Creating instances within the if statement of a loop and using them - rust

Question

I am trying to parse a string of multiple lines that contain information about points and polygons.
The issue I am facing is with ownership in Rust. In languages like C# or C++, I can declare a variable and then instantiate it at a certain point, then use that variable later on. But in Rust, this causes an ownership problem.
How can I handle this situation in Rust?
fn main() {
// Point struct
struct Point {
x: f64,
y: f64,
}
// Polygon struct that has points
struct Polygon {
points: Vec<Point>,
}
// Implement the Polygon struct
impl Polygon {
fn new() -> Self {
Self { points: Vec::new() }
}
}
// Surface struct that has polygons
struct Surface {
polygons: Vec<Polygon>,
}
// Implement the Surface struct
impl Surface {
fn new() -> Self {
Self { polygons: Vec::new() }
}
}
// string polygons
let strs = "
SB
PB
PT 350 130
PT 350 170
PT 450 170
PE
PB
PT 370 150
PT 370 160
PT 430 160
PE
SE";
let mut surface = Surface::new();
let mut poly:Polygon = Polygon::new();
let lines: Vec<&str> = strs.lines().collect();
for line in lines.iter().map(|s| s.trim()) {
if line == "SB" {
surface = Surface::new();
} else if line == "PB" {
poly = Polygon::new();
surface.polygons.push(poly);
} else if line.starts_with("PT") {
let mut parts = line.split_whitespace();
parts.next(); // skip "PT"
let x: f64 = parts.next().unwrap().parse().unwrap();
let y: f64 = parts.next().unwrap().parse().unwrap();
let pt = Point { x, y };
poly.points.push(pt); // <-- Error
// surface.polygons.last_mut().unwrap().points.push(pt); // <-- OK
}
}
// Print the Surface polygons points
for poly in surface.polygons.iter() {
println!("PB");
for point in poly.points.iter() {
println!("\tx: {}, y: {}", point.x, point.y);
}
println!("PE");
}
}
error[E0382]: borrow of moved value: `poly`
--> src/main.rs:65:13
|
48 | let mut poly:Polygon = Polygon::new();
| -------- move occurs because `poly` has type `Polygon`, which does not implement the `Copy` trait
...
57 | surface.polygons.push(poly);
| ---- value moved here, in previous iteration of loop
...
65 | poly.points.push(pt); // <-- Error
| ^^^^^^^^^^^^^^^^^^^^ value borrowed here after move

Answer 1

There is two problems with your code for line == "PB" the first is that you're deleting all the data you collected in
poly, create a new one and then move that into surface.polygons, the other is the test on == "PB" would require a test if this is the first polygon.
So Rust just caught a bug for you!
Instead try this:
//...
} else if line == "PE" {
surface.polygons.push(poly);
poly = Polygon::new();
}
//...