[meta] Wishlist for smallvec 1.0 about rust-smallvec

[mbrubeck]

I posted a PR for a 1.0.0 release in #175

I'd like to publish this as soon as possible, because lack of MaybeUninit in existing releases is UB waiting to happen, and is already impacting downstream projects that run MIRI tests. We can release a 2.0 version next year or whenever const generics are ready.

[mbrubeck]

Many of the unstable Rust features seem unlikely to stabilize any time soon. I think we should consider making a 1.0 release in the near future (sometime this year) which would include everything that is stable now (or in the very near future), and then make a 2.0 release much later (probably 2 or more years from now) with features that stabilize later.

The following would be delayed until 2.0 or later:

• const generics
• custom allocators
• specialization

The following might be ready in time for 1.0:

• MaybeUninit
• liballoc
• union

It might be worth waiting until MaybeUninit is stabilized, since that fixes a potentially critical soundness issue. Alternately we could use build-time rustc version detection to conditionally use MaybeUninit, but this would increase code complexity and we would need to live with it for potentially a long time.

[newpavlov]

I really hope that v1.0 version will use const generics. Yes, stabilization is unlikely to happen in the near future (6-12 months), but at least it's on horizon, which can not be said about specialization.

[llogiq]

Regarding specialization, I can see how the From implementation can work out, but wouldn't Extend and insert_many violate coherence rules?

[mbrubeck]

Specializing these would require adding an intermediate trait with a default impl and specialized impls, like libstd's impl Extend for Vec.

[cbarrick]

Might I suggest adding an updated drain to the list.

SmallVec::drain was implemented in #26, but does not accept a range argument like Vec::drain. At the time, RangeArgument (now RangeBounds) had not stabilized. RangeBounds is now stable since Rust 1.28.

[lambda-fairy]

MaybeUninit and alloc are now stable. Do we want to update this issue?

[llogiq]

There's a neat trick written up by @dtolnay to emulate specialization on stable. I haven't looked too deeply into how well that performs, but it's probably worth a shot trying to use it.

[matklad]

@mbrubeck thank you for putting work into this!

Seems like the issue can be closed and a new one, wishlist for 2.0, opened instead

[mbrubeck]

Filed #183 to track 2.0 progress.