What is ~const?

This is Option's contains() method signature:

pub const fn contains<U>(&self, x: &U) -> bool
where
    U: ~const PartialEq<T>

What exactly is that ~const?

Solution 1:

This is a new-ish experimental syntax for the const_trait_impl feature.

Remember that the std library can use experimental features even in the stable channel.

Basically, this allows you to implement a trait full all const functions and use it in a const context. Something like this:

trait MyTrait {
    fn foo(&self);
}

struct A;

impl const MyTrait for A {
    fn foo(&self) { /* is is a const function! */}
}

struct B;

impl MyTrait for B {
    fn foo(&self) { /* is is NOT a const function */}
}

const fn test<X>(x: &X)
   where X: ~const MyTrait
{
    x.foo();
}

Now a call to test(&A) can be done in a const context while a call to test(&B) cannot. Without the ~const it would never be a const function:

static THE_A: () = test(&A); //ok
static THE_B: () = test(&B); //error: B does not implement MyTrait constly

In the case of the Option implementation, Option::contains() is const if the implementation of T as PartialEq is const.