Add function core::iter::zip

This makes it a little easier to `zip` iterators:

```rust
for (x, y) in zip(xs, ys) {}
// vs.
for (x, y) in xs.into_iter().zip(ys) {}
```

You can `zip(&mut xs, &ys)` for the conventional `iter_mut()` and
`iter()`, respectively. This can also support arbitrary nesting, where
it's easier to see the item layout than with arbitrary `zip` chains:

```rust
for ((x, y), z) in zip(zip(xs, ys), zs) {}
for (x, (y, z)) in zip(xs, zip(ys, zs)) {}
// vs.
for ((x, y), z) in xs.into_iter().zip(ys).zip(xz) {}
for (x, (y, z)) in xs.into_iter().zip((ys.into_iter().zip(xz)) {}
```

It may also format more nicely, especially when the first iterator is a
longer chain of methods -- for example:

```rust
    iter::zip(
        trait_ref.substs.types().skip(1),
        impl_trait_ref.substs.types().skip(1),
    )
    // vs.
    trait_ref
        .substs
        .types()
        .skip(1)
        .zip(impl_trait_ref.substs.types().skip(1))
```

This replaces the tuple-pair `IntoIterator` in rust-lang#78204.
There is prior art for the utility of this in [`itertools::zip`].

[`itertools::zip`]: https://docs.rs/itertools/0.10.0/itertools/fn.zip.html

library/core/src/iter/adapters/mod.rs

@@ -51,6 +51,9 @@ pub use self::map_while::MapWhile;
 #[unstable(feature = "trusted_random_access", issue = "none")]
 pub use self::zip::TrustedRandomAccess;
 
+#[unstable(feature = "iter_zip", issue = "none")]
+pub use self::zip::zip;
+
 /// This trait provides transitive access to source-stage in an interator-adapter pipeline
 /// under the conditions that
 /// * the iterator source `S` itself implements `SourceIter<Source = S>`

library/core/src/iter/adapters/zip.rs

@@ -5,8 +5,8 @@ use crate::iter::{InPlaceIterable, SourceIter, TrustedLen};
 
 /// An iterator that iterates two other iterators simultaneously.
 ///
-/// This `struct` is created by [`Iterator::zip`]. See its documentation
-/// for more.
+/// This `struct` is created by [`zip`] or [`Iterator::zip`].
+/// See their documentation for more.
 #[derive(Clone)]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -33,6 +33,37 @@ impl<A: Iterator, B: Iterator> Zip<A, B> {
     }
 }
 
+/// Converts the arguments to iterators and zips them.
+///
+/// See the documentation of [`Iterator::zip`] for more.
+///
+/// # Examples
+///
+/// ```
+/// #![feature(iter_zip)]
+/// use std::iter::zip;
+///
+/// let xs = [1, 2, 3];
+/// let ys = [4, 5, 6];
+/// for (x, y) in zip(&xs, &ys) {
+///     println!("x:{}, y:{}", x, y);
+/// }
+///
+/// // Nested zips are also possible:
+/// let zs = [7, 8, 9];
+/// for ((x, y), z) in zip(zip(&xs, &ys), &zs) {
+///     println!("x:{}, y:{}, z:{}", x, y, z);
+/// }
+/// ```
+#[unstable(feature = "iter_zip", issue = "none")]
+pub fn zip<A, B>(a: A, b: B) -> Zip<A::IntoIter, B::IntoIter>
+where
+    A: IntoIterator,
+    B: IntoIterator,
+{
+    ZipImpl::new(a.into_iter(), b.into_iter())
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<A, B> Iterator for Zip<A, B>
 where

library/core/src/iter/mod.rs

@@ -389,6 +389,8 @@ pub use self::traits::{
     DoubleEndedIterator, ExactSizeIterator, Extend, FromIterator, IntoIterator, Product, Sum,
 };
 
+#[unstable(feature = "iter_zip", issue = "none")]
+pub use self::adapters::zip;
 #[stable(feature = "iter_cloned", since = "1.1.0")]
 pub use self::adapters::Cloned;
 #[stable(feature = "iter_copied", since = "1.36.0")]