Add hash_map::Entry.or_insert_with_key() method

[dnspies]

In some cases, I only have one key, and by calling

let e = my_hash_map.entry(k);

I'm giving up my one and only instance of k.
Now I need to use k to populate the entry (imagine my_hash_map memoizes the function my_fun):

e.or_insert_with(|| my_fun(&k))

but I'm out of luck because k has been stolen by the call to entry (k has been moved). By adding an extra function to entry, this can be rectified by

e.or_insert_with_key(|kref| my_fun(kref))

[Gankra]

I would really rather just expose a key method for retrieving the key on VacantEntry and OccupiedEntry.

See #1194

[dnspies]

I don't think that works.
As soon as I call e.get_key() I'm now holding an immutable ref into e so now I can't call any mutable methods on e (such as or_insert_with()) until I drop it.
I suppose one alternative approach would be to have a:

e.get_key_and_value()
which returns a pair of both an immutable ref to the key and also a mutable ref to the value.
And then call or_insert() on the value ref.
But that requires a much bigger change to the way Entry works.

[Diggsey]

@dnspies What you're trying to do is fundamentally unsafe - you can't put a value into a map which keeps a reference to its own key. You can use such a reference to construct the value, in which case @gankro's solution will work, because the borrow will expire after construction, and you can then call mutable methods on the entry again.

[dnspies]

That won't work with or_insert_with which clearly must be called with both borrows. Of course, I could avoid using that method and instead just match against the entry to see if it's vacant or not. In that case get_key() is sufficient.

[KodrAus]

As per rust-lang/rust#70996, we now have an unstable implementation of this in std, so I'll go ahead and close this issue now.

[ChaiTRex]

Note: Fully stabilized today in Rust 1.50 in HashMap and in BTreeMap.

[ChaiTRex]

Stabilized in 1.50.0. See the method's documentation.