`#[simd]` should work on functions without an explicit SIMD token parameter

[RunDevelopment]

Right now (#347), #[simd] requires the first non-receiver argument to a SIMD token. However, it's not uncommon for SIMD functions to not have an explicit SIMD token parameter, because any SIMD type (e.g. f32xN, etc) already carries a SIMD token with them. For example, this is what the sRGB example does:

fearless_simd/fearless_simd/examples/srgb.rs

Lines 52 to 67 in bd9f7c7

	#[inline(always)]
	fn copy_alpha<S: Simd>(a: f32x4<S>, b: f32x4<S>) -> f32x4<S> {
	#[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
	if let Some(sse4_2) = a.simd.level().as_sse4_2() {
	return copy_alpha_sse4_2(sse4_2, a.into(), b.into()).simd_into(a.simd);
	}
	#[cfg(target_arch = "aarch64")]
	if let Some(neon) = a.simd.level().as_neon() {
	return copy_alpha_neon(neon, a.into(), b.into()).simd_into(a.simd);
	}
	let mut result = a;
	result[3] = b[3];
	result
	}

Such functions should be supported to make the #[simd] macro easier to use.

Suggested solution

Instead of requiring that the first parameter is a SIMD token, require that the first parameter carries a SIMD token.

This could be done by adding a new trait like so:

pub trait ExtractToken { // bikeshed names
    type S: Simd;
    fn simd(&self) -> Self::S;
}

// all SIMD token types return self
impl ExtractToken for Avx2 {
    type S = Self;
    fn simd(&self) -> Self::S {
        *self
    }
}

// all data type return their SIMD token
impl<S: Simd> ExtractToken for f32x8<S> {
    type S = S;
    fn simd(&self) -> Self::S {
        self.simd
    }
}

and change the code gen of the #[simd] macro to use this trait to get a SIMD token from the first parameter.

By adding #[diagnostic::on_unimplemented] on the trait, we can also get nice compiler errors when users use #[simd] incorrectly. E.g.

#[simd]
fn foo<S: Simd>(a: i32);

error[E0277]: `i32` does not carry a SIMD token
   --> src\encode\bc4.rs:961:24
    |
961 |     ExtractToken::simd(&a)
    |     ------------------ ^^ the trait `fearless_simd::ExtractToken` is not implemented for `i32`
    |     |
    |     required by a bound introduced by this call
    |
    = note: If you are using the #[simd] macro, the first parameter must carry a SIMD token. See the docs of #[simd] for more information.
help: the following other types implement trait `fearless_simd::ExtractToken`
   --> ...

---

Note: I think the ExtractToken trait should be part of the public API of the main crate. I want to implement it on the newtype wrappers around fearless_simd types I use.