Macro optics::mark_known_source

macro_rules! mark_known_source {
    ($name:ident: $base:ty) => { ... };
    ($name:ident $(for<$($p:ident),+>)? : $base:ty) => { ... };
    ($name:ident if single ($(for<$($p:ident),+>)?, $base:ty)) => { ... };
    ($name:ident if single ($(for<$($p:ident),+>)?, $base:ty;
                            $($(for<$($p_rest:ident),+>)?, $rest:ty);+)) => { ... };
}

Mark an optics to have a single known source type by implementing OpticsKnownSource.