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Allow replacing HashMap entries #44278

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Merged
merged 3 commits into from
Sep 28, 2017
Merged

Allow replacing HashMap entries #44278

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e9f01bc
Added a way to retrieve the key out of a HashMap when it's being repl…
Binero Sep 2, 2017
a312b47
Marked `Entry::replace` as unstable.
Binero Sep 3, 2017
d3de465
Addressed @BurntSuchi's remarks regarding Entry::replace
Binero Sep 12, 2017
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30 changes: 30 additions & 0 deletions src/libstd/collections/hash/map.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2161,6 +2161,36 @@ impl<'a, K, V> OccupiedEntry<'a, K, V> {
fn take_key(&mut self) -> Option<K> {
self.key.take()
}

/// Replaces the entry, returning the old key and value.
///
/// # Examples
///
/// ```
/// #![feature(map_entry_replace)]
/// use std::collections::HashMap;
/// use std::collections::hash_map::Entry;
///
/// let mut map: HashMap<String, u32> = HashMap::new();
/// map.insert("poneyland".to_string(), 15);
///
/// if let Entry::Occupied(entry) = map.entry("poneyland".to_string()) {
/// let (old_key, old_value): (String, u32) = entry.replace(16);
/// assert_eq!(old_key, "poneyland");
/// assert_eq!(old_value, 15);
/// }
///
/// assert_eq!(map.get("poneyland"), Some(&16));
/// ```
#[unstable(feature = "map_entry_replace", issue = "44286")]
pub fn replace(mut self, value: V) -> (K, V) {
let (old_key, old_value) = self.elem.read_mut();

let old_key = mem::replace(old_key, self.key.unwrap());
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One can take the key out of the entry using take_key before replace, so this key.unwrap() can fail. It needs a custom message at least.

let old_value = mem::replace(old_value, value);

(old_key, old_value)
}
}

impl<'a, K: 'a, V: 'a> VacantEntry<'a, K, V> {
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