diff --git a/spec/function.dd b/spec/function.dd
index a6de616004..843f33b3a2 100644
--- a/spec/function.dd
+++ b/spec/function.dd
@@ -3329,6 +3329,16 @@ $(H3 $(LNAME2 trusted-functions, Trusted Functions))
         that they are easier to manually verify.
         )
 
+        $(BEST_PRACTICE Do not use `@trusted` nested functions in order to introduce
+        unsafe code into `@safe` code. Such functions tend to hide their presence
+        inside `@safe` code. Nested functions can manipulate variables in their
+        caller's scope leading to difficulty in manually verifying that it has an
+        `@safe` interface.
+        Instead, `@trusted` functions should be non-nested with a carefully designed
+        `@safe` interface.
+        `@trusted` nested functions may be deprecated in the future.
+        )
+
 $(H3 $(LNAME2 system-functions, System Functions))
 
         $(P System functions are functions not marked with $(CODE @safe) or