Rollup merge of #23559 - aturon:future-proof-map-index, r=Gankro

 This commit removes the `IndexMut` impls on `HashMap` and `BTreeMap`, in
order to future-proof the API against the eventual inclusion of an
`IndexSet` trait.

Ideally, we would eventually be able to support:

```rust
map[owned_key] = val;
map[borrowed_key].mutating_method(arguments);
&mut map[borrowed_key];
```

but to keep the design space as unconstrained as possible, we do not
currently want to support `IndexMut`, in case some other strategy will
eventually be needed.

Code currently using mutating index notation can use `get_mut` instead.

[breaking-change]

Closes #23448

r? @Gankro

1 files changed, 1 insertions, 10 deletions

diff --git a/src/libcollections/btree/map.rs b/src/libcollections/btree/map.rs
index c7e1e3c9176..a9e1ce8d7ce 100644
--- a/src/libcollections/btree/map.rs
+++ b/src/libcollections/btree/map.rs
@@ -24,7 +24,7 @@ use core::default::Default;
 use core::fmt::Debug;
 use core::hash::{Hash, Hasher};
 use core::iter::{Map, FromIterator, IntoIterator};
-use core::ops::{Index, IndexMut};
+use core::ops::{Index};
 use core::{iter, fmt, mem, usize};
 use Bound::{self, Included, Excluded, Unbounded};
 
@@ -925,15 +925,6 @@ impl<K: Ord, Q: ?Sized, V> Index<Q> for BTreeMap<K, V>
     }
 }
 
-#[stable(feature = "rust1", since = "1.0.0")]
-impl<K: Ord, Q: ?Sized, V> IndexMut<Q> for BTreeMap<K, V>
-    where K: Borrow<Q>, Q: Ord
-{
-    fn index_mut(&mut self, key: &Q) -> &mut V {
-        self.get_mut(key).expect("no entry found for key")
-    }
-}
-
 /// Genericises over how to get the correct type of iterator from the correct type
 /// of Node ownership.
 trait Traverse<N> {