Fix and extend ControlFlow `traverse_inorder` example

1. The existing example compiles on its own, but any usage fails
   to be monomorphised and so doesn't compile. Fix that by using
   a mutable reference as an input argument.
2. Added an example usage of `traverse_inorder` showing how we
   can terminate the traversal early.

Fixes #90063

library/core/src/ops/control_flow.rs

@@ -24,7 +24,7 @@ use crate::{convert, ops};
 /// ```
 ///
 /// A basic tree traversal:
-/// ```no_run
+/// ```
 /// use std::ops::ControlFlow;
 ///
 /// pub struct TreeNode<T> {
@@ -34,7 +34,7 @@ use crate::{convert, ops};
 /// }
 ///
 /// impl<T> TreeNode<T> {
-///     pub fn traverse_inorder<B>(&self, f: &impl Fn(&T) -> ControlFlow<B>) -> ControlFlow<B> {
+///     pub fn traverse_inorder<B>(&self, f: &mut impl FnMut(&T) -> ControlFlow<B>) -> ControlFlow<B> {
 ///         if let Some(left) = &self.left {
 ///             left.traverse_inorder(f)?;
 ///         }
@@ -44,30 +44,32 @@ use crate::{convert, ops};
 ///         }
 ///         ControlFlow::Continue(())
 ///     }
+///     fn leaf(value: T) -> Option<Box<TreeNode<T>>> {
+///         Some(Box::new(Self { value, left: None, right: None }))
+///     }
 /// }
 ///
 /// let node = TreeNode {
 ///     value: 0,
-///     left: Some(Box::new(TreeNode {
-///         value: 1,
-///         left: None,
-///         right: None
-///     })),
+///     left: TreeNode::leaf(1),
 ///     right: Some(Box::new(TreeNode {
-///         value: 2,
-///         left: None,
-///         right: None
+///         value: -1,
+///         left: TreeNode::leaf(5),
+///         right: TreeNode::leaf(2),
 ///     }))
 /// };
+/// let mut sum = 0;
 ///
-/// node.traverse_inorder(& |val| {
-///     println!("{}", val);
-///     if *val <= 0 {
-///         ControlFlow::Break(())
+/// let res = node.traverse_inorder(&mut |val| {
+///     if *val < 0 {
+///         ControlFlow::Break(*val)
 ///     } else {
+///         sum += *val;
 ///         ControlFlow::Continue(())
 ///     }
 /// });
+/// assert_eq!(res, ControlFlow::Break(-1));
+/// assert_eq!(sum, 6);
 /// ```
 #[stable(feature = "control_flow_enum_type", since = "1.55.0")]
 #[derive(Debug, Clone, Copy, PartialEq)]