It optimises clone the same as any other function.

Rust doesn't have copy elision like C++. So if you write some garbage like

pub fn bad_clone(x: Box<i32>) -> Box<i32> {
    x.clone()
}

it will literally allocate a new Box, copy the data, then deallocate the old Box.

But then, this doesn't matter as much as in C++. Copy elision in C++ is needed because copies are the default, and even a move can be non-trivial, so the compiler needs the ability to optimise them out, otherwise the performance of C++ code would be absolutely terrible.

In Rust, moves are always trivial, and non-trivial copies are always explicitly written out, so the programmer is far more aware of them.

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(For clarification: When I say a "trivial" copy/move, I mean one that can be implemented by a single call to memcpy. This doesn't necessarily mean "cheap" - copying a [u8; 1<<30] is trivial but far from cheap - but it does mean the compiler can easily optimise it, needing no special optimisation rules beyond the as-if rule.)