Macro sp_core::impl_maybe_marker

macro_rules! impl_maybe_marker {
    ($($(#[$doc : meta]) + trait $trait_name : ident : $($trait_bound : path), +
   ;) +) => { ... };
}

Macro for creating Maybe* marker traits.

Such a maybe-marker trait requires the given bound when feature = std and doesn’t require the bound on no_std. This is useful for situations where you require that a type implements a certain trait with feature = std, but not on no_std.

Example

sp_core::impl_maybe_marker! {
    /// A marker for a type that implements `Debug` when `feature = std`.
    trait MaybeDebug: std::fmt::Debug;
    /// A marker for a type that implements `Debug + Display` when `feature = std`.
    trait MaybeDebugDisplay: std::fmt::Debug, std::fmt::Display;
}