## Behavior considered undefined

Rust code, including within `unsafe` blocks and `unsafe` functions is incorrect if it exhibits any of the behaviors in the following list. It is the programmer's responsibility when writing `unsafe` code that it is not possible to let `safe` code exhibit these behaviors.

Warning: The following list is not exhaustive. There is no formal model of Rust's semantics for what is and is not allowed in unsafe code, so there may be more behavior considered unsafe. The following list is just what we know for sure is undefined behavior. Please read the Rustonomicon before writing unsafe code.

- Data races.
- Dereferencing a null or dangling raw pointer.
- Unaligned pointer reading and writing outside of `read_unaligned` and `write_unaligned`.
- Reads of undef (uninitialized) memory.
- Breaking the pointer aliasing rules on accesses through raw pointers; a subset of the rules used by C.
- `&mut T` and `&T` follow LLVM’s scoped noalias model, except if the `&T` contains an `UnsafeCell<U>`.
- Mutating non-mutable data — that is, data reached through a shared reference or data owned by a `let` binding), unless that data is contained within an `UnsafeCell<U>`.
- Invoking undefined behavior via compiler intrinsics:
  - Indexing outside of the bounds of an object with `offset` with the exception of one byte past the end of the object.
  - Using `std::ptr::copy_nonoverlapping_memory`, a.k.a. the `memcpy32`and `memcpy64` intrinsics, on overlapping buffers.
- Invalid values in primitive types, even in private fields and locals:
  - Dangling or null references and boxes.
  - A value other than `false` (`0`) or `true` (`1`) in a `bool`.
  - A discriminant in an `enum` not included in the type definition.
  - A value in a `char` which is a surrogate or above `char::MAX`.
  - Non-UTF-8 byte sequences in a `str`.