Future-proof indexing on maps: remove IndexMut

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

Author: aturon

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> {

src/librustc_resolve/lib.rs

@@ -900,7 +900,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
             return;
         }
         if self.glob_map.contains_key(&import_id) {
-            self.glob_map[import_id].insert(name);
+            self.glob_map.get_mut(&import_id).unwrap().insert(name);
             return;
         }
 

src/librustc_resolve/resolve_imports.rs

@@ -603,7 +603,7 @@ impl<'a, 'b:'a, 'tcx:'b> ImportResolver<'a, 'b, 'tcx> {
 
         // We've successfully resolved the import. Write the results in.
         let mut import_resolutions = module_.import_resolutions.borrow_mut();
-        let import_resolution = &mut (*import_resolutions)[target];
+        let import_resolution = import_resolutions.get_mut(&target).unwrap();
 
         {
             let mut check_and_write_import = |namespace, result: &_, used_public: &mut bool| {

src/librustc_typeck/check/upvar.rs

@@ -380,7 +380,7 @@ impl<'a,'tcx> AdjustBorrowKind<'a,'tcx> {
                 // borrow_kind of the upvar to make sure it
                 // is inferred to mutable if necessary
                 let mut upvar_capture_map = self.fcx.inh.upvar_capture_map.borrow_mut();
-                let ub = &mut upvar_capture_map[upvar_id];
+                let ub = upvar_capture_map.get_mut(&upvar_id).unwrap();
                 self.adjust_upvar_borrow_kind(upvar_id, ub, borrow_kind);
 
                 // also need to be in an FnMut closure since this is not an ImmBorrow

src/libstd/collections/hash/map.rs

@@ -23,7 +23,7 @@ use hash::{Hash, SipHasher};
 use iter::{self, Iterator, ExactSizeIterator, IntoIterator, IteratorExt, FromIterator, Extend, Map};
 use marker::Sized;
 use mem::{self, replace};
-use ops::{Deref, FnMut, Index, IndexMut};
+use ops::{Deref, FnMut, Index};
 use option::Option::{self, Some, None};
 use rand::{self, Rng};
 use result::Result::{self, Ok, Err};
@@ -1258,18 +1258,6 @@ impl<K, Q: ?Sized, V, S> Index<Q> for HashMap<K, V, S>
     }
 }
 
-#[stable(feature = "rust1", since = "1.0.0")]
-impl<K, V, S, Q: ?Sized> IndexMut<Q> for HashMap<K, V, S>
-    where K: Eq + Hash + Borrow<Q>,
-          Q: Eq + Hash,
-          S: HashState,
-{
-    #[inline]
-    fn index_mut<'a>(&'a mut self, index: &Q) -> &'a mut V {
-        self.get_mut(index).expect("no entry found for key")
-    }
-}
-
 /// HashMap iterator.
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct Iter<'a, K: 'a, V: 'a> {