Added feature const_new which enables SmallVec::new_const()

[TimLuq]

As I wasn't alone in finding a use for having a const version of new (see #263) I decided to make a PR for it.

This adds a new feature flag const_new (nightly only) which exposes an alternative new function: pub const unsafe fn new_const() -> SmallVec<A>.

Nightly is needed for the binding A: Array, which could be worked around if the strict requirement is dropped from the struct declarations and moved only to the existing impls.

If the maintainers wish for another solution for this I'll be happy to make a few more commits.

[TimLuq]

I added from_const which behaves like from_buf for arrays ([_; N]). I also added a macro smallvec_inline that's just sugar for from_const.

My thought was that new_const and from_const should be visible in the docs. So the most recent commit also includes the changes necessary for displaying feature hints on docs.rs.

[mbrubeck]

Could new_const be made safe by using const generics to implement it only for SmallVec<[T; N]>?

[TimLuq]

Could new_const be made safe by using const generics to implement it only for SmallVec<[T; N]>?

Yes, that is another option that should work. It will then be added to the same place as from_const which seems reasonable.

I'll be able to push a new commit in 8 hours or so. Is there anything else I should take a look at regarding this PR?

[TimLuq]

Limiting the const functions to SmallVec<[T; N]> also got rid of the need for the trait bound in addition to the unsafe keyword. This means it now builds in stable 1.51+ with the const_new feature active.

[mbrubeck]

@bors-servo r+

[bors-servo]

📌 Commit 8320871 has been approved by mbrubeck

[bors-servo]

⌛ Testing commit 8320871 with merge 18b720f...

[mbrubeck]

Thanks! This has been released in smallvec 1.7.0: https://crates.io/crates/smallvec/1.7.0

[bors-servo]

💥 Test timed out