Right, I think Julia would spot that if we get a Thing back, it's clearly not the Thing inside our array, because those are necessarily different Things. If th

Right, I think Julia would spot that if we get a Thing back, it's clearly not the Thing inside our array, because those are necessarily different Things. If the function does return Thing, in Rust we can implement the mistake, returning a copy, but there's no plausible way to implement the intended functionality, we can get the matching thing out of an array we're allowed to mutate -- by swapping it, but now some different Thing is in the array of Things instead, that's what swapping means. If we're given the actual array, not a (mutable) reference as a parameter, we can destroy it and keep just the matching Thing to return, but now our caller hasn't got a Things array, it was destructively moved to give us a parameter.