Assert covariance of BTree{Map,Set} and associated iterators

[nbdd0121]

Difference etc are not yet covariant, see #30642.

[rust-highfive]

r? @m-ou-se

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[steffahn]

There’s also the variance of the lifetime parameter that could be asserted for types like Iter<'a, K, V>. You also don’t really need separate tests for the key and value, e.g. a single

fn iter<'a>(x: Iter<'static, &'static (), &'static ()>) -> Iter<'a, &'a (), &'a ()> {
    x
}

for btree_map::Iter should be enough.

[nbdd0121]

I followed HashMap, which has separate key and value assertions.

[steffahn]

Alright, still the covariance of the lifetime argument of Iter, Keys, Values, Range, and even IterMut, RangeMut and ValuesMut could be included in the tests. Whether they’re tested separately or not.

[steffahn]

If HashMap doesn’t check those either, then that may be a reasonable addition there as well, I guess.

[nbdd0121]

In that case I'll just merge the checks into one... otherwise it'll too verbose.

As for HashMap and LinkedList I think it'll be better to file it under a separate PR. I'd like to keep this PR limited to just adding new checks for BTree{Map,Set}.

[nbdd0121]

It turns out there are already tests for variances, just that they are named test_variance instead of assert_covariance so my grep doesn't detect them. Sigh 🤦

[nbdd0121]

Of course these tests don't cover lifetime parameters, so it might worth doing that.