utils VS tinyvec

I didn't use any inline asm, instead I wrote the simplest if statement possible, which I confirmed on x86_64, x86, and aarch64 compiles down to use the platform's supported conditional move instructions. If this changes in the future and there's a need to write inline asm, you can do so in a portable way, e.g. the cmov crate.

There's also cpufeatures which might suit your use case better (depending on exactly how you want to do it).

I have seen that RustCrypto also offers a create for securely zeroing memory with: zeroize

The zeroing routine itself looks fine to me. You might want to compare your code with the (already mentioned) zeroize crate, which does something similar (or maybe identical?).

To save a few clicks, this was found to be, indeed, a Y2K issue, and der v0.4.1 has just been released to fix it. Citing the comment added to the issue after the fact:

I opened a tracking issue for that.

https://github.com/RustCrypto/utils/pull/325#issuecomment-791503136

funny indeed. i changed all my projects to use tinyvec

I've seen the cost of zeroing memory be measurable, or maybe significant enough to care about, but I've never seen 90% of cycles spent on it, the only case I know of that gets close is creating an empty TinyVec versus an empty SmallVec with an inline buffer of 256 bytes. In my opinion that's an unreasonably large inline buffer. At inline buffers of 128 bytes and below, the overhead is less than 50%, and that's on a microbenchmark of the Default impl; the effect is rapidly diluted in a real program.

My point is that "implementation that doesn't use unsafe" is not necessarily always slower than "implementation that does use unsafe". Often people assume that this is the case, and it isn't. tinyvec currently beats smallvec in more than a few benchmarks. Not all, but some. And this sometimes visible to users. The point is that if you want speed, you don't necessarily need to give up any safety at all. Most differences in performance are due to the amount of effort or expertise that has been spent on the codebase, not the amount of unsafe in it.

https://github.com/Lokathor/tinyvec will definitely benefit, although not as much as something currently relying on typenum.