I have a for loop that iterates over a slice of Point structs. The Points will have some fields modified in the loop, so the function containing the loop requires a mutable reference to the slice.
The problem arises when I need to pass a (immutable) reference pointing to the slice to a function within the for loop that iterates over the mutable reference:
#[derive(Debug)]
struct Point {
x: i32,
y: i32,
}
fn main() {
let mut grid = vec![];
grid.push(Point { x: 10, y: 10 });
grid.push(Point { x: -1, y: 7 });
calculate_neighbors(&mut grid);
}
fn calculate_neighbors(grid: &mut [Point]) {
for pt in grid.iter_mut() {
pt.x = nonsense_calc(grid);
}
}
#[allow(unused_variables)]
fn nonsense_calc(grid: &[Point]) -> i32 {
unimplemented!();
}
Playground
error[E0502]: cannot borrow `*grid` as immutable because it is also borrowed as mutable
--> src/main.rs:18:30
|
17 | for pt in grid.iter_mut() {
| ---------------
| |
| mutable borrow occurs here
| mutable borrow used here, in later iteration of loop
18 | pt.x = nonsense_calc(grid);
| ^^^^ immutable borrow occurs here
The compiler complains that grid cannot be borrowed as immutable, because a mutable borrow already exists. This is correct, and I can see the problem it is trying to prevent, but how do I achieve what I need to do? Ideally, I do not have to create a copy of the grid, as this can be expensive.
A solution to avoid borrow the array for the iteration would be to use indexes:
fn calculate_neighbors(grid: &mut [Point]) {
for i in 0..grid.len() {
grid[i].x = nonsense_calc(grid);
}
}
Related
Just a question so I can better understand Rust.
Why does this fail:
for event in rulesengine::RuleEvent::into_enum_iter()
{
let mut event_stats: payloads::EventStats;
event_stats.name = rulesengine::RuleEvent::to_string(&event);
event_stats.event_count = event_counters.event_count[event as usize];
event_stats.event_participant_count = event_counters.event_participant_count[event as usize];
event_stats.event_trigger_count = event_counters.event_trigger_count[event as usize];
rules_stats.event_stats.push(event_stats);
}
With:
error[E0382]: assign to part of moved value: `event_stats`
--> src/motion.rs:317:37
|
316 | ... let mut event_stats: payloads::EventStats;
| --------------- move occurs because `event_stats` has type `payloads::EventStats`, which does not implement the `Copy` trait
317 | ... event_stats.name = rulesengine::RuleEvent::to_string(&event).clone();
| ^^^^^^^^^^^^^^^^ value partially assigned here after move
...
323 | ... },
| - value moved here, in previous iteration of loop
error: aborting due to previous error
When this works:
for event in rulesengine::RuleEvent::into_enum_iter()
{
let name = rulesengine::RuleEvent::to_string(&event);
let event_count = event_counters.event_count[event as usize];
let event_participant_count = event_counters.event_participant_count[event as usize];
let event_trigger_count = event_counters.event_trigger_count[event as usize];
let event_stats = payloads::EventStats { name, event_count, event_participant_count, event_trigger_count };
rules_stats.event_stats.push(event_stats);
}
or even better
for event in rulesengine::RuleEvent::into_enum_iter()
{
let event_stats = payloads::EventStats
{
name: rulesengine::RuleEvent::to_string(&event),
event_count: event_counters.event_count[event as usize],
event_participant_count: event_counters.event_participant_count[event as usize],
event_trigger_count: event_counters.event_trigger_count[event as usize],
};
rules_stats.event_stats.push(event_stats);
}
The variable event_stats is mutable yes, but local. It doesn't exist in the "previous iteration of loop" to be moved.
I believe the error is a mis-diagnosis of the problem. There have been compiler bugs in the past where the message incorrectly blames the loop for uninitialized variables. Perhaps this is another one of those cases.
The core issue is you are trying to assign a field to an uninitialized value:
let mut event_stats: payloads::EventStats;
event_stats.name = ...;
You have declared event_stats but have not initialized it. Rust does not allow you to piece-wise initialize a struct, at least not without a lot of extra juggling.
Both your working solutions create event_stats from a struct literal:
let event_stats = payloads::EventStats { name: ..., ... };
which is the correct way to initialize a struct.
I have an iterator in Rust that loops over a Vec<u8> and applies the same function at two different stages. I do this by chaining a couple of map functions together. Here is the relevant code (where example, example_function_1, and example_function_2 are stand-in variables and functions respectively):
NOTE: example.chunks() is a custom function! Not the default one on slices!
let example = vec![0, 1, 2, 3];
let mut hashers = Cycler::new([example_function_1, example_function_2].iter());
let ret: Vec<u8> = example
//...
.chunks(hashers.len())
.map(|buf| hashers.call(buf))
//...
.map(|chunk| hashers.call(chunk))
.collect();
Here is the code for Cycler:
pub struct Cycler<I> {
orig: I,
iter: I,
len: usize,
}
impl<I> Cycler<I>
where
I: Clone + Iterator,
I::Item: Fn(Vec<u8>) -> Vec<u8>,
{
pub fn new(iter: I) -> Self {
Self {
orig: iter.clone(),
len: iter.clone().count(),
iter,
}
}
pub fn len(&self) -> usize {
self.len
}
pub fn reset(&mut self) {
self.iter = self.orig.clone();
}
pub fn call(&mut self, buf: Bytes) -> Bytes {
// It is safe to unwrap because it should indefinietly continue without stopping
self.next().unwrap()(buf)
}
}
impl<I> Iterator for Cycler<I>
where
I: Clone + Iterator,
I::Item: Fn(Vec<u8>) -> Vec<u8>,
{
type Item = I::Item;
fn next(&mut self) -> Option<I::Item> {
match self.iter.next() {
next => next,
None => {
self.reset();
self.iter.next()
}
}
}
// No size_hint, try_fold, or fold methods
}
What confuses me is that the second time I reference hashers it says this:
error[E0499]: cannot borrow `hashers` as mutable more than once at a time
--> libpressurize/src/password/password.rs:28:14
|
21 | .map(|buf| hashers.call(buf))
| ----- ------- first borrow occurs due to use of `hashers` in closure
| |
| first mutable borrow occurs here
...
28 | .map(|chunk| hashers.call(chunk))
| --- ^^^^^^^ ------- second borrow occurs due to use of `hashers` in closure
| | |
| | second mutable borrow occurs here
| first borrow later used by call
Shouldn't this work because the mutable reference is not used at the same time?
Please let me know if more info/code is needed to answer this.
.map(|buf| hashers.call(buf))
You're probably thinking that in the above line, hashers is mutably borrowed to call it. That's true (since Cycler::call takes &mut self) but it's not what the compiler error is about. In this line, hashers is mutably borrowed to construct the closure |buf| hashers.call(buf), and that borrow lasts as long as the closure does.
Thus, when you write
.map(|buf| hashers.call(buf))
//...
.map(|chunk| hashers.call(chunk))
you are constructing two closures which live at the same time (assuming this is std::iter::Iterator::map) and mutably borrowing hashers for each of them, which is not allowed.
This error is actually protecting you against a side-effect hazard: it's not obvious (in a purely local analysis) what order the side effects of the two call()s will be performed in, because the map()s could do anything they like with the closures. Given the code you wrote, I assume you're doing this on purpose, but the compiler doesn't know that you know what you're doing.
(We can't even predict what the interleaving will be just because they're iterators. Inside of your //... there could be, say a .filter() step which leads to hashers.call(buf) being called several times between each call to hashers.call(chunk), or something else that produces a different number of outputs than inputs.)
If you know that you want the interleaving of side-effects that is “whenever either map() decides to call it”, then you can gain that freedom with a RefCell or other interior mutability, as dianhenglau's answer demonstrates.
Shouldn't this work because the mutable reference is not used at the same time?
No. The rules of references stated that "At any given time, you can have either one mutable reference or any number of immutable references", no matter if it is or isn't used at the same time. See this answer for the reason behind the rules.
As for workaround, since you're sure that the mutations do not occur simultaneously, you can use std::cell::RefCell as explained in this chapter. Modify the code into:
use std::cell::RefCell;
let example = vec![0, 1, 2, 3];
// Remove the "mut", wrap Cycler in RefCell.
let hashers = RefCell::new(Cycler::new([example_function_1, example_function_2].iter()));
let ret: Vec<u8> = example
//...
.chunks(hashers.borrow().len())
// Borrow hashers as immutable inside the closure, then borrow the Cycler as mutable.
.map(|buf| hashers.borrow_mut().call(buf))
//...
.map(|chunk| hashers.borrow_mut().call(chunk))
.collect();
I have Instance struct with HashSet field for storing nonordered numbers with easy removing and adding with duplicates control.
Then there is another struct called Matrix, which holds an two-dimensional array of Instance struct.
I create the new() method in trait for Instance to use in array initialization, but using this method gives error during compilation, saying that HashSet does not implement the Copy trait.
Here is the code:
#![allow(unused)]
use std::collections::HashSet;
struct Instance {
ids: HashSet<u8>,
value: u8,
}
trait IsInstance {
fn new() -> Instance;
}
impl IsInstance for Instance {
fn new() -> Instance {
Instance {
ids: [1, 2, 3, 5].iter().cloned().collect(),
value: 0,
}
}
}
/*
Line below is commented due to error:
error[E0204]: the trait `Copy` may not be implemented for this type
--> src/main.rs:26:6
|
5 | ids: HashSet,
| ---------------- this field does not implement `Copy`
...
26 | impl Copy for Instance {}
| ^^^^
*/
//impl Copy for Instance {}
impl Clone for Instance {
fn clone(&self) -> Instance {
Instance {
ids: self.ids,
value: self.value,
}
}
}
struct Matrix {
instances: [[Instance; 4]; 4],
status: u8,
}
fn main() {
let mut m = Matrix {
instances: [[Instance::new(); 4]; 4],
status: 0,
};
}
Compiling this gives error:
error[E0507]: cannot move out of `self.ids` which is behind a shared reference
--> src/main.rs:42:18
|
42 | ids: self.ids,
| ^^^^^^^^ move occurs because `self.ids` has type `std::collections::HashSet<u8>`, which does not implement the `Copy` trait
error[E0277]: the trait bound `Instance: std::marker::Copy` is not satisfied
--> src/main.rs:60:22
|
60 | instances : [[Instance::new(); 4]; 4],
| ^^^^^^^^^^^^^^^^^^^^ the trait `std::marker::Copy` is not implemented for `Instance`
|
= note: the `Copy` trait is required because the repeated element will be copied
error[E0277]: the trait bound `[Instance; 4]: std::marker::Copy` is not satisfied
--> src/main.rs:60:21
|
60 | instances : [[Instance::new(); 4]; 4],
| ^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::marker::Copy` is not implemented for `[Instance; 4]`
|
= note: the `Copy` trait is required because the repeated element will be copied
Is there any way to properly initialize this array? I clearly missed something, but didn't find HashSet copy implementation working with arrays. Maybe there is another way to implement this?
I clearly missed something, but didn't find HashSet copy implementation working with arrays.
That's because there isn't one! Copy means that a type is copyable bit-for-bit. That cannot be the case of a type like HashSet<T>. However what HashSet<T> is is default-initializable. Combine with the fact that arrays (under 32 elements for now) are also default-initializable, you can use the following:
let mut m = Matrix {
instances: Default::default(),
status: 0,
};
if you add
impl Default for Instance {
fn default() -> Self {
Self::new()
}
}
(Permalink to the playground)
But for example in the following code, I want to insert a value into a mutable BTreeMap/Hashmap in each loop, my simple solution triggered an error, what is the elegant way to do that in Rust?
Do I need clone the BTreeMap at the beginning of each loop?
pub fn build_string(word: &str, trie: &mut BTreeMap<&str, &Node>, word_index: usize) {
for char in word.chars() {
let char_label = char.to_string();
let insert_node = Node::new(&char_label, word_index);
let insert_result = trie.insert(&char_label, &insert_node);
println!("char: {}, at index", char);
}
}
error is
error[E0597]: `insert_node` does not live long enough
--> src/lib.rs:69:54
|
65 | pub fn build_string(word: &str, trie: &mut BTreeMap<&str, &Node>, word_index: usize) {
| - let's call the lifetime of this reference `'1`
...
69 | let insert_result = trie.insert(&char_label, &insert_node);
| -------------------------^^^^^^^^^^^^-
| | |
| | borrowed value does not live long enough
| argument requires that `insert_node` is borrowed for `'1`
70 | println!("char: {}, at index", char);
71 | }
| - `insert_node` dropped here while still borrowed
The issue is not with your BTreeMap but rather with the lifetime of your Node. You are creating a node inside the method build_string and trying to insert a reference to it into the map. However, once the method build_string goes out of scope, the local insert_node gets dropped but the reference would still exist in the tree. This is not allowed.
Therefore, you could change the BTreeMap to have values of Node instead of &Node. Then you would simply insert the insert_node into the tree which would take ownership of the node thus avoiding any reference leaks
This question is motivated by this CodinGame puzzle.
I am implementing a basic pathfinding algorithm using Dijkstra's method. It uses a boundary HashMap and a finished HashMap to hold pathfinding-related node info. In a particular loop, I find the highest-valued node in boundary, remove the node, add the node to finished, and add/update the node's neighbors' info in boundary.
Attempting to mutate boundary while looping over it is making Rust's borrow checker queasy, but the logic of the loop seems sound to me. How do I rewrite it so that the compiler shares my confidence? (Or fix the errors I'm missing, if that's the issue.)
Code:
On Rust Playground here
use std::io;
use std::collections::{HashSet, HashMap};
use std::cmp::Ordering;
use std::cell::RefCell;
struct NodeInfo {
nbrs: HashSet<i32>,
gwlinks: i32,
}
#[derive(PartialEq,PartialOrd)]
struct PFInfo {
avg: f32,
cum: i32,
dist: i32,
prev: Option<i32>,
}
impl Eq for PFInfo {}
impl Ord for PFInfo {
fn cmp(&self, other: &PFInfo) -> Ordering {
match self.partial_cmp(other) {
Some(ord) => ord,
None => Ordering::Equal
}
}
}
type Graph = HashMap<i32, RefCell<NodeInfo>>;
type PFGraph = HashMap<i32, PFInfo>;
// Find the path that passes the most gateway links per distance traveled,
// starting at a given node. This is meant to simulate the behavior of an
// "agent" which traverses the graph in the puzzle mentioned above.
fn generate_path(si: &i32, graph: &Graph) -> Vec<i32> {
let n = graph.len();
let mut boundary = PFGraph::with_capacity(n);
let mut finished = PFGraph::with_capacity(n);
boundary.insert( si.clone(),
PFInfo {
avg: 0.,
cum: graph.get(&si).unwrap().borrow().gwlinks,
dist: 0,
prev: None } );
// Keep grabbing the key corresponding the highest value until `boundary` is
// empty
while let Some( (currid, _) ) = boundary.iter().max_by_key(|x| x.1) {
// Move the node from `boundary` to `finished`
let val = boundary.remove(&currid).unwrap();
finished.insert(currid.clone(), val);
// Add or update all adjacent nodes that are not in `finished`
for nbrid in graph.get(&currid).unwrap()
.borrow()
.nbrs.iter()
.filter(|x| !finished.contains_key(x)) {
let currval = finished.get(&currid).unwrap();
let prev = Some(currid.clone());
let dist = currval.dist + 1;
let cum = currval.cum + graph.get(nbrid).unwrap().borrow().gwlinks;
let avg = cum as f32 / dist as f32;
boundary.insert(
nbrid.clone(),
PFInfo {
avg: avg,
cum: cum,
dist: dist,
prev: prev,
}
);
}
}
let mut path = Vec::new();
let mut currid = finished.iter().max_by_key(|x| x.1).unwrap().0.clone();
path.push(currid.clone());
while let Some(previd) = finished.get(&currid).unwrap().prev {
path.push(previd.clone());
currid = previd.clone();
}
path.reverse();
path
}
macro_rules! parse_input {
($x:expr, $t:ident) => ($x.trim().parse::<$t>().unwrap())
}
#[test]
fn test_generate_path() {
let mut inputs = "8 13 2
6 2
7 3
6 3
5 3
3 4
7 1
2 0
0 1
0 3
1 3
2 3
7 4
6 5
4
5".lines();
let header = inputs.next().unwrap().split_whitespace().collect::<Vec<_>>();
let n = parse_input!(header[0], i32); // the total number of nodes in the level, including the gateways
let l = parse_input!(header[1], i32); // the number of links
let e = parse_input!(header[2], i32); // the number of exit gateways
let mut graph = Graph::with_capacity(n as usize);
for node in 0..n {
graph.insert(node, RefCell::new(NodeInfo{ nbrs: HashSet::new(), gwlinks: 0 }));
}
let graph = graph;
for _ in 0..l as usize {
let link = inputs.next().unwrap();
let nodes = link.split(" ").collect::<Vec<_>>();
let n1 = parse_input!(nodes[0], i32); // N1 and N2 defines a link between these nodes
let n2 = parse_input!(nodes[1], i32);
graph.get(&n1).unwrap().borrow_mut().nbrs.insert(n2);
graph.get(&n2).unwrap().borrow_mut().nbrs.insert(n1);
}
let mut gateways = HashSet::new();
for _ in 0..e as usize {
let ei = parse_input!(inputs.next().unwrap(), i32); // the index of a gateway node
gateways.insert(ei);
}
let gateways = gateways;
for gwid in &gateways {
for gwnbr in &graph.get(gwid).unwrap().borrow().nbrs {
(&graph).get(&gwnbr).unwrap().borrow_mut().gwlinks += 1;
}
}
assert_eq!(generate_path(&0, &graph), vec![0, 3]);
}
Errors:
rustc 1.18.0 (03fc9d622 2017-06-06)
error[E0502]: cannot borrow `boundary` as mutable because it is also borrowed as immutable
--> <anon>:53:19
|
50 | while let Some( (currid, _) ) = boundary.iter().max_by_key(|x| x.1) {
| -------- immutable borrow occurs here
...
53 | let val = boundary.remove(&currid).unwrap();
| ^^^^^^^^ mutable borrow occurs here
...
76 | }
| - immutable borrow ends here
error[E0502]: cannot borrow `boundary` as mutable because it is also borrowed as immutable
--> <anon>:66:13
|
50 | while let Some( (currid, _) ) = boundary.iter().max_by_key(|x| x.1) {
| -------- immutable borrow occurs here
...
66 | boundary.insert(
| ^^^^^^^^ mutable borrow occurs here
...
76 | }
| - immutable borrow ends here
error: aborting due to 2 previous errors
I found a solution to my issue, and it's somewhat generalizable, which is what I was hoping for. The problem was that an implicit reference created in the while let statement was living to the end of the loop even though it was only needed on that one line. The borrow begins at .iter() and is no longer needed once the referenced value is cloned at the end of the expression.
while let Some( (currid, _) ) = boundary.iter().max_by_key(|x| x.1).clone() {
// ^---where borrow begins ^---where borrow could end
// Move the node from `boundary` to `finished`
let val = boundary.remove(&currid).unwrap();
finished.insert(currid.clone(), val);
...
} // <--- where borrow does end
The trick was moving the binding of currid into the loop. When the value is borrowed in the while let statement, the borrow checker apparently thinks the borrow needs to last throughout the loop. If, instead, the implicit borrow is made in a regular let binding, the borrow checker is smart enough to realize the borrow can be safely discarded at the end of the line.
while !boundary.is_empty() {
let currid = boundary.iter().max_by_key(|x| x.1).unwrap().0.clone();
// ^---where borrow begins ^---where borrow ends
// Move the node from `boundary` to `finished`
let val = boundary.remove(&currid).unwrap();
finished.insert(currid.clone(), val);
...
}
I guess the take away here is that if you need to mutate a structure in a loop that depends on it, put any borrows of the structure inside the loop and keep those borrows as short as possible – for example, by using clone.
This might be one of the situations eventually mitigated by the proposed non-lexical lifetimes.