Mutable indexing is strange.

[HeroesGrave]

Mutably indexing a Vec (and probably lots of other collections) is weird.
On one hand, you can dereference a mutable pointer:

*vec.get_mut(index) = value;

Which is ugly and inconsistent with immutable indexing (vec[index])

With the new slicing syntax you can do this:

vec[mut][index] = value;

Which is arguably nicer. But wouldn't something like:

vec[mut index] = value;

Be even better and more consistent?

[ftxqxd]

#12825 is preventing implementing IndexMut for Vec, HashMap, &c., but once that’s fixed, simply doing vec[index] = value; will work. The reason slicing uses an explicit mut but not indexing is because slicing directly returns a reference/slice (e.g., vec[a..b] returns &[T], vec[mut a..b] returns &mut [T]), while indexing just returns a value (e.g., vec[a] returns T). However, it doesn’t move the value out of the collection, it just dereferences a reference to it (i.e., vec[a] = *vec.index(&a) or *vec.index_mut(&a)). The problem is figuring out which of index and index_mut to use, which is represented by the issue #12825.

[HeroesGrave]

Ah, okay. Thanks for clearing that up.

[ftxqxd]

The vec[idx] = val; syntax now works for Vec, so I think this issue can be closed.