Change core::iter::Fuse's Default impl to do what its docs say it does

[zachs18]

The docs on impl<I: Default> Default for core::iter::Fuse<I> say (as the I: Default bound implies) that Fuse::<I>::default "Creates a Fuse iterator from the default value of I". However, the implementation creates a Fuse with Fuse { iter: Default::default() }, and since the iter field is an Option<I>, this is actually Fuse { iter: None }, not Fuse { iter: Some(I::default()) }, so Fuse::<I>::default() always returns an empty iterator, even if I::default() would not be empty.

This PR changes Fuse's Default implementation to match the documentation. This will be a behavior change for anyone currently using Fuse::<I>::default() where I::default() is not an empty iterator¹, as Fuse::<I>::default() will now also not be an empty iterator.

(Alternately, the docs could be updated to reflect what the current implementation actually does, i.e. returns an always-exhausted iterator that never yields any items (even if I::default() would have yielded items). With this option, the I: Default bound could also be removed to reflect that no I is ever created.)

Current behavior example (maybe an example like this should be added to the docs either way?)

This PR changes publicly observable behavior, so I think requires at least a T-libs-api FCP?

r? libs-api

cc #140961

impl<I: Default> Default for Fuse<I> was added in 1.70.0 (#99929), and it's docs and behavior do not appear to have changed since (Fuse's iter field has been an Option since before the impl was added).

Footnotes

1. IIUC it is a "de facto" guideline for the stdlib that an iterator type's default() should be empty (and for iterators where that would not make sense, they should not implement Default): cc https://github.com/rust-lang/libs-team/issues/77#issuecomment-1194681709 , so for stdlib iterators, I don't think this would change anything. However, if a user has a custom Iterator type I, and they are using Fuse<I>, and they call Fuse::<I>::default(), this may change the behavior of their code. ↩

[hanna-kruppe]

Either way (fix the impl or fix the docs) this should probably gain a test since it’s extremely non-obvious from the code alone.

[scottmcm]

And maybe change the code to either Option::default() just None or to I::default() to help emphasize, too.