This isn’t actually unsafe unless shared across threads right? Maybe the borrow checker needs to be more nuanced to differentiate this rather then outright banning it all together. It would increase the logic of the borrow checker by a lot though.
> This isn’t actually unsafe unless shared across threads right?
Multiple mutable borrows do not need multiple threads to cause UB. Consider the following:
fn main() {
let mut v = vec![0, 1, 2];
let e = &mut v[0];
v.push(3);
*e = 4;
}
rustc refuses to compile this code due to multiple mutable borrows of `v` [0]:
error[E0499]: cannot borrow `v` as mutable more than once at a time
--> <source>:4:5
|
3 | let e = &mut v[0];
| - first mutable borrow occurs here
4 | v.push(3);
| ^ second mutable borrow occurs here
5 | *e = 4;
| ------ first borrow later used here
If rustc allowed multiple mutable borrows here, `v.push(3)` would cause the underlying vec to reallocate, which invalidates `e`, and so `*e = 4` would cause UB.
