Macro zerogc::impl_trace_for_nulltrace

macro_rules! impl_trace_for_nulltrace {
    (impl $(<$($lt:lifetime,)* $($param:ident),*>)? Trace for $target:path $(where $($where_clause:tt)*)?) => { ... };
}

Implement Trace and TraceImmutable as a no-op, based on the fact that a type implements NullTrace

Safety

Because this verifies that Self: NullTrace, it is known there are no values inside that need to be traced (allowing a no-op trace)

In other words, the unsafety is delegated to NullTrace