Add "Logic errors" as behavior not considered unsafe

In rust-lang/rust#80657 and
rust-lang/rust#80681 it is discussed
how to clarify/define what a "logic error" is and what are
their consequences. The reference should mention them as well.

Signed-off-by: Miguel Ojeda <[email protected]>

Author: ojeda

src/behavior-not-considered-unsafe.md

@@ -36,4 +36,21 @@ semantics.
 See [RFC 560] for error conditions, rationale, and more details about
 integer overflow.
 
+##### Logic errors
+
+Safe code may impose extra logical constraints that can be checked
+at neither compile-time nor runtime. If a program breaks such
+a constraint, the behavior may be unspecified but will not result in
+undefined behavior. This could include panics, incorrect results,
+aborts, and non-termination. The behavior may also differ between
+runs, builds, or kinds of build.
+
+For example, implementing both `Hash` and `Eq` requires that values
+considered equal have equal hashes. Another example are data structures
+like `BinaryHeap`, `BTreeMap`, `BTreeSet`, `HashMap` and `HashSet`
+which describe constraints on the modification of their keys while
+they are in the data structure. Violating such constraints is not
+considered unsafe, yet the program is considered erroneous and
+its behavior unpredictable.
+
 [RFC 560]: https://github.com/rust-lang/rfcs/blob/master/text/0560-integer-overflow.md