For panicking with formatting in const contexts.

This library exists because the panic macro was stabilized for use in const contexts in Rust 1.57.0, without formatting support.

All of the types that implement the PanicFmt trait can be formatted in panics.

Examples

Basic
Custom Types

Basic

use const_panic::concat_assert;

const FOO: u32 = 10;
const BAR: u32 = 0;
const _: () = assert_non_zero(FOO, BAR);

#[track_caller]
const fn assert_non_zero(foo: u32, bar: u32) {
    concat_assert!{
        foo != 0 && bar != 0,
        "\nneither foo nor bar can be zero!\nfoo: ", foo, "\nbar: ", bar
    }
}

The above code fails to compile with this error:

error[E0080]: evaluation of constant value failed
 --> src/lib.rs:20:15
  |
8 | const _: () = assert_non_zero(FOO, BAR);
  |               ^^^^^^^^^^^^^^^^^^^^^^^^^ the evaluated program panicked at '
neither foo nor bar can be zero!
foo: 10
bar: 0', src/lib.rs:8:15

When called at runtime

use const_panic::concat_assert;

assert_non_zero(10, 0);

#[track_caller]
const fn assert_non_zero(foo: u32, bar: u32) {
    concat_assert!{
        foo != 0 && bar != 0,
        "\nneither foo nor bar can be zero!\nfoo: ", foo, "\nbar: ", bar
    }
}

it prints this:

thread 'main' panicked at '
neither foo nor bar can be zero!
foo: 10
bar: 0', src/lib.rs:6:1
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Custom types

Panic formatting for custom types can be done in these ways (in increasing order of verbosity):

Using the PanicFmt derive macro (requires the opt-in "derive" feature)
Using the impl_panicfmt macro (requires the default-enabled "non_basic" feature)
Using the flatten_panicvals macro (requires the default-enabled "non_basic" feature)
Manually implementing the PanicFmt trait as described in its docs.

This example uses the PanicFmt derive approach.

use const_panic::{PanicFmt, concat_panic};

const LAST: u8 = {
    Foo{
        x: &[],
        y: Bar(false, true),
        z: Qux::Left(23),
    }.pop().1
};

impl Foo<'_> {
    /// Pops the last element
    ///
    /// # Panics
    ///
    /// Panics if `self.x` is empty
    #[track_caller]
    const fn pop(mut self) -> (Self, u8) {
        if let [rem @ .., last] = self.x {
            self.x = rem;
            (self, *last)
        } else {
            concat_panic!(
                "\nexpected a non-empty Foo, found: \n",
                // uses alternative Debug formatting for `self`,
                // otherwise this would use regular Debug formatting.
                alt_debug: self
            )
        }
    }
}

#[derive(PanicFmt)]
struct Foo<'a> {
    x: &'a [u8],
    y: Bar,
    z: Qux,
}

#[derive(PanicFmt)]
struct Bar(bool, bool);

#[derive(PanicFmt)]
enum Qux {
    Up,
    Down { x: u32, y: u32 },
    Left(u64),
}

The above code fails to compile with this error:

error[E0080]: evaluation of constant value failed
  --> src/lib.rs:57:5
   |
7  | /     Foo{
8  | |         x: &[],
9  | |         y: Bar(false, true),
10 | |         z: Qux::Left(23),
11 | |     }.pop().1
   | |___________^ the evaluated program panicked at '
expected a non-empty Foo, found:
Foo {
    x: [],
    y: Bar(
        false,
        true,
    ),
    z: Left(
        23,
    ),
}', src/lib.rs:11:7

Limitations

Arguments to the formatting/panicking macros must have a fully inferred concrete type, because const_panic macros use duck typing to call methods on those arguments.

One effect of that limitation is that you will have to pass suffixed integer literals (eg: 100u8) when those integers aren't inferred to be a concrete type.

Panic message length

The panic message can only be up to MAX_PANIC_MSG_LEN long, after which it is truncated.

Cargo features

"non_basic"(enabled by default): Enables support for formatting structs, enums, and arrays.

Without this feature, you can effectively only format primitive types (custom types can manually implement formatting with more difficulty).

"rust_1_64"(disabled by default): Enables formatting of additional items that require Rust 1.64.0 to do so.
"derive"(disabled by default): Enables the PanicFmt derive macro.

Plans

None for now

No-std support

const_panic is #![no_std], it can be used anywhere Rust can be used.

Minimum Supported Rust Version

This requires Rust 1.57.0, because it uses the panic macro in a const context.

TTWNO/const_panic