Smarter default implementation of Extend::extend_reserve

[dtolnay]

Following up on #72631 (comment): there is a way for extend_reserve to perform a sensible reserve by default, without always needing to be overridden by every Extend impl, as long as extend checks its argument's size_hint as many already do. For example the following code in String's Extend impl:

rust/library/alloc/src/string.rs

Lines 1927 to 1933 in c9db3e0

	impl Extend<char> for String {
	fn extend<I: IntoIterator<Item = char>>(&mut self, iter: I) {
	let iterator = iter.into_iter();
	let (lower_bound, _) = iterator.size_hint();
	self.reserve(lower_bound);
	iterator.for_each(move |c| self.push(c));
	}

In a future PR we can clean up handwritten implementations of extend_reserve which are no longer providing value over this default implementation.

[rust-highfive]

r? @kennytm

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[dtolnay]

I have used this in the past to work around lack of stable extend_reserve successfully, but it may be possible there are Extend implementations in which this would trigger a panic.

[cuviper]

Iterator::size_hint docs say:

It is not enforced that an iterator implementation yields the declared number of elements. A buggy iterator may yield less than the lower bound or more than the upper bound of elements.

How comfortable are we with intentionally using a "buggy" iterator?

but it may be possible there are Extend implementations in which this would trigger a panic.

Funny, @scottmcm just demonstrated that in the users forum:
https://users.rust-lang.org/t/is-string-splitn-missing-a-huge-easy-optimisation/64555/3

[the8472]

I have seen suggestions similar to the one in the urlo thread before. Since specialization isn't stable it might be used as a poor man's TrustedLen when the length is fixed.

[dtolnay]

I think I am going to withdraw this. While it works well in practice for a wide range of Extend impls, there is nothing that makes it incorrect for an Extend impl to panic in response to this situation, which would become a bad/surprising extend_reserve default behavior.