Add `LocalKey<Cell>::update`

[camsteffen]

Proposal

Problem statement

Now that Cell::update is stabilized, it seems like LocalKey<Cell>::update ought to exist as well. I looked for it and didn't find it today.

Motivating examples or use cases

Not much to say. Cell::update is already accepted. Other LocalKey<Cell> methods exist. So it just makes sense.

Solution sketch

...

Alternatives

Don't add it.

Links and related work

...

What happens now?

This issue contains an API change proposal (or ACP) and is part of the libs-api team feature lifecycle. Once this issue is filed, the libs-api team will review open proposals as capability becomes available. Current response times do not have a clear estimate, but may be up to several months.

Possible responses

The libs team may respond in various different ways. First, the team will consider the problem (this doesn't require any concrete solution or alternatives to have been proposed):

• We think this problem seems worth solving, and the standard library might be the right place to solve it.
• We think that this probably doesn't belong in the standard library.

Second, if there's a concrete solution:

• We think this specific solution looks roughly right, approved, you or someone else should implement this. (Further review will still happen on the subsequent implementation PR.)
• We're not sure this is the right solution, and the alternatives or other materials don't give us enough information to be sure about that. Here are some questions we have that aren't answered, or rough ideas about alternatives we'd want to see discussed.

[Amanieu]

We discussed this in the @rust-lang/libs-api meeting and are happy to accept this.

Please open a tracking issue and open a PR to rust-lang/rust to add it as an unstable feature. You can close this ACP once the tracking issue has been created.