Macro impl_downcast

macro_rules! impl_downcast {
    (@impl_full
        $trait_:ident [$($param_types:tt)*]
        for [$($forall_types:ident),*]
        where [$($preds:tt)*]
    ) => { ... };
    (@impl_full_sync
        $trait_:ident [$($param_types:tt)*]
        for [$($forall_types:ident),*]
        where [$($preds:tt)*]
    ) => { ... };
    (@impl_body $trait_:ident [$($types:tt)*]) => { ... };
    (@impl_body_sync $trait_:ident [$($types:tt)*]) => { ... };
    (@inject_where [$($before:tt)*] types [] where [] [$($after:tt)*]) => { ... };
    (@inject_where [$($before:tt)*] types [$($types:ident),*] where [] [$($after:tt)*]) => { ... };
    (@inject_where [$($before:tt)*] types [$($types:ident),*] where [$($preds:tt)+] [$($after:tt)*]) => { ... };
    (@as_item $i:item) => { ... };
    ($trait_:ident   ) => { ... };
    ($trait_:ident <>) => { ... };
    (sync $trait_:ident   ) => { ... };
    (sync $trait_:ident <>) => { ... };
    ($trait_:ident < $($types:ident),* >) => { ... };
    (sync $trait_:ident < $($types:ident),* >) => { ... };
    ($trait_:ident < $($types:ident),* > where $($preds:tt)+) => { ... };
    (sync $trait_:ident < $($types:ident),* > where $($preds:tt)+) => { ... };
    ($trait_:ident assoc $($atypes:ident),*) => { ... };
    (sync $trait_:ident assoc $($atypes:ident),*) => { ... };
    ($trait_:ident assoc $($atypes:ident),* where $($preds:tt)+) => { ... };
    (sync $trait_:ident assoc $($atypes:ident),* where $($preds:tt)+) => { ... };
    ($trait_:ident < $($types:ident),* > assoc $($atypes:ident),*) => { ... };
    (sync $trait_:ident < $($types:ident),* > assoc $($atypes:ident),*) => { ... };
    ($trait_:ident < $($types:ident),* > assoc $($atypes:ident),* where $($preds:tt)+) => { ... };
    (sync $trait_:ident < $($types:ident),* > assoc $($atypes:ident),* where $($preds:tt)+) => { ... };
    (concrete $trait_:ident < $($types:ident),* >) => { ... };
    (sync concrete $trait_:ident < $($types:ident),* >) => { ... };
    (concrete $trait_:ident assoc $($atypes:ident = $aty:ty),*) => { ... };
    (sync concrete $trait_:ident assoc $($atypes:ident = $aty:ty),*) => { ... };
    (concrete $trait_:ident < $($types:ident),* > assoc $($atypes:ident = $aty:ty),*) => { ... };
    (sync concrete $trait_:ident < $($types:ident),* > assoc $($atypes:ident = $aty:ty),*) => { ... };
}

Adds downcasting support to traits that extend downcast::Downcast by defining forwarding methods to the corresponding implementations on std::any::Any in the standard library.

See https://users.rust-lang.org/t/how-to-create-a-macro-to-impl-a-provided-type-parametrized-trait/5289 for why this is implemented this way to support templatized traits.