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struct Point { x: isize, y: isize }
fn a() {
let mut p = Point {x: 3, y: 4};
let q = &p;
// This assignment is illegal because the field x is not
// inherently mutable; since `p` was made immutable, `p.x` is now
// immutable. Otherwise the type of &_q.x (&isize) would be wrong.
p.x = 5; //~ ERROR cannot assign to `p.x` because it is borrowed
q.x;
}
fn c() {
// this is sort of the opposite. We take a loan to the interior of `p`
// and then try to overwrite `p` as a whole.
let mut p = Point {x: 3, y: 4};
let q = &p.y;
p = Point {x: 5, y: 7};//~ ERROR cannot assign to `p` because it is borrowed
p.x; // silence warning
*q; // stretch loan
}
fn d() {
// just for completeness's sake, the easy case, where we take the
// address of a subcomponent and then modify that subcomponent:
let mut p = Point {x: 3, y: 4};
let q = &p.y;
p.y = 5; //~ ERROR cannot assign to `p.y` because it is borrowed
*q;
}
fn main() {
}
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