How can I modify self in a closure called from a member function?
I am trying to calculate legal chess moves and am having problems satisfying the borrow checker. I have a struct Chess that implements these methods (non-important code replaced by ... ):
// internal iterator over (possibly not legal) moves
fn get_moves<F>(&self, func: F)
where
F: Fn(/* ... */),
{
func(/* ... */); // move 1
func(/* ... */); // move 2
func(/* ... */); // etc...
}
fn is_legal_move(&mut self) -> bool {
// notice this takes a mutable self. For performance
// reasons, the move is made, legality is checked, then I
// undo the move, so it must be mutable to be able to move pieces
make_move(/* ... */);
// check if legal
undo_move(/* ... */);
//return true if legal
}
fn get_legal_moves(&self) /* -> ... */ {
self.get_moves(|/* ... */| {
if self.is_legal_move(/* ... */) { // <-- error here
// do something with legal move
}
})
}
I get a compilation error in get_legal_moves because I am modifying self inside the closure while 'get_moves' is still borrowing self .
I created a simplified example showing the problem I am trying to solve:
struct Tester {
x: i8,
}
impl Tester {
fn traverse<Func>(&mut self, mut f: Func)
where
Func: FnMut(),
{
//in real-world, this would probably iterate over something
f();
}
}
fn main() {
let mut tester = Tester { x: 8 };
tester.traverse(|| {
tester.x += 1; //I want to be able to modify tester here
});
println!("{}", tester.x);
}
Playground
The error:
error[E0499]: cannot borrow `tester` as mutable more than once at a time
--> src/main.rs:17:21
|
17 | tester.traverse(|| {
| ------ ^^ second mutable borrow occurs here
| |
| first mutable borrow occurs here
18 | tester.x += 1; //I want to be able to modify tester here
| ------ borrow occurs due to use of `tester` in closure
19 | });
| - first borrow ends here
How can I satisfy the borrow checker so the code can compile?
The simplest change you can make is to pass the reference to the closure:
struct Tester {
x: i8,
}
impl Tester {
fn traverse<F>(&mut self, mut f: F)
where
F: FnMut(&mut Tester),
{
f(self);
}
}
fn main() {
let mut tester = Tester { x: 8 };
tester.traverse(|z| z.x += 1);
println!("{}", tester.x);
}
This prevents having multiple mutable references (also known as aliasing), which are disallowed in Rust.
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