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Exceptions have high cost so I don't see where you are coming from. Rust's RAII goes further along than C++'s. C++ is playing a game of catchup to steal the move semantics from Rust.

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According to http://stackoverflow.com/a/13836329/1517969 modern c++ exceptions are free when no throw occurs.

In addition, Rust Results/Options are free when no throw occurs. So the argument is moot.

No they aren't, you must issue a compare instruction to verify the result was successful.

Do you know whether branch prediction makes them effectively free, in places where the result is almost always successful?

Exception handling in C++ is absolutely zero-cost when no exception occurs. Look up the Itanium ABI. In Rust, even if no error occurs, the return value must still be checked.

Additionally, Move semantics were borrowed from C++, not the other way around.

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