## Behavior considered undefined

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` 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 any safe code interacting with the `unsafe` code cannot trigger these behaviors. `unsafe` code that satisfies this property for any safe client is called sound; if `unsafe` code can be misused by safe code to exhibit undefined behavior, it is unsound.

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`.
- Executing code compiled with platform features that the current platform does not support (see `target_feature`).

> Note: Undefined behavior affects the entire program. For example, calling a function in C that exhibits undefined behavior of C means your entire program contains undefined behaviour that can also affect the Rust code. And vice versa, undefined behavior in Rust can cause adverse affects on code executed by any FFI calls to other languages.