Could someone explain why the following snippet does not work? It is not intuitive for me.
struct X;
struct Y {
x: X
}
impl Y {
pub fn foo(&mut self) {
self.x = self.x;
}
}
The error is “cannot move out of borrowed content” on self.x = self.x. I’d understand if I was assigning it to something else.
I am aware that the function is not meaningful, but it is a minimized example that I do not understand.
Thanks.
I’m guessing the compiler stops when it detects a move out of a borrow, and does not check the target of the move. You mention this is a minimal example; is there a “real” case that’s failing for you that doesn’t involve a “self” move?
Also, does Rust guarantee that moves can’t panic and are otherwise atomic operations?
Moves are guaranteed to not panic. They’re just a bitwise memory copy.
They’re not atomic, since the memcpy can be interrupted by a context switch.
Moves are guaranteed to not panic. They’re just a bitwise memory copy.
Thanks. I knew they were implemented as bitwise mem copies, but wasn’t sure if that’s a guarantee or the compiler could theoretically do something else that could panic (whether that would be a good implementation choice is a different matter).
They’re not atomic, since the memcpy can be interrupted by a context switch.
Sorry, by “atomic” I really meant “cannot fail”, an expansion on the panic statement - poor wording on my part.
Evaluating the source or destination can still panic (e.g. *vec.last_mut().unwrap() = vec2[0];). If it’s just a plain statement like self.x then it can’t.
Thanks for your replies.
I am attempting to make a simple game to learn Rust - the code in question was something similar to self.velocity = self.velocity * 2.0;, where velocity is a simple 2D vector struct. I solved the problem by defining an additional Mul implementation for vector references and then lending the reference to * instead, since it was actually a move. Trying to understand the issue brought me to the example above.
As you said, this is perhaps a case of the compiler choosing to be strict rather than doing some in-depth analysis.