Macro def_anyhow_variant

macro_rules! def_anyhow_variant {
    (
        $(#[$attr:meta])* $vis:vis $name:ident,
        $enum_name:ident $variant_name:ident
        $(, $from:ty => $context:tt)* $(,)?
    ) => { ... };
}

Define a newtype + it’s variant in the specified type. This is useful when you want to define a new error type, but also want to define a variant for it in another enum.