Deprecate the unstable Vec::resize_default

[scottmcm]

As a way to either get additional feedback to stabilize or help move nightly users off it.

Tracking issue: #41758 (comment)

r? @SimonSapin

[leonardo-m]

What's the point of this change?

[SimonSapin]

@leonardo-m The point is to emit a deprecation warning where this method is used, to leave some time for users to move to .resize_with(Default::default) instead, before we remove resize_default.

I’m ok with going in this direction, let’s see about the rest of the team:

@rfcbot fcp merge

[rfcbot]

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

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

No concerns currently listed.

Once a majority of reviewers approve (and none object), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[clarfonthey]

I think this is a great idea to encourage people who prefer resize_default to voice their opinions rather than removing it immediately. 👍

[leonardo-m]

I think I prefer resize_default because it's shorter and more handy. But if you remove it, I'll live without it.

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[avl]

I discovered Vec::resize_default yesterday. It was precisely what I wanted. First I found resize_with and it would have worked, but resize_default was exactly what I needed.

Arguments to keep it:
1: It makes for slightly shorter code in cases where it's needed (fewer symbols)
2: It is quite obvious how it works
3: It presumably doesn't cost very much in documentation or implementation to keep it?

That said, it is of course not an essential function.

[scottjmaddox]

Considering resize_with(Default::default) is not a particularly obvious pattern, especially for new Rust users, and considering the use case is fairly common (e.g. creating a zeroed byte buffer), I support stabilizing, rather than deprecating.

[SimonSapin]

I think that vec.resize(n, 0) would be easier to find than vec.resize_default(n) to create a zeroed byte buffer.

[hellow554]

I do like the deprecation. At first I thought it's a bad idea, but removing duplicate code is always-ish okay.
Yes it might be confusing for really new people, but IMHO it really tells you what it does, namely it resizes the vec with Default (values).
It removes the ambiguity between which one to choose, is then identical to the struct construction with Default (..Default::default).
Also a plus point the doc already has the correct example https://doc.rust-lang.org/std/vec/struct.Vec.html#method.resize_with

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

[scottmcm]

@SimonSapin looks like FCP has finished; are you good with the diff here?

[SimonSapin]

Thanks for the PR!

@bors r+

[bors]

📌 Commit 81cd1e6 has been approved by SimonSapin

[Manishearth]

@bors rollup