References (&T) are Copy.
Huon
On 01/06/14 09:42, Tommi wrote:
On 2014-06-01, at 1:02, Patrick Walton <[email protected]
<mailto:[email protected]>> wrote:
fn my_transmute<T:Clone,U>(value: T, other: U) -> U {
let mut x = Left(other);
let y = match x {
Left(ref mut y) => y,
Right(_) => fail!()
};
*x = Right(value);
(*y).clone()
}
If `U` implements `Copy`, then I don't see a (memory-safety) issue
here. And if `U` doesn't implement `Copy`, then it's same situation as
it was in the earlier example given by Matthieu, where there was an
assignment to an `Option<Box<str>>` variable while a different
reference pointing to that variable existed. The compiler shouldn't
allow that assignment just as in your example the compiler shouldn't
allow the assignment `x = Right(value);` (after a separate reference
pointing to the contents of `x` has been created) if `U` is not a
`Copy` type.
But, like I said in an earlier post, even though I don't see this
(transmuting a `Copy` type in safe code) as a memory-safety issue, it
is a code correctness issue. So it's a compromise between preventing
logic bugs (in safe code) and the convenience of more liberal mutation.
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