Macro const_panic::inline_macro

macro_rules! inline_macro {
    ($(($($args : tt) *)), * $(,) ? ; ($($params : tt) *) => $($code : tt) *) => { ... };
}

This is supported on crate feature non_basic only.

Helper macro for defining and using a macro_rules! macro inline.

The reason this was defined is to work around a limitation in stable const-eval, where you can’t call methods on generic types.

Example

Implementing panic formatting for a generic struct with flatten_panicvals.

use const_panic::{
    fmt::{self, ComputePvCount, FmtArg},
    ArrayString, PanicFmt, PanicVal, flatten_panicvals, inline_macro,
};

// Debug formatting
assert_eq!(
    ArrayString::<99>::from_panicvals(
        &Foo(10, 20).to_panicvals(FmtArg::DEBUG)
    ).unwrap(),
    "Foo(10, 20)"
);

// Alternate-Debug formatting
assert_eq!(
    ArrayString::<99>::from_panicvals(
        &Foo(false, true).to_panicvals(FmtArg::ALT_DEBUG)
    ).unwrap(),
    concat!(
        "Foo(\n",
        "    false,\n",
        "    true,\n",
        ")",
    ),
);

// Display formatting
assert_eq!(
    ArrayString::<99>::from_panicvals(
        &Foo("hmm", "what?").to_panicvals(FmtArg::DISPLAY)
    ).unwrap(),
    "Foo(hmm, what?)"
);




struct Foo<T>(T, T);

impl<T> PanicFmt for Foo<T>
where
    T: PanicFmt,
{
    type This = Self;
    type Kind = const_panic::IsCustomType;

    const PV_COUNT: usize = ComputePvCount{
        field_amount: 2,
        summed_pv_count: T::PV_COUNT * 2,
        delimiter: fmt::TypeDelim::Tupled,
    }.call();
}

// Because of limitations of stable const evaluation,
// this macro only implements panic formatting for
// - `Foo<bool>`
// - `Foo<u8>`
// - `Foo<&str>`
inline_macro!{
    (bool), (u8), (&str);
    ($T:ty)=>
    impl Foo<$T> {
        const fn to_panicvals(&self, fmtarg: FmtArg) -> [PanicVal<'_>; Foo::<$T>::PV_COUNT] {
            flatten_panicvals! {fmtarg;
                "Foo",
                open: fmt::OpenParen,
                    $T => self.0, fmt::COMMA_SEP,
                    $T => self.1, fmt::COMMA_TERM,
                close: fmt::CloseParen,
            }
        }
    }
}