Add `core::iter::chain` method

[rossmacarthur]

Proposal

Problem statement

Add a convenient IntoIterator enabled function for chaining two iterators.

Motivation, use-cases

• A similar function already exists core::iter::zip (added in Add function core::iter::zip rust#82917)
• There are many cases where it can be clearer to use a direct function instead of using the iterator method.
• Particularly useful in the case where you want to prepend to another iterator.
• There is prior art for the utility of this in itertools::chain.

for (x, y) in chain(xs, ys) {}
// vs
for (x, y) in xs.into_iter().chain(ys) {}

Some examples from the rust-lang/rust repo

infcx
    .type_implements_trait(p.def_id, [ty.into()].into_iter().chain(p.substs.iter()), cx.param_env)
    .must_apply_modulo_regions()
// vs
infcx
    .type_implements_trait(p.def_id, iter::chain([ty.into()], p.substs.iter()), cx.param_env)
    .must_apply_modulo_regions()

Some(t).into_iter().chain(slice::from_mut(u))
// vs
iter::chain([t], slice::from_mut(u)))

Solution sketches

// library/core/src/iter/adapters/chain.rs
pub fn chain<A, B>(a: A, b: B) -> Chain<A::IntoIter, B::IntoIter>
where
    A: IntoIterator,
    B: IntoIterator<Item = A::Item>,
{
    Chain::new(a.into_iter(), b.into_iter())
}

[clarfonthey]

Would love to see this added to libstd. Actually was going to file an ACP if this didn't have one already.

[Amanieu]

We discussed this in the libs-api meeting today. We're happy to accept this for the same reasons as zip.