Tweak example in chapter 10

[steveklabnik]

We were cheating due to Copy, and that made this example awkward.
Returning a reference is probably better here anyway, and it makes
everything flow a bit nicer.

FIxes #1761

[dalrrard]

The changes merged in this PR seem to have affected the continuity of this example into Chapter 10.2. None of the error codes associated with these two line up and the borrowing flips throughout the two chapters without rewriting the example code.

Chapter 10.2

fn largest<T: PartialOrd + Copy>(list: &[T]) -> T {
    let mut largest = list[0];

    for &item in list {
        if item > largest {
            largest = item;
        }
    }

    largest
}

Chapter 10.1

fn largest<T>(list: &[T]) -> &T {
    let mut largest = list[0];

    for item in list {
        if item > largest {
            largest = item;
        }
    }

    largest
}

Written error

$ cargo run
   Compiling chapter10 v0.1.0 (file:///projects/chapter10)
error[E0369]: binary operation `>` cannot be applied to type `T`
 --> src/main.rs:5:17
  |
5 |         if item > largest {
  |            ---- ^ ------- T
  |            |
  |            T
  |
  = note: `T` might need a bound for `std::cmp::PartialOrd`

Actual error

binary operation `>` cannot be applied to type `&T`
 --> generics/src/main.rs:5:17
  |
5 |         if item > largest {
  |            ---- ^ ------- T
  |            |
  |            &T
  |
help: consider restricting type parameter `T`
  |
1 | fn largest<T: std::cmp::PartialOrd>(list: &[T]) -> &T {
  |             ^^^^^^^^^^^^^^^^^^^^^^

error[E0308]: mismatched types
 --> generics/src/main.rs:6:23
  |
1 | fn largest<T>(list: &[T]) -> &T {
  |            - this type parameter
...
6 |             largest = item;
  |                       ^^^^ expected type parameter `T`, found `&T`
  |
  = note: expected type parameter `T`
                  found reference `&T`

error[E0308]: mismatched types
  --> generics/src/main.rs:10:5
   |
1  | fn largest<T>(list: &[T]) -> &T {
   |            - this type parameter
...
10 |     largest
   |     ^^^^^^^
   |     |
   |     expected `&T`, found type parameter `T`
   |     help: consider borrowing here: `&largest`
   |
   = note:   expected reference `&T`
           found type parameter `T`

error: aborting due to 3 previous errors

It seems like there was a PR that fixed most of the issues with Chapter 10.1 by adding let mut largest = &list[0];, but the compiler still throws a different error than written.

error[E0369]: binary operation `>` cannot be applied to type `&T`
 --> generics/src/main.rs:5:17
  |
5 |         if item > largest {
  |            ---- ^ ------- &T
  |            |
  |            &T
  |
help: consider restricting type parameter `T`
  |
1 | fn largest<T: std::cmp::PartialOrd>(list: &[T]) -> &T {
  |             ^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

(https://doc.rust-lang.org/book/ch10-02-traits.html#fixing-the-largest-function-with-trait-bounds)

listings/ch10-generic-types-traits-and-lifetimes/listing-10-05/src/main.rs

src/ch10-01-syntax.md

[steveklabnik]

Can you open a new issue for this please?