Hi,i tried to compile that code:

let mut x = 10;
let x_ref = &mut x;
let y = x;
*x_ref = 5;

and i got a error bout i cant use "x" while there is mut reference to x.

my question is, why i got that error? (i know about rust borrow rules, and about when we have &mut we cannot use this variable), but why? okay i understand that if we writing in a multi-thread code, there can be a data races, but this is single thread code, what bad can happen??

i think like that:
x is i32, i32 using a Copy trait, this is a single-thread code, what bad can happen??

i read a similar posts about my question, they talking about optimization in answers about reordering a instructions, and by making optimization in my code that can looks like that:

let mut x = 10;
let x_ref = &mut x;
*x_ref = 5;
let y = x;

y is 5, but we expected that y gonna be 10 etc...

but really? optimization is that "stupid"? okay by using that logic i can tell that code is unsafe:

let mut vector = vec![1];
vector.push(2);
vector.push(3);
vector.push(4);

cause optimization can reorder push calling and we can get a vector that looks like that:

1,4,2,3

but we expected:

1,2,3,4

that sounds stupid, and i don't trust to that answer.

maybe compiler doesn't know bout we writing in single thread code? that sounds better than optimization answer, but i don't think so.

maybe my code is too simple to be unsafe in single-thread, but why compiler doesn't know that my code is too simple to be unsafe in single-thread?

well please give me answer to this question, if you can provide me a single thread code that looks like mine and explain why its gonna be unsafe if its gonna compile, that will be super answer to my question.

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