## 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 (using the `*` operator on) a dangling or unaligned raw pointer.
- Breaking the pointer aliasing rules. `&mut T` and `&T` follow LLVM’s scoped noalias model, except if the `&T` contains an `UnsafeCell<U>`.
- Mutating immutable data. All data inside a `const` item is immutable. Moreover, all data reached through a shared reference or data owned by an immutable binding is immutable, unless that data is contained within an `UnsafeCell<U>`.
- Invoking undefined behavior via compiler intrinsics.
- Executing code compiled with platform features that the current platform does not support (see `target_feature`).
- Calling a function with the wrong call ABI or unwinding from a function with the wrong unwind ABI.
- Producing an invalid value, even in private fields and locals. "Producing" a value happens any time a value is assigned to or read from a place, passed to a function/primitive operation or returned from a function/primitive operation. The following values are invalid (at their respective type):

  - A value other than `false` (`0`) or `true` (`1`) in a `bool`.

  - A discriminant in an `enum` not included in the type definition.

  - A null `fn` pointer.

  - A value in a `char` which is a surrogate or above `char::MAX`.

  - A `!` (all values are invalid for this type).

  - An integer (`i*`/`u*`), floating point value (`f*`), or raw pointer obtained from uninitialized memory.

  - A reference or `Box<T>` that is dangling, unaligned, or points to an invalid value.

  - Invalid metadata in a wide reference, `Box<T>`, or raw pointer:

    - `dyn Trait` metadata is invalid if it is not a pointer to a vtable for `Trait` that matches the actual dynamic trait the pointer or reference points to.
    - Slice metadata is invalid if if the length is not a valid `usize` (i.e., it must not be read from uninitialized memory).

  - Non-UTF-8 byte sequences in a `str`.

  - Invalid values for a type with a custom definition of invalid values. In the standard library, this affects `NonNull<T>` and `NonZero*`.

    > Note: `rustc` achieves this with the unstable `rustc_layout_scalar_valid_range_*` attributes.

A reference/pointer is "dangling" if it is null or not all of the bytes it points to are part of the same allocation (so in particular they all have to be part of some allocation). The span of bytes it points to is determined by the pointer value and the size of the pointee type. As a consequence, if the span is empty, "dangling" is the same as "non-null". Note that slices point to their entire range, so it is important that the length metadata is never too large. In particular, allocations and therefore slices cannot be bigger than `isize::MAX` bytes.

> 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.