style-guide: Narrow guidance about references and dereferencing

The style guide advises "prefer dereferencing to taking references", but
doesn't give guidance on when that "preference" should get overridden by
other considerations. Give an example of when it's fine to ignore
that advice.

Author: joshtriplett

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