add more-evocative examples for Shl and Shr

r? @steveklabnik

add examples that lift `<<` and `>>` to a trivial struct

replace `Scalar` structs with struct tuples

add `fn main` wrappers to enable Rust Playground "Run" button

Author: matthew-piziak

src/libcore/ops.rs

@@ -852,25 +852,54 @@ bitxor_impl! { bool usize u8 u16 u32 u64 isize i8 i16 i32 i64 }
 ///
 /// # Examples
 ///
-/// A trivial implementation of `Shl`. When `Foo << Foo` happens, it ends up
-/// calling `shl`, and therefore, `main` prints `Shifting left!`.
+/// An implementation of `Shl` that lifts the `<<` operation on integers to a
+/// `Scalar` struct.
 ///
 /// ```
 /// use std::ops::Shl;
 ///
-/// struct Foo;
+/// #[derive(PartialEq, Debug)]
+/// struct Scalar(usize);
 ///
-/// impl Shl<Foo> for Foo {
-///     type Output = Foo;
+/// impl Shl<Scalar> for Scalar {
+///     type Output = Self;
 ///
-///     fn shl(self, _rhs: Foo) -> Foo {
-///         println!("Shifting left!");
-///         self
+///     fn shl(self, Scalar(rhs): Self) -> Scalar {
+///         let Scalar(lhs) = self;
+///         Scalar(lhs << rhs)
+///     }
+/// }
+/// fn main() {
+///     assert_eq!(Scalar(4) << Scalar(2), Scalar(16));
+/// }
+/// ```
+///
+/// An implementation of `Shl` that spins a vector leftward by a given amount.
+///
+/// ```
+/// use std::ops::Shl;
+///
+/// #[derive(PartialEq, Debug)]
+/// struct SpinVector<T: Clone> {
+///     vec: Vec<T>,
+/// }
+///
+/// impl<T: Clone> Shl<usize> for SpinVector<T> {
+///     type Output = Self;
+///
+///     fn shl(self, rhs: usize) -> SpinVector<T> {
+///         // rotate the vector by `rhs` places
+///         let (a, b) = self.vec.split_at(rhs);
+///         let mut spun_vector: Vec<T> = vec![];
+///         spun_vector.extend_from_slice(b);
+///         spun_vector.extend_from_slice(a);
+///         SpinVector { vec: spun_vector }
 ///     }
 /// }
 ///
 /// fn main() {
-///     Foo << Foo;
+///     assert_eq!(SpinVector { vec: vec![0, 1, 2, 3, 4] } << 2,
+///                SpinVector { vec: vec![2, 3, 4, 0, 1] });
 /// }
 /// ```
 #[lang = "shl"]
@@ -924,25 +953,54 @@ shl_impl_all! { u8 u16 u32 u64 usize i8 i16 i32 i64 isize }
 ///
 /// # Examples
 ///
-/// A trivial implementation of `Shr`. When `Foo >> Foo` happens, it ends up
-/// calling `shr`, and therefore, `main` prints `Shifting right!`.
+/// An implementation of `Shr` that lifts the `>>` operation on integers to a
+/// `Scalar` struct.
 ///
 /// ```
 /// use std::ops::Shr;
 ///
-/// struct Foo;
+/// #[derive(PartialEq, Debug)]
+/// struct Scalar(usize);
 ///
-/// impl Shr<Foo> for Foo {
-///     type Output = Foo;
+/// impl Shr<Scalar> for Scalar {
+///     type Output = Self;
 ///
-///     fn shr(self, _rhs: Foo) -> Foo {
-///         println!("Shifting right!");
-///         self
+///     fn shr(self, Scalar(rhs): Self) -> Scalar {
+///         let Scalar(lhs) = self;
+///         Scalar(lhs >> rhs)
+///     }
+/// }
+/// fn main() {
+///     assert_eq!(Scalar(16) >> Scalar(2), Scalar(4));
+/// }
+/// ```
+///
+/// An implementation of `Shr` that spins a vector rightward by a given amount.
+///
+/// ```
+/// use std::ops::Shr;
+///
+/// #[derive(PartialEq, Debug)]
+/// struct SpinVector<T: Clone> {
+///     vec: Vec<T>,
+/// }
+///
+/// impl<T: Clone> Shr<usize> for SpinVector<T> {
+///     type Output = Self;
+///
+///     fn shr(self, rhs: usize) -> SpinVector<T> {
+///         // rotate the vector by `rhs` places
+///         let (a, b) = self.vec.split_at(self.vec.len() - rhs);
+///         let mut spun_vector: Vec<T> = vec![];
+///         spun_vector.extend_from_slice(b);
+///         spun_vector.extend_from_slice(a);
+///         SpinVector { vec: spun_vector }
 ///     }
 /// }
 ///
 /// fn main() {
-///     Foo >> Foo;
+///     assert_eq!(SpinVector { vec: vec![0, 1, 2, 3, 4] } >> 2,
+///                SpinVector { vec: vec![3, 4, 0, 1, 2] });
 /// }
 /// ```
 #[lang = "shr"]