Auto merge of rust-lang#75163 - canova:map_into_keys_values, r=dtolnay

Implement `into_keys` and `into_values` for associative maps

This PR implements `into_keys` and `into_values` for HashMap and BTreeMap types. They are implemented as unstable, under `map_into_keys_values` feature.
Fixes rust-lang#55214.
r? @dtolnay

Author: bors

library/alloc/src/collections/btree/map.rs

@@ -1,3 +1,5 @@
+// ignore-tidy-filelength
+
 use core::borrow::Borrow;
 use core::cmp::Ordering;
 use core::fmt::Debug;
@@ -355,6 +357,30 @@ pub struct ValuesMut<'a, K: 'a, V: 'a> {
     inner: IterMut<'a, K, V>,
 }
 
+/// An owning iterator over the keys of a `BTreeMap`.
+///
+/// This `struct` is created by the [`into_keys`] method on [`BTreeMap`].
+/// See its documentation for more.
+///
+/// [`into_keys`]: BTreeMap::into_keys
+#[unstable(feature = "map_into_keys_values", issue = "75294")]
+#[derive(Debug)]
+pub struct IntoKeys<K, V> {
+    inner: IntoIter<K, V>,
+}
+
+/// An owning iterator over the values of a `BTreeMap`.
+///
+/// This `struct` is created by the [`into_values`] method on [`BTreeMap`].
+/// See its documentation for more.
+///
+/// [`into_values`]: BTreeMap::into_values
+#[unstable(feature = "map_into_keys_values", issue = "75294")]
+#[derive(Debug)]
+pub struct IntoValues<K, V> {
+    inner: IntoIter<K, V>,
+}
+
 /// An iterator over a sub-range of entries in a `BTreeMap`.
 ///
 /// This `struct` is created by the [`range`] method on [`BTreeMap`]. See its
@@ -1291,6 +1317,52 @@ impl<K: Ord, V> BTreeMap<K, V> {
 
         self.length = dfs(self.root.as_ref().unwrap().as_ref());
     }
+
+    /// Creates a consuming iterator visiting all the keys, in sorted order.
+    /// The map cannot be used after calling this.
+    /// The iterator element type is `K`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(map_into_keys_values)]
+    /// use std::collections::BTreeMap;
+    ///
+    /// let mut a = BTreeMap::new();
+    /// a.insert(2, "b");
+    /// a.insert(1, "a");
+    ///
+    /// let keys: Vec<i32> = a.into_keys().collect();
+    /// assert_eq!(keys, [1, 2]);
+    /// ```
+    #[inline]
+    #[unstable(feature = "map_into_keys_values", issue = "75294")]
+    pub fn into_keys(self) -> IntoKeys<K, V> {
+        IntoKeys { inner: self.into_iter() }
+    }
+
+    /// Creates a consuming iterator visiting all the values, in order by key.
+    /// The map cannot be used after calling this.
+    /// The iterator element type is `V`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(map_into_keys_values)]
+    /// use std::collections::BTreeMap;
+    ///
+    /// let mut a = BTreeMap::new();
+    /// a.insert(1, "hello");
+    /// a.insert(2, "goodbye");
+    ///
+    /// let values: Vec<&str> = a.into_values().collect();
+    /// assert_eq!(values, ["hello", "goodbye"]);
+    /// ```
+    #[inline]
+    #[unstable(feature = "map_into_keys_values", issue = "75294")]
+    pub fn into_values(self) -> IntoValues<K, V> {
+        IntoValues { inner: self.into_iter() }
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -1780,6 +1852,82 @@ impl<'a, K, V> Range<'a, K, V> {
     }
 }
 
+#[unstable(feature = "map_into_keys_values", issue = "75294")]
+impl<K, V> Iterator for IntoKeys<K, V> {
+    type Item = K;
+
+    fn next(&mut self) -> Option<K> {
+        self.inner.next().map(|(k, _)| k)
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.inner.size_hint()
+    }
+
+    fn last(mut self) -> Option<K> {
+        self.next_back()
+    }
+
+    fn min(mut self) -> Option<K> {
+        self.next()
+    }
+
+    fn max(mut self) -> Option<K> {
+        self.next_back()
+    }
+}
+
+#[unstable(feature = "map_into_keys_values", issue = "75294")]
+impl<K, V> DoubleEndedIterator for IntoKeys<K, V> {
+    fn next_back(&mut self) -> Option<K> {
+        self.inner.next_back().map(|(k, _)| k)
+    }
+}
+
+#[unstable(feature = "map_into_keys_values", issue = "75294")]
+impl<K, V> ExactSizeIterator for IntoKeys<K, V> {
+    fn len(&self) -> usize {
+        self.inner.len()
+    }
+}
+
+#[unstable(feature = "map_into_keys_values", issue = "75294")]
+impl<K, V> FusedIterator for IntoKeys<K, V> {}
+
+#[unstable(feature = "map_into_keys_values", issue = "75294")]
+impl<K, V> Iterator for IntoValues<K, V> {
+    type Item = V;
+
+    fn next(&mut self) -> Option<V> {
+        self.inner.next().map(|(_, v)| v)
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.inner.size_hint()
+    }
+
+    fn last(mut self) -> Option<V> {
+        self.next_back()
+    }
+}
+
+#[unstable(feature = "map_into_keys_values", issue = "75294")]
+impl<K, V> DoubleEndedIterator for IntoValues<K, V> {
+    fn next_back(&mut self) -> Option<V> {
+        self.inner.next_back().map(|(_, v)| v)
+    }
+}
+
+#[unstable(feature = "map_into_keys_values", issue = "75294")]
+impl<K, V> ExactSizeIterator for IntoValues<K, V> {
+    fn len(&self) -> usize {
+        self.inner.len()
+    }
+}
+
+#[unstable(feature = "map_into_keys_values", issue = "75294")]
+impl<K, V> FusedIterator for IntoValues<K, V> {}
+
 #[stable(feature = "btree_range", since = "1.17.0")]
 impl<'a, K, V> DoubleEndedIterator for Range<'a, K, V> {
     fn next_back(&mut self) -> Option<(&'a K, &'a V)> {

library/alloc/tests/btree/map.rs

@@ -1461,3 +1461,27 @@ fn test_into_iter_drop_leak_height_1() {
         assert_eq!(DROPS.load(Ordering::SeqCst), size);
     }
 }
+
+#[test]
+fn test_into_keys() {
+    let vec = vec![(1, 'a'), (2, 'b'), (3, 'c')];
+    let map: BTreeMap<_, _> = vec.into_iter().collect();
+    let keys: Vec<_> = map.into_keys().collect();
+
+    assert_eq!(keys.len(), 3);
+    assert!(keys.contains(&1));
+    assert!(keys.contains(&2));
+    assert!(keys.contains(&3));
+}
+
+#[test]
+fn test_into_values() {
+    let vec = vec![(1, 'a'), (2, 'b'), (3, 'c')];
+    let map: BTreeMap<_, _> = vec.into_iter().collect();
+    let values: Vec<_> = map.into_values().collect();
+
+    assert_eq!(values.len(), 3);
+    assert!(values.contains(&'a'));
+    assert!(values.contains(&'b'));
+    assert!(values.contains(&'c'));
+}

library/alloc/tests/lib.rs

@@ -4,6 +4,7 @@
 #![feature(drain_filter)]
 #![feature(exact_size_is_empty)]
 #![feature(map_first_last)]
+#![feature(map_into_keys_values)]
 #![feature(new_uninit)]
 #![feature(pattern)]
 #![feature(str_split_once)]

library/std/src/collections/hash/map.rs

@@ -872,6 +872,52 @@ where
     {
         self.base.retain(f)
     }
+
+    /// Creates a consuming iterator visiting all the keys in arbitrary order.
+    /// The map cannot be used after calling this.
+    /// The iterator element type is `K`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(map_into_keys_values)]
+    /// use std::collections::HashMap;
+    ///
+    /// let mut map = HashMap::new();
+    /// map.insert("a", 1);
+    /// map.insert("b", 2);
+    /// map.insert("c", 3);
+    ///
+    /// let vec: Vec<&str> = map.into_keys().collect();
+    /// ```
+    #[inline]
+    #[unstable(feature = "map_into_keys_values", issue = "75294")]
+    pub fn into_keys(self) -> IntoKeys<K, V> {
+        IntoKeys { inner: self.into_iter() }
+    }
+
+    /// Creates a consuming iterator visiting all the values in arbitrary order.
+    /// The map cannot be used after calling this.
+    /// The iterator element type is `V`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(map_into_keys_values)]
+    /// use std::collections::HashMap;
+    ///
+    /// let mut map = HashMap::new();
+    /// map.insert("a", 1);
+    /// map.insert("b", 2);
+    /// map.insert("c", 3);
+    ///
+    /// let vec: Vec<i32> = map.into_values().collect();
+    /// ```
+    #[inline]
+    #[unstable(feature = "map_into_keys_values", issue = "75294")]
+    pub fn into_values(self) -> IntoValues<K, V> {
+        IntoValues { inner: self.into_iter() }
+    }
 }
 
 impl<K, V, S> HashMap<K, V, S>
@@ -1154,6 +1200,28 @@ pub struct ValuesMut<'a, K: 'a, V: 'a> {
     inner: IterMut<'a, K, V>,
 }
 
+/// An owning iterator over the keys of a `HashMap`.
+///
+/// This `struct` is created by the [`into_keys`] method on [`HashMap`].
+/// See its documentation for more.
+///
+/// [`into_keys`]: HashMap::into_keys
+#[unstable(feature = "map_into_keys_values", issue = "75294")]
+pub struct IntoKeys<K, V> {
+    inner: IntoIter<K, V>,
+}
+
+/// An owning iterator over the values of a `HashMap`.
+///
+/// This `struct` is created by the [`into_values`] method on [`HashMap`].
+/// See its documentation for more.
+///
+/// [`into_values`]: HashMap::into_values
+#[unstable(feature = "map_into_keys_values", issue = "75294")]
+pub struct IntoValues<K, V> {
+    inner: IntoIter<K, V>,
+}
+
 /// A builder for computing where in a HashMap a key-value pair would be stored.
 ///
 /// See the [`HashMap::raw_entry_mut`] docs for usage examples.
@@ -1827,6 +1895,66 @@ where
     }
 }
 
+#[unstable(feature = "map_into_keys_values", issue = "75294")]
+impl<K, V> Iterator for IntoKeys<K, V> {
+    type Item = K;
+
+    #[inline]
+    fn next(&mut self) -> Option<K> {
+        self.inner.next().map(|(k, _)| k)
+    }
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.inner.size_hint()
+    }
+}
+#[unstable(feature = "map_into_keys_values", issue = "75294")]
+impl<K, V> ExactSizeIterator for IntoKeys<K, V> {
+    #[inline]
+    fn len(&self) -> usize {
+        self.inner.len()
+    }
+}
+#[unstable(feature = "map_into_keys_values", issue = "75294")]
+impl<K, V> FusedIterator for IntoKeys<K, V> {}
+
+#[unstable(feature = "map_into_keys_values", issue = "75294")]
+impl<K: Debug, V: Debug> fmt::Debug for IntoKeys<K, V> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_list().entries(self.inner.iter().map(|(k, _)| k)).finish()
+    }
+}
+
+#[unstable(feature = "map_into_keys_values", issue = "75294")]
+impl<K, V> Iterator for IntoValues<K, V> {
+    type Item = V;
+
+    #[inline]
+    fn next(&mut self) -> Option<V> {
+        self.inner.next().map(|(_, v)| v)
+    }
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.inner.size_hint()
+    }
+}
+#[unstable(feature = "map_into_keys_values", issue = "75294")]
+impl<K, V> ExactSizeIterator for IntoValues<K, V> {
+    #[inline]
+    fn len(&self) -> usize {
+        self.inner.len()
+    }
+}
+#[unstable(feature = "map_into_keys_values", issue = "75294")]
+impl<K, V> FusedIterator for IntoValues<K, V> {}
+
+#[unstable(feature = "map_into_keys_values", issue = "75294")]
+impl<K: Debug, V: Debug> fmt::Debug for IntoValues<K, V> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_list().entries(self.inner.iter().map(|(_, v)| v)).finish()
+    }
+}
+
 #[stable(feature = "drain", since = "1.6.0")]
 impl<'a, K, V> Iterator for Drain<'a, K, V> {
     type Item = (K, V);
@@ -3084,6 +3212,30 @@ mod test_map {
         assert!(values.contains(&6));
     }
 
+    #[test]
+    fn test_into_keys() {
+        let vec = vec![(1, 'a'), (2, 'b'), (3, 'c')];
+        let map: HashMap<_, _> = vec.into_iter().collect();
+        let keys: Vec<_> = map.into_keys().collect();
+
+        assert_eq!(keys.len(), 3);
+        assert!(keys.contains(&1));
+        assert!(keys.contains(&2));
+        assert!(keys.contains(&3));
+    }
+
+    #[test]
+    fn test_into_values() {
+        let vec = vec![(1, 'a'), (2, 'b'), (3, 'c')];
+        let map: HashMap<_, _> = vec.into_iter().collect();
+        let values: Vec<_> = map.into_values().collect();
+
+        assert_eq!(values.len(), 3);
+        assert!(values.contains(&'a'));
+        assert!(values.contains(&'b'));
+        assert!(values.contains(&'c'));
+    }
+
     #[test]
     fn test_find() {
         let mut m = HashMap::new();