Added break-continue section. (#1793)

This is a contribution of a break-continue section for Comprehensive
Rust.


![image](https://github.com/google/comprehensive-rust/assets/65899331/e68bf439-bcbd-43c9-88bd-f66470a5956f)

![image](https://github.com/google/comprehensive-rust/assets/65899331/1644a460-8373-4878-b6b9-0ce498e8c95d)

---------

Co-authored-by: Dustin J. Mitchell <dustin@v.igoro.us>

src/SUMMARY.md

@@ -35,6 +35,7 @@
     - [`for`](control-flow-basics/loops/for.md)
     - [`loop`](control-flow-basics/loops/loop.md)
   - [`break` and `continue`](control-flow-basics/break-continue.md)
+    - [Labels](control-flow-basics/break-continue/labels.md)
   - [Blocks and Scopes](control-flow-basics/blocks-and-scopes.md)
   - [Functions](control-flow-basics/functions.md)
   - [Macros](control-flow-basics/macros.md)

src/control-flow-basics/break-continue.md

@@ -4,56 +4,26 @@ minutes: 4
 
 # `break` and `continue`
 
+If you want to immediately start the next iteration use
+[`continue`](https://doc.rust-lang.org/reference/expressions/loop-expr.html#continue-expressions).
+
 If you want to exit any kind of loop early, use
 [`break`](https://doc.rust-lang.org/reference/expressions/loop-expr.html#break-expressions).
 For `loop`, this can take an optional expression that becomes the value of the
 `loop` expression.
 
-If you want to immediately start the next iteration use
-[`continue`](https://doc.rust-lang.org/reference/expressions/loop-expr.html#continue-expressions).
-
 ```rust,editable
 fn main() {
-    let (mut a, mut b) = (100, 52);
-    let result = loop {
-        if a == b {
-            break a;
-        }
-        if a < b {
-            b -= a;
-        } else {
-            a -= b;
-        }
-    };
-    println!("{result}");
-}
-```
-
-Both `continue` and `break` can optionally take a label argument which is used
-to break out of nested loops:
-
-```rust,editable
-fn main() {
-    'outer: for x in 1..5 {
-        println!("x: {x}");
     let mut i = 0;
-        while i < x {
+    loop {
-            println!("x: {x}, i: {i}");
         i += 1;
-            if i == 3 {
+        if i > 5 {
-                break 'outer;
+            break;
         }
+        if i % 2 == 0 {
+            continue;
         }
+        println!("{}", i);
     }
 }
 ```
-
-In this case we break the outer loop after 3 iterations of the inner loop.
-
-<details>
-
-- Note that `loop` is the only looping construct which returns a non-trivial
-  value. This is because it's guaranteed to be entered at least once (unlike
-  `while` and `for` loops).
-
-</details>

src/control-flow-basics/break-continue/labels.md

@@ -0,0 +1,29 @@
+# Labels
+
+Both `continue` and `break` can optionally take a label argument which is used
+to break out of nested loops:
+
+```rust,editable
+fn main() {
+    let s = [[5, 6, 7], [8, 9, 10], [21, 15, 32]];
+    let mut found = false;
+    let target_value = 10;
+    'outer: for i in 0..=2 {
+        for j in 0..=2 {
+            if s[i][j] == target_value {
+                found = true;
+                break 'outer;
+            }
+        }
+    }
+    print!("{}", found)
+}
+```
+
+<details>
+
+- Note that `loop` is the only looping construct which returns a non-trivial
+  value. This is because it's guaranteed to be entered at least once (unlike
+  `while` and `for` loops).
+
+</details>