Auto merge of #27305 - KieranHunt:master, r=steveklabnik

I found that the book had little information for `loop`s and `loop` label so I've added some.

Author: bors

src/doc/trpl/SUMMARY.md

@@ -26,8 +26,7 @@
     * [Primitive Types](primitive-types.md)
     * [Comments](comments.md)
     * [if](if.md)
-    * [for loops](for-loops.md)
-    * [while loops](while-loops.md)
+    * [Loops](loops.md)
     * [Ownership](ownership.md)
     * [References and Borrowing](references-and-borrowing.md)
     * [Lifetimes](lifetimes.md)

src/doc/trpl/for-loops.md

@@ -0,0 +1,209 @@
+% Loops
+
+Rust currently provides three approaches to performing some kind of iterative activity. They are: `loop`, `while` and `for`. Each approach has its own set of uses.
+
+## loop
+
+The infinite `loop` is the simplest form of loop available in Rust. Using the keyword `loop`, Rust provides a way to loop indefinitely until some terminating statement is reached. Rust's infinite `loop`s look like this:
+
+```rust,ignore
+loop {
+    println!("Loop forever!");
+}
+```
+
+## while
+
+Rust also has a `while` loop. It looks like this:
+
+```rust
+let mut x = 5; // mut x: i32
+let mut done = false; // mut done: bool
+
+while !done {
+    x += x - 3;
+
+    println!("{}", x);
+
+    if x % 5 == 0 {
+        done = true;
+    }
+}
+```
+
+`while` loops are the correct choice when you’re not sure how many times
+you need to loop.
+
+If you need an infinite loop, you may be tempted to write this:
+
+```rust,ignore
+while true {
+```
+
+However, `loop` is far better suited to handle this case:
+
+```rust,ignore
+loop {
+```
+
+Rust’s control-flow analysis treats this construct differently than a `while
+true`, since we know that it will always loop. In general, the more information
+we can give to the compiler, the better it can do with safety and code
+generation, so you should always prefer `loop` when you plan to loop
+infinitely.
+
+## for
+
+The `for` loop is used to loop a particular number of times. Rust’s `for` loops
+work a bit differently than in other systems languages, however. Rust’s `for`
+loop doesn’t look like this “C-style” `for` loop:
+
+```c
+for (x = 0; x < 10; x++) {
+    printf( "%d\n", x );
+}
+```
+
+Instead, it looks like this:
+
+```rust
+for x in 0..10 {
+    println!("{}", x); // x: i32
+}
+```
+
+In slightly more abstract terms,
+
+```ignore
+for var in expression {
+    code
+}
+```
+
+The expression is an [iterator][iterator]. The iterator gives back a series of
+elements. Each element is one iteration of the loop. That value is then bound
+to the name `var`, which is valid for the loop body. Once the body is over, the
+next value is fetched from the iterator, and we loop another time. When there
+are no more values, the `for` loop is over.
+
+[iterator]: iterators.html
+
+In our example, `0..10` is an expression that takes a start and an end position,
+and gives an iterator over those values. The upper bound is exclusive, though,
+so our loop will print `0` through `9`, not `10`.
+
+Rust does not have the “C-style” `for` loop on purpose. Manually controlling
+each element of the loop is complicated and error prone, even for experienced C
+developers.
+
+### Enumerate
+
+When you need to keep track of how many times you already looped, you can use the `.enumerate()` function.
+
+#### On ranges:
+
+```rust
+for (i,j) in (5..10).enumerate() {
+    println!("i = {} and j = {}", i, j);
+}
+```
+
+Outputs:
+
+```text
+i = 0 and j = 5
+i = 1 and j = 6
+i = 2 and j = 7
+i = 3 and j = 8
+i = 4 and j = 9
+```
+
+Don't forget to add the parentheses around the range.
+
+#### On iterators:
+
+```rust
+# let lines = "hello\nworld".lines();
+for (linenumber, line) in lines.enumerate() {
+    println!("{}: {}", linenumber, line);
+}
+```
+
+Outputs:
+
+```text
+0: Content of line one
+1: Content of line two
+2: Content of line tree
+3: Content of line four
+```
+
+## Ending iteration early
+
+Let’s take a look at that `while` loop we had earlier:
+
+```rust
+let mut x = 5;
+let mut done = false;
+
+while !done {
+    x += x - 3;
+
+    println!("{}", x);
+
+    if x % 5 == 0 {
+        done = true;
+    }
+}
+```
+
+We had to keep a dedicated `mut` boolean variable binding, `done`, to know
+when we should exit out of the loop. Rust has two keywords to help us with
+modifying iteration: `break` and `continue`.
+
+In this case, we can write the loop in a better way with `break`:
+
+```rust
+let mut x = 5;
+
+loop {
+    x += x - 3;
+
+    println!("{}", x);
+
+    if x % 5 == 0 { break; }
+}
+```
+
+We now loop forever with `loop` and use `break` to break out early. Issuing an explicit `return` statement will also serve to terminate the loop early.
+
+`continue` is similar, but instead of ending the loop, goes to the next
+iteration. This will only print the odd numbers:
+
+```rust
+for x in 0..10 {
+    if x % 2 == 0 { continue; }
+
+    println!("{}", x);
+}
+```
+
+## Loop labels
+
+You may also encounter situations where you have nested loops and need to
+specify which one your `break` or `continue` statement is for. Like most
+other languages, by default a `break` or `continue` will apply to innermost
+loop. In a sitation where you would like to a `break` or `continue` for one
+of the outer loops, you can use labels to specify which loop the `break` or
+ `continue` statement applies to. This will only print when both `x` and `y` are
+ odd:
+
+```rust
+'outer: for x in 0..10 {
+    'inner: for y in 0..10 {
+        if x % 2 == 0 { continue 'outer; } // continues the loop over x
+        if y % 2 == 0 { continue 'inner; } // continues the loop over y
+        println!("x: {}, y: {}", x, y);
+    }
+}
+```