Update unsafe in const functions documentation

[oli-obk]

fixes #482

cc @Centril

[Centril]

Thanks!

[BatmanAoD]

Just for my own understanding: it looks like this doesn't actually permit anything truly "unsafe" inside const fn, except maybe inside the standard library?

Is there ongoing discussion somewhere about additional unsafe features we might want to permit inside const fn? I'm interested in what the definition of undefined behavior would be inside a const fn. (Come to think of it, I'm now also wondering what the implications of undefined behavior inside a C++ constexpr function are, and whether the standard even addresses that...)

[oli-obk]

A few truly unsafe things are tracked in rust-lang/rust#57563 but you are right, we need to figure out our story around unsafe and const safety. This is tracked somewhat in rust-lang/const-eval#14

[Centril]

Just for my own understanding: it looks like this doesn't actually permit anything truly "unsafe" inside const fn, except maybe inside the standard library?

https://doc.rust-lang.org/nightly/std/num/struct.NonZeroU8.html#method.new_unchecked

[BatmanAoD]

@Centril Ah, okay, I can see how calling that in user code would be truly "unsafe". Thanks.