Entry APIs should have or_default for V: Default

[jonhoo]

The various Entry APIs (e.g., hash_map::Entry, btree_map::Entry) currently have or_insert, and or_insert_with. However, when V: Default, I believe they should also have an or_default. This would make it nicer to work with things like maps of maps, maps of lists, etc. While it's true that this is equivalent to or_insert_with(Default::default), I would argue that or_default is much more readable.

I'd be happy to take a stab at implementing this, but wanted to air the idea first.

[steveklabnik]

Generally, the way the libs team works is "open a PR if a change is small, open an RFC if a change is big." I'm not on the libs team, but this feels pretty small to me; you already almost have an implementation in the comment!

[jonhoo]

@steveklabnik PR submitted!

[dtolnay]

An unstable or_default() method was added in #44344. This issue is now the tracking issue for stabilization.

[khuey]

Is there any reason not to just stabilize this?

[SimonSapin]

In Nightly for 6 months, looks good to me to stabilize.

@rfcbot fcp merge

[rfcbot]

Team member @SimonSapin has proposed to merge this. The next step is review by the rest of the tagged teams:

• @Kimundi
• @SimonSapin
• @alexcrichton
• @aturon
• @dtolnay
• @sfackler
• @withoutboats

No concerns currently listed.

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