Type-erasure does have an inherent overhead. Sure, optimizations can be made, but they can be fickle and specialization is basically implicit monomorphization.

Using C macros to replicate Rust's monomorphism has several drawbacks: they are inherently unhygienic, even in comparison to Rust's own; you can't set type-bounds; they aren't even a part of C proper, etc.

I prefer Rust's approach with the choice between generics, macros, dyn and Any.