Relax Debug constraints when debugging {HashMap,BTreeMap}::{Keys,Values}.

Fixed #41924.

Author: razielgn

src/liballoc/btree/map.rs

@@ -334,7 +334,7 @@ pub struct Keys<'a, K: 'a, V: 'a> {
 }
 
 #[stable(feature = "collection_debug", since = "1.17.0")]
-impl<'a, K: 'a + fmt::Debug, V: 'a + fmt::Debug> fmt::Debug for Keys<'a, K, V> {
+impl<'a, K: 'a + fmt::Debug, V: 'a> fmt::Debug for Keys<'a, K, V> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         f.debug_list().entries(self.clone()).finish()
     }
@@ -353,7 +353,7 @@ pub struct Values<'a, K: 'a, V: 'a> {
 }
 
 #[stable(feature = "collection_debug", since = "1.17.0")]
-impl<'a, K: 'a + fmt::Debug, V: 'a + fmt::Debug> fmt::Debug for Values<'a, K, V> {
+impl<'a, K: 'a, V: 'a + fmt::Debug> fmt::Debug for Values<'a, K, V> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         f.debug_list().entries(self.clone()).finish()
     }

src/libstd/collections/hash/map.rs

@@ -1407,7 +1407,7 @@ impl<'a, K, V> Clone for Keys<'a, K, V> {
 }
 
 #[stable(feature = "std_debug", since = "1.16.0")]
-impl<'a, K: Debug, V: Debug> fmt::Debug for Keys<'a, K, V> {
+impl<'a, K: Debug, V> fmt::Debug for Keys<'a, K, V> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         f.debug_list()
             .entries(self.clone())
@@ -1436,7 +1436,7 @@ impl<'a, K, V> Clone for Values<'a, K, V> {
 }
 
 #[stable(feature = "std_debug", since = "1.16.0")]
-impl<'a, K: Debug, V: Debug> fmt::Debug for Values<'a, K, V> {
+impl<'a, K, V: Debug> fmt::Debug for Values<'a, K, V> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         f.debug_list()
             .entries(self.clone())