const simd_extract/insert are no longer usable in latest nightly (2020-01-02)

[novacrazy]

A few months ago #64738 introduced const simd_insert/simd_extract, which I then integrated into my application via a fork of packed_simd.

However, upon upgrading to the latest nightly (rustc 1.42.0-nightly (0a58f5864 2020-01-02)), it was suddenly non-const.

Furthermore, It seems the ability to make it const via rustc_const_unstable is not possible in user applications, making the const versions of this effectively rustc internal-only now, breaking my fork of packed_simd and my application.

[mati865]

cc @oli-obk ?

[oli-obk]

As mentioned on #64738 you can use stability attributes to get this working again.

If this requires you to add too many annotations to your crate, you can create a crate that solely consists of the intrinsic declarations and exports them. Then your main crate doesn't need any annotations. I do not think there's anything on the compiler end that we will do to further support using very unstable internals of the standard library.

If you want to speed up the stabilization of this, @gnzlbg mentioned moving packed_simd into the libstd. Though I do not know what the status, progress or plans for that are

[workingjubilee]

@rustbot modify labels: +A-simd
Current status of portable SIMD: Coalescing project group.
EDIT: whoops, wrong link. rust-lang/rfcs#2977