[−][src]Derive Macro const_format::ConstDebug

#[derive(ConstDebug)]
{
    // Attributes available to this derive:
    #[cdeb]
}
This is supported on crate feature derive only.

Derives const debug formatting for a type.

Derives the FormatMarker trait, and defines an const_debug_fmt inherent method to format a type at compile-time.

Features

This derive macro is only available with the "derive" feature, and the nightly compiler, because at the time of writing these docs (2020-08-XX) mutable references in const fn require the unstable const_mut_refs feature..

Limitations

Compile-time formatting currently imposes these limitations on users, this derive macro has some mitigations for some of them.

Generic impls

Because the formatting of custom types is implemented with duck typing, it's not possible to format generic types, instead you must do either of these:

  • Provide all the implementations ahead of time, what the impls attribute is for.

  • Provide a macro that formats the type. The call_debug_fmt macro is a version of this that formats generic std types, nothing equivalent is provided in this crate for user defined types.

These are the things that this macro does to mitigate the limitations:

  • Allows users to provide a function/macro/wrapper to format a field.

  • Automatically detect some builtin/standard library types that are generic.

  • Allow users to ignore a field.

Container Attributes

These attributes go on the type itself, rather than the fields.

#[cdeb(debug_print)]

Panics with the output of the expanded derive.

#[cdeb(impls(....))]

Allows users to implement debug formatting for multiple different concrete instances of the type.

When this attribute is used it disables the default implementation that uses the type parameters generically.

#[cdeb(crate = "foo::bar")]

The path to the const_format crate, useful if you want to reexport the ConstDebug macro, or rename the const_format crate in the Cargo.toml .

Example of renaming the const_format crate in the Cargo.toml file:

cfmt = {version = "0.*", package = "const_format"}

Example:

#[derive(const_format::ConstDebug)]
#[cdeb(impls(
    "Foo<u8, u64>",
    "<T> Foo<u16, T>",
    "<T> Foo<u32, T> where T: 'static",
))]
struct Foo<A, B>(A, *const B);

In this example, there's exactly three impls of the const_debug_fmt method and FormatMarker trait.

Field attributes

#[cdeb(ignore)]

Ignoes the field, pretending that it doesn't exist.

#[cdeb(with = "module::function")]

Uses the function at the passed-in path to format the field,

The function is expected to have this signature:

const fn(&FieldType, &mut const_format::Formatter<'_>) -> Result<(), const_format::Error>

#[cdeb(with_macro = "module::the_macro")]

Uses the macro at the passed-in path to format the field,

The macro is expected to be callable like a function with this signature: 

const fn(&FieldType, &mut const_format::Formatter<'_>) -> Result<(), const_format::Error>

#[cdeb(with_wrapper = "module::Wrapper")]

Uses the wrapper type to print the field.

The wrapper is expected to wrap a reference to the field type, to have an implementation of the FormatMarker trait, and have a method with this signature:

const fn const_debug_fmt(
    self,
    &mut const_format::Formatter<'_>,
) -> Result<(), const_format::Error>

(self can be taken by reference or by value)

#[cdeb(is_a(....))]

Gives the derive macro a hint of what the type is.

For standard library types, this is necessary if you're using a type alias, since the derive macro detects those types syntactically.

These are the valid ways to use this attribute:

  • #[cdeb(is_a(array))]/#[cdeb(is_a(slice))]: Treats the field as being a slice/array, printing the elements with debug formatting.

  • #[cdeb(is_a(Option))]/#[cdeb(is_a(option))]: Treats the field as being an Option, printing the contents with debug formatting.

  • #[cdeb(is_a(newtype))]: Treats the field as being being a single field tuple struct, using the identifier of the field type as the name of the struct, then printing the single field with debug formatting.

  • #[cdeb(is_a(non_std))]/#[cdeb(is_a(not_std))]: This acts as an opt-out for the automatic detection of std types, most likely needed for types named Option.

Examples

Basic

This example demonstrates using the derive without using any helper attributes.

#![feature(const_mut_refs)]
 
use const_format::{ConstDebug, formatc};
 
use std::cmp::Ordering;
 
const E_FOO: &str = formatc!("{:?}", Enum::Foo);
const E_BAR: &str = formatc!("{:?}", Enum::Bar(10));
const E_BAZ: &str = formatc!("{:?}", Enum::Baz{order: Ordering::Less});
 
const S_UNIT: &str = formatc!("{:?}", Unit);
const S_BRACED: &str = formatc!("{:?}", Braced{is_true: false, optional: Some(Unit)});
 
assert_eq!(E_FOO, "Foo");
assert_eq!(E_BAR, "Bar(10)");
assert_eq!(E_BAZ, "Baz { order: Less }");
 
assert_eq!(S_UNIT, "Unit");
assert_eq!(S_BRACED, "Braced { is_true: false, optional: Some(Unit) }");
 
 
#[derive(ConstDebug)]
enum Enum {
    Foo,
    Bar(u32),
    Baz{
        order: Ordering,
    },
}
 
#[derive(ConstDebug)]
struct Unit;
 
#[derive(ConstDebug)]
struct Braced {
    is_true: bool,
    optional: Option<Unit>,
}

Generic type

This example demonstrates the #[cdeb(impls)] attribute, a workaround for deriving this trait for generic types, specifying a list of impls of types that uncnoditionally implement debug formatting

#![feature(const_mut_refs)]
 
use const_format::{ConstDebug, formatc};
 
use std::marker::PhantomData;
 
 
const S_U32: &str = formatc!("{:?}", Foo(10));

const S_STR: &str = formatc!("{:?}", Foo("hello"));
 
const S_PHANTOM: &str = formatc!("{:?}", Foo(PhantomData::<()>));
 
assert_eq!(S_U32, r#"Foo(10)"#);
assert_eq!(S_STR, r#"Foo("hello")"#);
assert_eq!(S_PHANTOM, r#"Foo(PhantomData)"#);
 
 
// This type implements debug formatting three times:
// - `Foo<u32>`
// - `Foo<&str>`
// - `Foo<PhantomData<T>>`: with a generic `T`
#[derive(ConstDebug)]
#[cdeb(impls(
    "Foo<u32>",
    "Foo<&str>",
    "<T> Foo<PhantomData<T>>",
))]
struct Foo<T>(T);

is_a attributes

This example demonstrates when you would use the is_a attributes.

#![feature(const_mut_refs)]
 
use const_format::{ConstDebug, formatc};
 
use std::{
    cmp::Ordering,
    marker::PhantomData,
    num::Wrapping,
};
 
const STRUCT: &Struct = &Struct {
    arr: [3, 5, 8, 13],
    opt: Some(Unit),
    wrap: Wrapping(21),
    not_option: Option(PhantomData), // This is not the standard library `Option`
};
 
const S_STRUCT: &str = formatc!("{STRUCT:#?}");
 
const EXPECTED: &str = "\
Struct {
    arr: [
        3,
        5,
        8,
        13,
    ],
    opt: Some(
        Unit,
    ),
    wrap: Wrapping(
        21,
    ),
    not_option: Option(
        PhantomData,
    ),
}";
 
fn main(){
    assert_eq!(S_STRUCT, EXPECTED);
}
 
#[derive(ConstDebug)]
struct Struct {
    #[cdeb(is_a(array))]
    arr: Array,
     
    #[cdeb(is_a(option))]
    opt: Opt,
     
    #[cdeb(is_a(newtype))]
    wrap: Wrapping<usize>,

    #[cdeb(is_a(not_std))]
    not_option: Option<u32>, 
     
}
 
type Array = [u32; 4];
type Opt = std::option::Option<Unit>;
 
#[derive(ConstDebug)]
struct Unit;
 
#[derive(ConstDebug)]
struct Option<T>(PhantomData<T>);

Renamed import

This example demonstrates that you can use all the macros when the const_format crate is renamed.

#![feature(const_mut_refs)]
use cfmt::{
    for_examples::Unit,
    ConstDebug, formatc,
};
 
#[derive(ConstDebug)]
#[cdeb(crate = "cfmt")]
struct Foo {
    bar: &'static str,
    baz: Unit
}
 
const TEXT: &str = formatc!("{:?}", Foo{ bar: "hello", baz: Unit });
 
assert_eq!(TEXT, r#"Foo { bar: "hello", baz: Unit }"#);