Interestingly copy elision in C++ originally exists as a language feature because copy and move constructors are user-overridable and thus observable, so an explicit elision rule was required to allow the compiler to call the copy constructor fewer times than expected.

Rust doesn't have overridable copy/move, so elision could be treated as QoI issue.

More interestingly, now that C++ has guaranteed copy elision in some cases, in addition to allow returning non copyable objects, code that doesn't override constructors can rely on the address of an returned object not changing that open up some possibilities. I assume rust wants this.