Mention "null pointer optimization" in option docs.

[jsha]

I had seen people discuss "null pointer optimization," but when I tried to find official documentation (using Google), the std::option page didn't show up, because it doesn't use that term. Hopefully adding the term will help others find it in the future.

[rust-highfive]

r? @scottmcm

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[joshtriplett]

Thanks for adding this!

I've never seen this written as "NPO" before; do you have any reference for that acronym being used?

[jsha]

Sure! Some examples I picked up by searching just now:

https://users.rust-lang.org/t/unsafe-rust-is-tough/48881
https://internals.rust-lang.org/t/pre-rfc-unsafe-enums-now-including-a-poll/2873/4?u=jsha
#27730 (comment)
https://www.reddit.com/r/rust/comments/cbttmh/null_pointer_optimization_only_applies_to_the/
https://discord.com/channels/442252698964721669/443150878111694848/820726682078937088

In my searches it was surprisingly less common than I expected. But I do recall having the experience of seeing someone write "NPO" in some chat environment (not the Discord link above; it was many months ago), and having to ask what it meant.

[joshtriplett]

@jsha Thanks for the examples! That's more than enough to make it clear that the acronym is in use.

@bors r+ rollup

[bors]

📌 Commit 79575a1 has been approved by joshtriplett