Can you explain?

Rust has both static (based on monomorphization, similar to a template instantiation in C++) and dynamic (based on vtables, similar to inheritance in C++) dispatch: https://doc.rust-lang.org/1.8.0/book/trait-objects.html. Both Rust and C++ are moving in the direction of encouraging static dispatch.

It may be similar, but it is implemented very differently, in exactly the "so can always work on unmodified structures" sense.

Trait objects are a double pointer: one to the vtable, and one to the data. C++'s dynamic dispatch uses a single pointer to a structure that has the vtable and the data. The memory layouts are very different.

"The memory layouts are very different."

Clarification: the memory layouts between C++ dynamic dispatch and rust dynamic dispatch are very different.

Rust imposes no requirements on the structure layout, whereas C++ adds some magic data. The magic data means it's awkward to make C++ do dynamic dispatch on a C struct pointer; but rust can do dynamic dispatch on a C struct pointer with no problem.