Keyword unsafe

Code or interfaces whose memory safety cannot be verified by the type system.

The unsafe keyword has two uses:

• to declare the existence of contracts the compiler can’t check (unsafe fn and unsafe trait),
• and to declare that a programmer has checked that these contracts have been upheld (unsafe {} and unsafe impl, but also unsafe fn – see below).

They are not mutually exclusive, as can be seen in unsafe fn: the body of an unsafe fn is, by default, treated like an unsafe block. The unsafe_op_in_unsafe_fn lint can be enabled to change that.

Unsafe abilities

No matter what, Safe Rust can’t cause Undefined Behavior. This is referred to as soundness: a well-typed program actually has the desired properties. The Nomicon has a more detailed explanation on the subject.

To ensure soundness, Safe Rust is restricted enough that it can be automatically checked. Sometimes, however, it is necessary to write code that is correct for reasons which are too clever for the compiler to understand. In those cases, you need to use Unsafe Rust.

Here are the abilities Unsafe Rust has in addition to Safe Rust:

• Dereference raw pointers
• Implement unsafe traits
• Call unsafe functions
• Mutate statics (including external ones)
• Access fields of unions

However, this extra power comes with extra responsibilities: it is now up to you to ensure soundness. The unsafe keyword helps by clearly marking the pieces of code that need to worry about this.

The different meanings of unsafe

Not all uses of unsafe are equivalent: some are here to mark the existence of a contract the programmer must check, others are to say “I have checked the contract, go ahead and do this”. The following discussion on Rust Internals has more in-depth explanations about this but here is a summary of the main points:

• unsafe fn: calling this function means abiding by a contract the compiler cannot enforce.
• unsafe trait: implementing the trait means abiding by a contract the compiler cannot enforce.
• unsafe {}: the contract necessary to call the operations inside the block has been checked by the programmer and is guaranteed to be respected.
• unsafe impl: the contract necessary to implement the trait has been checked by the programmer and is guaranteed to be respected.

By default, unsafe fn also acts like an unsafe {} block around the code inside the function. This means it is not just a signal to the caller, but also promises that the preconditions for the operations inside the function are upheld. Mixing these two meanings can be confusing, so the unsafe_op_in_unsafe_fn lint can be enabled to warn against that and require explicit unsafe blocks even inside unsafe fn.

See the Rustonomicon and the Reference for more information.

Examples

Marking elements as unsafe

unsafe can be used on functions. Note that functions and statics declared in extern blocks are implicitly marked as unsafe (but not functions declared as extern "something" fn ...). Mutable statics are always unsafe, wherever they are declared. Methods can also be declared as unsafe:

static mut FOO: &str = "hello";

unsafe fn unsafe_fn() {}

extern "C" {
    fn unsafe_extern_fn();
    static BAR: *mut u32;
}

trait SafeTraitWithUnsafeMethod {
    unsafe fn unsafe_method(&self);
}

struct S;

impl S {
    unsafe fn unsafe_method_on_struct() {}
}

Traits can also be declared as unsafe:

unsafe trait UnsafeTrait {}

Since unsafe fn and unsafe trait indicate that there is a safety contract that the compiler cannot enforce, documenting it is important. The standard library has many examples of this, like the following which is an extract from Vec::set_len. The # Safety section explains the contract that must be fulfilled to safely call the function.

/// Forces the length of the vector to `new_len`.
///
/// This is a low-level operation that maintains none of the normal
/// invariants of the type. Normally changing the length of a vector
/// is done using one of the safe operations instead, such as
/// `truncate`, `resize`, `extend`, or `clear`.
///
/// # Safety
///
/// - `new_len` must be less than or equal to `capacity()`.
/// - The elements at `old_len..new_len` must be initialized.
pub unsafe fn set_len(&mut self, new_len: usize)

Using unsafe {} blocks and impls

Performing unsafe operations requires an unsafe {} block:

#![deny(unsafe_op_in_unsafe_fn)]

/// Dereference the given pointer.
///
/// # Safety
///
/// `ptr` must be aligned and must not be dangling.
unsafe fn deref_unchecked(ptr: *const i32) -> i32 {
    // SAFETY: the caller is required to ensure that `ptr` is aligned and dereferenceable.
    unsafe { *ptr }
}

let a = 3;
let b = &a as *const _;
// SAFETY: `a` has not been dropped and references are always aligned,
// so `b` is a valid address.
unsafe { assert_eq!(*b, deref_unchecked(b)); };

unsafe and traits

The interactions of unsafe and traits can be surprising, so let us contrast the two combinations of safe fn in unsafe trait and unsafe fn in safe trait using two examples:

/// # Safety
///
/// `make_even` must return an even number.
unsafe trait MakeEven {
    fn make_even(&self) -> i32;
}

// SAFETY: Our `make_even` always returns something even.
unsafe impl MakeEven for i32 {
    fn make_even(&self) -> i32 {
        self << 1
    }
}

fn use_make_even(x: impl MakeEven) {
    if x.make_even() % 2 == 1 {
        // SAFETY: this can never happen, because all `MakeEven` implementations
        // ensure that `make_even` returns something even.
        unsafe { std::hint::unreachable_unchecked() };
    }
}

Note how the safety contract of the trait is upheld by the implementation, and is itself used to uphold the safety contract of the unsafe function unreachable_unchecked called by use_make_even. make_even itself is a safe function because its callers do not have to worry about any contract, only the implementation of MakeEven is required to uphold a certain contract. use_make_even is safe because it can use the promise made by MakeEven implementations to uphold the safety contract of the unsafe fn unreachable_unchecked it calls.

It is also possible to have unsafe fn in a regular safe trait:

#![deny(unsafe_op_in_unsafe_fn)]

trait Indexable {
    const LEN: usize;

    /// # Safety
    ///
    /// The caller must ensure that `idx < LEN`.
    unsafe fn idx_unchecked(&self, idx: usize) -> i32;
}

// The implementation for `i32` doesn't need to do any contract reasoning.
impl Indexable for i32 {
    const LEN: usize = 1;

    unsafe fn idx_unchecked(&self, idx: usize) -> i32 {
        debug_assert_eq!(idx, 0);
        *self
    }
}

// The implementation for arrays exploits the function contract to
// make use of `get_unchecked` on slices and avoid a run-time check.
impl Indexable for [i32; 42] {
    const LEN: usize = 42;

    unsafe fn idx_unchecked(&self, idx: usize) -> i32 {
        // SAFETY: As per this trait's documentation, the caller ensures
        // that `idx < 42`.
        unsafe { *self.get_unchecked(idx) }
    }
}

// The implementation for the never type declares a length of 0,
// which means `idx_unchecked` can never be called.
impl Indexable for ! {
    const LEN: usize = 0;

    unsafe fn idx_unchecked(&self, idx: usize) -> i32 {
        // SAFETY: As per this trait's documentation, the caller ensures
        // that `idx < 0`, which is impossible, so this is dead code.
        unsafe { std::hint::unreachable_unchecked() }
    }
}

fn use_indexable<I: Indexable>(x: I, idx: usize) -> i32 {
    if idx < I::LEN {
        // SAFETY: We have checked that `idx < I::LEN`.
        unsafe { x.idx_unchecked(idx) }
    } else {
        panic!("index out-of-bounds")
    }
}

This time, use_indexable is safe because it uses a run-time check to discharge the safety contract of idx_unchecked. Implementing Indexable is safe because when writing idx_unchecked, we don’t have to worry: our callers need to discharge a proof obligation (like use_indexable does), but the implementation of get_unchecked has no proof obligation to contend with. Of course, the implementation of Indexable may choose to call other unsafe operations, and then it needs an unsafe block to indicate it discharged the proof obligations of its callees. (We enabled unsafe_op_in_unsafe_fn, so the body of idx_unchecked is not implicitly an unsafe block.) For that purpose it can make use of the contract that all its callers must uphold – the fact that idx < LEN.

Formally speaking, an unsafe fn in a trait is a function with preconditions that go beyond those encoded by the argument types (such as idx < LEN), whereas an unsafe trait can declare that some of its functions have postconditions that go beyond those encoded in the return type (such as returning an even integer). If a trait needs a function with both extra precondition and extra postcondition, then it needs an unsafe fn in an unsafe trait.