diff --git a/src/doc/rustc-dev-guide/src/diagnostics.md b/src/doc/rustc-dev-guide/src/diagnostics.md
index 51d6c27ae395..2212fc790dbf 100644
--- a/src/doc/rustc-dev-guide/src/diagnostics.md
+++ b/src/doc/rustc-dev-guide/src/diagnostics.md
@@ -54,7 +54,7 @@ LL | more code
 The text should be matter of fact and avoid capitalization and periods, unless
 multiple sentences are _needed_:
 
-```
+```txt
 error: the fobrulator needs to be krontrificated
 ```