dyn Trait vs Generics

Start with the idea

A generic bound is monomorphised: one specialised copy per concrete type, direct calls, inlinable, larger binary. dyn Trait keeps one copy and dispatches through a vtable, which is smaller and lets heterogeneous types share a collection at the cost of an indirection.

With a generic T: Trait, callers supply a concrete type and the compiler can specialize code for that type. This often permits inlining and avoids a per-call virtual dispatch, but different concrete instantiations can increase code size and compile time. With dyn Trait, a trait object pairs data with a vtable for methods allowed by object safety. Calls through it use dynamic dispatch, and a collection can hold different concrete implementations behind one pointer type.

Give the interviewer a design example.