Auto merge of #128711 - clarfonthey:default-iters-hash, r=dtolnay

impl `Default` for `HashMap`/`HashSet` iterators that don't already have it

This is a follow-up to #128261 that isn't included in that PR because it depends on:

* [x] rust-lang/hashbrown#542 (`Default`)
* [x] `hashbrown` release containing above

It also wasn't included in #128261 initially and should have its own FCP, since these are also insta-stable.

Changes added:

* `Default for hash_map::{Iter, IterMut, IntoIter, IntoKeys, IntoValues, Keys, Values, ValuesMut}`
* `Default for hash_set::{Iter, IntoIter}`

Changes that were added before FCP, but are being deferred to later:

* `Clone for hash_map::{IntoIter, IntoKeys, IntoValues} where K: Clone, V: Clone`
* `Clone for hash_set::IntoIter where K: Clone`

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

@@ -1438,6 +1438,14 @@ fn clone(&self) -> Self {
     }
 }
 
+#[stable(feature = "default_iters_hash", since = "CURRENT_RUSTC_VERSION")]
+impl<K, V> Default for Iter<'_, K, V> {
+    #[inline]
+    fn default() -> Self {
+        Iter { base: Default::default() }
+    }
+}
+
 #[stable(feature = "std_debug", since = "1.16.0")]
 impl<K: Debug, V: Debug> fmt::Debug for Iter<'_, K, V> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
@@ -1476,6 +1484,14 @@ pub(super) fn iter(&self) -> Iter<'_, K, V> {
     }
 }
 
+#[stable(feature = "default_iters_hash", since = "CURRENT_RUSTC_VERSION")]
+impl<K, V> Default for IterMut<'_, K, V> {
+    #[inline]
+    fn default() -> Self {
+        IterMut { base: Default::default() }
+    }
+}
+
 /// An owning iterator over the entries of a `HashMap`.
 ///
 /// This `struct` is created by the [`into_iter`] method on [`HashMap`]
@@ -1506,6 +1522,14 @@ pub(super) fn iter(&self) -> Iter<'_, K, V> {
     }
 }
 
+#[stable(feature = "default_iters_hash", since = "CURRENT_RUSTC_VERSION")]
+impl<K, V> Default for IntoIter<K, V> {
+    #[inline]
+    fn default() -> Self {
+        IntoIter { base: Default::default() }
+    }
+}
+
 /// An iterator over the keys of a `HashMap`.
 ///
 /// This `struct` is created by the [`keys`] method on [`HashMap`]. See its
@@ -1538,6 +1562,14 @@ fn clone(&self) -> Self {
     }
 }
 
+#[stable(feature = "default_iters_hash", since = "CURRENT_RUSTC_VERSION")]
+impl<K, V> Default for Keys<'_, K, V> {
+    #[inline]
+    fn default() -> Self {
+        Keys { inner: Default::default() }
+    }
+}
+
 #[stable(feature = "std_debug", since = "1.16.0")]
 impl<K: Debug, V> fmt::Debug for Keys<'_, K, V> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
@@ -1577,6 +1609,14 @@ fn clone(&self) -> Self {
     }
 }
 
+#[stable(feature = "default_iters_hash", since = "CURRENT_RUSTC_VERSION")]
+impl<K, V> Default for Values<'_, K, V> {
+    #[inline]
+    fn default() -> Self {
+        Values { inner: Default::default() }
+    }
+}
+
 #[stable(feature = "std_debug", since = "1.16.0")]
 impl<K, V: Debug> fmt::Debug for Values<'_, K, V> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
@@ -1665,6 +1705,14 @@ pub struct ValuesMut<'a, K: 'a, V: 'a> {
     inner: IterMut<'a, K, V>,
 }
 
+#[stable(feature = "default_iters_hash", since = "CURRENT_RUSTC_VERSION")]
+impl<K, V> Default for ValuesMut<'_, K, V> {
+    #[inline]
+    fn default() -> Self {
+        ValuesMut { inner: Default::default() }
+    }
+}
+
 /// An owning iterator over the keys of a `HashMap`.
 ///
 /// This `struct` is created by the [`into_keys`] method on [`HashMap`].
@@ -1687,6 +1735,14 @@ pub struct IntoKeys<K, V> {
     inner: IntoIter<K, V>,
 }
 
+#[stable(feature = "default_iters_hash", since = "CURRENT_RUSTC_VERSION")]
+impl<K, V> Default for IntoKeys<K, V> {
+    #[inline]
+    fn default() -> Self {
+        IntoKeys { inner: Default::default() }
+    }
+}
+
 /// An owning iterator over the values of a `HashMap`.
 ///
 /// This `struct` is created by the [`into_values`] method on [`HashMap`].
@@ -1709,6 +1765,14 @@ pub struct IntoValues<K, V> {
     inner: IntoIter<K, V>,
 }
 
+#[stable(feature = "default_iters_hash", since = "CURRENT_RUSTC_VERSION")]
+impl<K, V> Default for IntoValues<K, V> {
+    #[inline]
+    fn default() -> Self {
+        IntoValues { inner: Default::default() }
+    }
+}
+
 /// 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.

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

@@ -1244,6 +1244,14 @@ pub struct Iter<'a, K: 'a> {
     base: base::Iter<'a, K>,
 }
 
+#[stable(feature = "default_iters_hash", since = "CURRENT_RUSTC_VERSION")]
+impl<K> Default for Iter<'_, K> {
+    #[inline]
+    fn default() -> Self {
+        Iter { base: Default::default() }
+    }
+}
+
 /// An owning iterator over the items of a `HashSet`.
 ///
 /// This `struct` is created by the [`into_iter`] method on [`HashSet`]
@@ -1265,6 +1273,14 @@ pub struct IntoIter<K> {
     base: base::IntoIter<K>,
 }
 
+#[stable(feature = "default_iters_hash", since = "CURRENT_RUSTC_VERSION")]
+impl<K> Default for IntoIter<K> {
+    #[inline]
+    fn default() -> Self {
+        IntoIter { base: Default::default() }
+    }
+}
+
 /// A draining iterator over the items of a `HashSet`.
 ///
 /// This `struct` is created by the [`drain`] method on [`HashSet`].