diff --git a/src/doc/style-guide/src/expressions.md b/src/doc/style-guide/src/expressions.md
index 8271b42da4c70..f5e37b6c46f69 100644
--- a/src/doc/style-guide/src/expressions.md
+++ b/src/doc/style-guide/src/expressions.md
@@ -288,7 +288,8 @@ and other assignment operators such as `+=` or `*=`).
 
 For comparison operators, because for `T op U`, `&T op &U` is also implemented:
 if you have `t: &T`, and `u: U`, prefer `*t op u` to `t op &u`. In general,
-within expressions, prefer dereferencing to taking references.
+within expressions, prefer dereferencing to taking references, unless necessary
+(e.g. to avoid an unnecessarily expensive operation).
 
 Use parentheses liberally, do not necessarily elide them due to precedence.
 Tools should not automatically insert or remove parentheses. Do not use spaces