
use const_format::utils::str_eq;
macro_rules! check_valid_pizza{
($user:expr, $topping:expr) => {
assertc_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");
# fn main(){}
This is the compiler output, the first compilation error is there to have an indicator of what assertion failed, and the second is the assertion failure:
error: any use of this value will cause an error
--> src/lib.rs:140:1
|
22 | check_valid_pizza!("Bob", "pineapple");
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ exceeded interpreter step limit (see `#[const_eval_limit]`)
|
= note: `#[deny(const_err)]` on by default
= note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
error[E0080]: could not evaluate constant
--> /const_format/src/panicking.rs:32:5
|
32 | .
| ^ the evaluated program panicked at '
--------------------------------------------------------------------------------
module_path: rust_out
line: 22
assertion failed: LEFT != RIGHT
left: "pineapple"
right: "pineapple"
You can't put pineapple on pizza, Bob
--------------------------------------------------------------------------------
', /const_format/src/panicking.rs:31:1
|
= note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
error: aborting due to 2 previous errors
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 aType::<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!;
assert_eq!;
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:
= { = "0.*", = "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 theformatc
/writec
formatting macros. -
"derive": implies the "fmt" feature, provides the
ConstDebug
derive macro to format user-defined types at compile-time. This implicitly uses thesyn
crate, so clean compiles take a bit longer than without the feature. -
"assert": implies the "fmt" feature, enables the assertion macros. This is a separate cargo feature because:
-
It uses nightly Rust features that are less stable than the "fmt" feature does.
-
It requires the
std
crate, becausecore::panic
requires a string literal argument. -
"constant_time_as_str": implies the "fmt" feature. An optimization that requires a few additional nightly features, allowing the
as_bytes_alt
methods andslice_up_to_len_alt
methods to run in constant time, rather than linear time proportional to the truncated part of the slice.
"const_generics": Enables impls that use const generics, currently only used for ergonomics. Use this when const generics are usable in stable Rust.
"nightly_const_generics": Enables impls that use const generics, currently only used for ergonomics. This requires a nightly Rust compiler.
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.