Remove most `unsafe` via #[target_feature] v1.1

[Shnatsel]

Rust now RustFFT relies on #[target_feature=...] unsafe fn functions for runtime detection of SSE and AVX. This obscures which operations are actually unsafe, such as loads/stores by raw pointer, and which are perfectly safe as long as the CPU supports the required instructions.

Rust 1.86 has stabilized the target-feature-1.1 RFC. Starting with Rust 1.86 it is no longer necessary to use unsafe fn for functions annotated with #[target_feature=...]. They are safe to call as long as they are being called from a context with the same target features, so e.g. AVX functions can freely call other AVX functions without unsafe.

Removing the now-redundant unsafe would make the codebase much easier to understand and audit.

[ejmahler]

I agree that reducing the size of unsafe blocks and/or eliminating them entirely is a priority, bunt unfortunately target feature isn't enough without a massive rewrite. We implement traits for vector types in order to be generic over f32 vs f64, and target feature 1.1 still can't piece through trait implementations.

See here for an example, we have the trait AvxVector, and we implement it for __m256.

Macros could probably cover parts of our use case instead of traits, but like I said it would be a massive rewrite.

I started a RFC for this, but it hasn't gotten much attention.

[Shnatsel]

Ah, I see. A significant limitation of the RFC is that it can't be made to work with std operations such as +, which is a shame.

FWIW I'm working on an FFT implementation built on top of fearless_simd crate, and I'm running into the same issue with lack of generics. I also solved that with declarative macros, this is what it ends up looking like: https://github.com/QuState/PhastFT/blob/d8b83d64bc3b23a69fe5b0cd8783e9dc7eab152c/src/algorithms/bravo.rs#L15-L185