Add a new example to the "Logging and Debugging" chapter.

Author: nnethercote

src/logging-and-debugging.md

@@ -5,7 +5,8 @@ Either the logging/debugging code itself is slow, or data collection code that
 feeds into logging/debugging code is slow. Make sure that no unnecessary work
 is done for logging/debugging purposes when logging/debugging is not enabled.
 [**Example 1**](https://github.com/rust-lang/rust/pull/50246/commits/2e4f66a86f7baa5644d18bb2adc07a8cd1c7409d),
-[**Example 2**](https://github.com/rust-lang/rust/pull/75133/commits/eeb4b83289e09956e0dda174047729ca87c709fe).
+[**Example 2**](https://github.com/rust-lang/rust/pull/75133/commits/eeb4b83289e09956e0dda174047729ca87c709fe),
+[**Example 3**](https://github.com/rust-lang/rust/pull/147293/commits/cb0f969b623a7e12a0d8166c9a498e17a8b5a3c4).
 
 Note that [`assert!`] calls always run, but [`debug_assert!`] calls only run in
 dev builds. If you have an assertion that is hot but is not necessary for