no excuse

Author: RalfJung

src/behavior-considered-undefined.md

@@ -2,7 +2,9 @@
 
 Rust code is incorrect if it exhibits any of the behaviors in the following
 list. This includes code within `unsafe` blocks and `unsafe` functions.
-`unsafe` is not an excuse to cause undefined behavior.
+`unsafe` only means that avoiding undefined behavior is on the programmer; it
+does not change anything about the fact that Rust programs must never cause
+undefined behavior.
 
 It is the programmer's responsibility when writing `unsafe` code to ensure that it is not
 possible to let `safe` code exhibit these behaviors. `unsafe` code that