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Add "Logic errors" as behavior not considered unsafe #919

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Jan 11, 2021
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Add "Logic errors" as behavior not considered unsafe #919

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17 changes: 17 additions & 0 deletions src/behavior-not-considered-unsafe.md
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Expand Up @@ -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