[Proposal] std/mem: add a more generic eql function

[ifreund]

I had to write some very ugly code recently to compare two optional, sentinel-terminated pointers in river: https://github.com/ifreund/river/blob/cd005e15f8bcb6852b35fd3d89248e72611a5d5f/river/Option.zig#L87-L90

    } else if (self.value == .string and
        // TODO: std.mem needs a good way to compare optional sentinel pointers
        ((self.value.string == null and value.string == null) or
        (self.value.string != null and value.string != null and
        std.cstr.cmp(self.value.string.?, value.string.?) != 0)))
    {

As I wrote in the comment, this made me feel like we needed a function to do a better job of this in std.mem. I've Implemented such a function in this patch as a proposal. I'm not sure if this function should replace the current std.mem.eql() as that would be a widely breaking change and the anytype based API is not as simple when working with slices. However, this would match the API of std.meta.eql() so perhaps simply replacing std.mem.eql() is worth considering.

The new function is temporarily named eqlGeneric(), but if we decide not to replace current mem.eql() the new function should have a better name. Naming suggestions welcome.

Edit: std.meta.deepEql() or similar would also be a reasonable place to put this function.

[ifreund]

One other feature we should consider here is easy and efficient comparison of sentinel terminated pointers and slices. The current pattern is to use for example:

mem.eql(u8, my_slice, mem.span(my_sentinel_terminated_ptr));

However this is inefficient as the full length of the sentinel terminated pointer is always iterated even when the slice is shorter. Ideally the comparison would look something like this:

fn better(a: []const u8, b: [*:0]const u8) bool {
    for (a) |a_elem, idx| {
        if (b[idx] == 0 or b[idx] != a_elem) return false;
    }
    return b[a.len] == 0;
}

Comparison in godbolt
I'm no expert in asm, but I've been led to believe that fewer instructions are better

[andrewrk]

Closing old drafts; please feel free to re-open if you decide to pick this back up.

[andrewrk]

I see this was intended to be a proposal; filed #8414 on your behalf