Capturing a local variable in a local function [duplicate] - rust

Question

This question already has answers here:
How can I implement Ord when the comparison depends on data not part of the compared items?
(2 answers)
Closed 2 years ago.
I have this maze-like thing with 2 points in it and I want the distance between those points through the maze. To do this I will use A* (unless there is a better algorithm I don't know about), but to use A* I need my SN (search node) struct to implement the Ord trait. I want to use the Manhattan distance from 1 location to another as heuristic, so I had code like this (playground):
use std::cmp::Ordering;
fn main() {
let something_i_need_in_cmp = 5;
struct SN {
loc: u16,
step: u16,
}
impl Ord for SN {
fn cmp(&self, other: &Self) -> Ordering {
let tc = self.step + something_i_need_in_cmp;
let oc = other.step + something_i_need_in_cmp;
tc.cmp(&oc)
}
}
}
This does not work because the to variable is not captured:
error[E0434]: can't capture dynamic environment in a fn item
--> src/main.rs:13:34
|
13 | let tc = self.step + something_i_need_in_cmp;
| ^^^^^^^^^^^^^^^^^^^^^^^
|
= help: use the `|| { ... }` closure form instead
error[E0434]: can't capture dynamic environment in a fn item
--> src/main.rs:14:35
|
14 | let oc = other.step + something_i_need_in_cmp;
| ^^^^^^^^^^^^^^^^^^^^^^^
|
= help: use the `|| { ... }` closure form instead
error[E0277]: can't compare `SN` with `SN`
--> src/main.rs:11:10
|
11 | impl Ord for SN {
| ^^^ no implementation for `SN < SN` and `SN > SN`
|
= help: the trait `PartialOrd` is not implemented for `SN`
error[E0277]: the trait bound `SN: Eq` is not satisfied
--> src/main.rs:11:10
|
11 | impl Ord for SN {
| ^^^ the trait `Eq` is not implemented for `SN`
Is there any way around this?
The error hints use the `|| { ... }` closure form instead, but I don't seem to be able to get this right. Can this help fix it?
An answer I found online suggested to add the extra data (the to variable in my case) to the struct (here SN but for searching like this that would mean that the to variable (or a reference to it) is copied a 100 times, which is not efficient.

Answer 1

It looks to me like you're wanting to use a dynamic ordering for the points, but the big problem here is that Ord (and all other Rust traits) need to be defined statically, at compile-time.
My suggestion is that instead of using bare SN structs in your code, use SN structs 'decorated' with a distance, i.e. (u32, SN). These tuples derive Ord so they ought to work in your priority queues, etc.
You still need an impl Ord for SN implementation for the (Distance, SN) trick to work. The good news here is that any arbitrary Ord definition, including a default one (#[derive(...)]) will work.
use std::cmp::Ordering;
fn main() {
let something_i_need_in_cmp = 5;
#[derive(Eq, PartialEq, Ord, PartialOrd)]
struct SN {
loc: u16,
step: u16,
}
}