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2

1

I'm writing a function that works with a HashMap and at various points in the function I want to check if any of the values in the map meet a condition and if any does return the key for just one of the entries that meets the condition.

So far, I tried to first iterate over references to check my condition, and then use remove_entry to get an owned copy of the key rather than a borrow:

use std::collections::HashMap, hash::Hash;

fn remove_first_odd<K: Hash + Eq>(map: &mut HashMap<K, u64>) -> Option<K> (_, &value)

but this fails borrow checking, because I can't mutate the map while I am still holding a reference to one of the keys in it:

error[E0502]: cannot borrow `*map` as mutable because it is also borrowed as immutable
 --> src/lib.rs:8:45
 |
4 | let maybe_entry = map.iter().find(|(_, &value)| value & 1 == 1);
 | --- immutable borrow occurs here
...
8 | let (key_owned, _value_owned) = map.remove_entry(key_ref).unwrap();
 | ^^^^------------^^^^^^^^^
 | | |
 | | immutable borrow later used by call
 | mutable borrow occurs here

I believe that this is possible, because as long as remove_entry only dereferences my key before mutation and not after, this should be sound.

So is there a safe API to do this? If not, can one be created?

Note that:

• I can't clone my keys because they are generic without a clone bound


• I can't consume the HashMap with something like .into_iter()
  


• I only need to remove a single entry, not all of the entries that match

rust hashmap borrow-checker

share|improve this question

edited Mar 31 at 18:28

Shepmaster

179k2222 gold badges395395 silver badges560560 bronze badges

asked Mar 28 at 3:44

timotreetimotree

50522 silver badges2020 bronze badges

• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  "I can't clone my keys" in which world ? I don't think you can in safe mode.
  
  – Stargateur
  Mar 28 at 5:01
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  I'm afraid it's not possible, at least not if you want to return the key which has been removed. If you do not need to return the value you could use retain to remove the pair that fits your criteria.
  
  – hellow
  Mar 28 at 7:15
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Stargateur I edited it to make it more clear
  
  – timotree
  Mar 28 at 12:10
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @timotree so let me be more clear, key in a hashmap should be clonable in 99% of use case. That very odd situation you have here. I advice you to describe why you can't clone the key.
  
  – Stargateur
  Mar 28 at 12:13
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  See also How to iterate through a Hashmap, print the key/value and remove the value in Rust?; Removing items from a BTreeMap or BTreeSet found through iteration; Rust loop on HashMap while borrowing self;
  
  – Shepmaster
  Mar 31 at 18:34
  
  

  
  
  
  

add a comment | 

2

1

I'm writing a function that works with a HashMap and at various points in the function I want to check if any of the values in the map meet a condition and if any does return the key for just one of the entries that meets the condition.

So far, I tried to first iterate over references to check my condition, and then use remove_entry to get an owned copy of the key rather than a borrow:

use std::collections::HashMap, hash::Hash;

fn remove_first_odd<K: Hash + Eq>(map: &mut HashMap<K, u64>) -> Option<K> (_, &value)

but this fails borrow checking, because I can't mutate the map while I am still holding a reference to one of the keys in it:

error[E0502]: cannot borrow `*map` as mutable because it is also borrowed as immutable
 --> src/lib.rs:8:45
 |
4 | let maybe_entry = map.iter().find(|(_, &value)| value & 1 == 1);
 | --- immutable borrow occurs here
...
8 | let (key_owned, _value_owned) = map.remove_entry(key_ref).unwrap();
 | ^^^^------------^^^^^^^^^
 | | |
 | | immutable borrow later used by call
 | mutable borrow occurs here

I believe that this is possible, because as long as remove_entry only dereferences my key before mutation and not after, this should be sound.

So is there a safe API to do this? If not, can one be created?

Note that:

• I can't clone my keys because they are generic without a clone bound


• I can't consume the HashMap with something like .into_iter()
  


• I only need to remove a single entry, not all of the entries that match

rust hashmap borrow-checker

share|improve this question

edited Mar 31 at 18:28

Shepmaster

179k2222 gold badges395395 silver badges560560 bronze badges

asked Mar 28 at 3:44

timotreetimotree

50522 silver badges2020 bronze badges

• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  "I can't clone my keys" in which world ? I don't think you can in safe mode.
  
  – Stargateur
  Mar 28 at 5:01
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  I'm afraid it's not possible, at least not if you want to return the key which has been removed. If you do not need to return the value you could use retain to remove the pair that fits your criteria.
  
  – hellow
  Mar 28 at 7:15
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Stargateur I edited it to make it more clear
  
  – timotree
  Mar 28 at 12:10
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @timotree so let me be more clear, key in a hashmap should be clonable in 99% of use case. That very odd situation you have here. I advice you to describe why you can't clone the key.
  
  – Stargateur
  Mar 28 at 12:13
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  See also How to iterate through a Hashmap, print the key/value and remove the value in Rust?; Removing items from a BTreeMap or BTreeSet found through iteration; Rust loop on HashMap while borrowing self;
  
  – Shepmaster
  Mar 31 at 18:34
  
  

  
  
  
  

add a comment | 

I'm writing a function that works with a HashMap and at various points in the function I want to check if any of the values in the map meet a condition and if any does return the key for just one of the entries that meets the condition.

So far, I tried to first iterate over references to check my condition, and then use remove_entry to get an owned copy of the key rather than a borrow:

use std::collections::HashMap, hash::Hash;

fn remove_first_odd<K: Hash + Eq>(map: &mut HashMap<K, u64>) -> Option<K> (_, &value)

but this fails borrow checking, because I can't mutate the map while I am still holding a reference to one of the keys in it:

error[E0502]: cannot borrow `*map` as mutable because it is also borrowed as immutable
 --> src/lib.rs:8:45
 |
4 | let maybe_entry = map.iter().find(|(_, &value)| value & 1 == 1);
 | --- immutable borrow occurs here
...
8 | let (key_owned, _value_owned) = map.remove_entry(key_ref).unwrap();
 | ^^^^------------^^^^^^^^^
 | | |
 | | immutable borrow later used by call
 | mutable borrow occurs here

I believe that this is possible, because as long as remove_entry only dereferences my key before mutation and not after, this should be sound.

So is there a safe API to do this? If not, can one be created?

Note that:

• I can't clone my keys because they are generic without a clone bound


• I can't consume the HashMap with something like .into_iter()
  


• I only need to remove a single entry, not all of the entries that match

rust hashmap borrow-checker

share|improve this question

edited Mar 31 at 18:28

Shepmaster

179k2222 gold badges395395 silver badges560560 bronze badges

asked Mar 28 at 3:44

timotreetimotree

50522 silver badges2020 bronze badges

use std::collections::HashMap, hash::Hash;

fn remove_first_odd<K: Hash + Eq>(map: &mut HashMap<K, u64>) -> Option<K> (_, &value)

error[E0502]: cannot borrow `*map` as mutable because it is also borrowed as immutable
 --> src/lib.rs:8:45
 |
4 | let maybe_entry = map.iter().find(|(_, &value)| value & 1 == 1);
 | --- immutable borrow occurs here
...
8 | let (key_owned, _value_owned) = map.remove_entry(key_ref).unwrap();
 | ^^^^------------^^^^^^^^^
 | | |
 | | immutable borrow later used by call
 | mutable borrow occurs here

share|improve this question

edited Mar 31 at 18:28

Shepmaster

179k2222 gold badges395395 silver badges560560 bronze badges

asked Mar 28 at 3:44

timotreetimotree

50522 silver badges2020 bronze badges

share|improve this question

edited Mar 31 at 18:28

Shepmaster

179k2222 gold badges395395 silver badges560560 bronze badges

asked Mar 28 at 3:44

timotreetimotree

50522 silver badges2020 bronze badges

edited Mar 31 at 18:28

Shepmaster

179k2222 gold badges395395 silver badges560560 bronze badges

edited Mar 31 at 18:28

Shepmaster

179k2222 gold badges395395 silver badges560560 bronze badges

asked Mar 28 at 3:44

timotreetimotree

50522 silver badges2020 bronze badges

asked Mar 28 at 3:44

timotreetimotree

50522 silver badges2020 bronze badges