Add documentation about Undefined Behavior

docs/book/src/SUMMARY.md

@@ -70,6 +70,7 @@
   - [Attributes](./reference/attributes.md)
   - [Style Guide](./reference/style_guide.md)
   - [Known Issues and Workarounds](./reference/known_issues_and_workarounds.md)
+  - [Behavior Considered Undefined](./reference/undefined_behavior.md)
   - [Differences From Solidity](./reference/solidity_differences.md)
   - [Differences From Rust](./reference/rust_differences.md)
   - [Contributing To Sway](./reference/contributing_to_sway.md)

docs/book/src/reference/undefined_behavior.md

@@ -0,0 +1,15 @@
+# Behavior Considered Undefined
+
+Sway code that contains any of the following behavior is considered undefined.
+The compiler is allowed to treat undefined Sway code however it desires,
+including removing it or replacing it with any other Sway code.
+
+This is not an exhaustive list, it may grow or shrink, there is no formal model
+of Sway's semantics so there may be more behavior considered undefined. We
+reserve the right to make some of the listed behavior defined in the future.
+
+* Invalid arithmetic operations (overflows, underflows, division by zero, etc)
+* Misuse of compiler intrinsics
+* Incorrect use of inline assembly
+* Reading and writing `raw_ptr` and `raw_slice`
+* Slicing and indexing out of bounds by directly using compiler intrinsics.