This is allowed to fail, but might produce useful results to check on.

...and copy the notes for why they're legal.

Now that we are thoroughly embedded in libcore, we don't need these on by default.
Indeed, their presence may provide confusing results during integration attempts.

Another approach that fixes rust-lang/portable-simd#223, as an alternative to rust-lang/portable-simd#238.

This adds the `ToBitMask` trait, which is implemented on a vector for each bitmask type it supports.  This includes all unsigned integers with enough bits to contain it.  The byte array variant has been separated out for now into rust-lang/portable-simd#246 and still requires `generic_const_exprs`, but the integer variants no longer require it and can make it to nightly.

* Explain unsafe contracts of core::simd

This permeates the module with remarks on safety for pub methods,
layout of the Simd type, correct use of intrinsics, et cetera.
This is mostly to help others curious about how core::simd works,
including other Rust contributors, `unsafe` library authors,
and eventually ourselves.

Remove overflow panic from divrem and add basic docs to Simd<T, N>

This finishes normalizing Simd<T, N> to being approximately equivalent to Simd<Wrapping<T>, N> for all implemented operations I can think of. It also documents this fact, allowing this to close rust-lang/portable-simd#56.

Co-authored-by: NCaleb Zulawski <caleb.zulawski@gmail.com>

and other quirks like panicking
and the equivalence to zipping and mapping binary ops

Includes some remarks in intrinsics.rs,
generated while auditing the interface for remaining UB.

Add spectral_norm example from packed_simd

Add Simd::cast
Add core_simd/tests/cast.rs

fix documentation typo

Co-authored-by: NAlexander Ronald Altman <alexanderaltman@me.com>

Remove redundant "neither" in the documentation comment.

The way the macro expands, it may sometimes infer
"this is a uint, but doesn't impl Neg???"
Also, I made the "wrong path for intrinsics" error.
These fixes allow integration into libcore.

This significantly simplifies codegen and should improve mask perf.
Co-authored-by: NJacob Lifshay <programmerjake@gmail.com>

Use intrinsic for min/max

 impl std::simd::StdFloat

 This introduces an extension trait to allow use of floating point methods
 that need runtime support. It is *excessively* documented because its mere
 existence is quite vexing, as the entire thing constitutes a leakage of
 implementation details into user observable space. Eventually the entire
 thing will ideally be folded into core and restructured to match the rest
 of the library, whatever that structure might look like at the time. This
 is preferred in lieu of the "lang item" path because any energy the lang
 items require (and it will be significant, by Simulacrum's estimation) is
 better spent on implementing our libmvec.

While consulting with Simulacrum on how to make available the float
functions that currently require runtime support for `Simd<f32, N>` and
`Simd<f64, N>`, we realized breaking coherence with the classic approach
of lang items was, since `{core,std}::simd::Simd` is a `ty::Adt`, likely
to be quite a bit nasty. The project group has a long-term plan for how
to get around this kind of issue and move the associated functions into
libcore, but that will likely take time as well. Since all routes
forward are temporally costly, we probably will skip the lang item
approach entirely and go the "proper" route, but in the interests of
having something this year for people to play around with, this
extension trait was whipped up.

For now, while it involves a lot of fairly internal details most users
shouldn't have to care about, I went ahead and fully documented the
situation for any passerby to read on the trait, as the situation is
quite unusual and puzzling to begin with.

Refactor ops.rs with wrapping shifts

This approaches reducing macro nesting in a slightly different way. Instead of just flattening details, make one macro apply another. This allows specifying all details up-front in the first macro invocation, making it easier to audit and refactor in the future.

This refactor also has some functional changes. Only one is a true behavior change, however:
- The visible one is that SIMD shifts are now wrapping, not panicking on overflow
- `core::simd` now has a lot more instances of `#[must_use]`, which merely lints
- div/rem now perform a SIMD check but remain as before, which should improve performance but be invisible