Struct PhantomData

pub struct PhantomData<T>
where
    T: ?Sized;

core Zero-sized type used to mark things that “act like” they own a T.

Adding a PhantomData<T> field to your type tells the compiler that your type acts as though it stores a value of type T, even though it doesn’t really. This information is used when computing certain safety properties.

For a more in-depth explanation of how to use PhantomData<T>, please see the Nomicon.

A ghastly note 👻👻👻

Though they both have scary names, PhantomData and ‘phantom types’ are related, but not identical. A phantom type parameter is simply a type parameter which is never used. In Rust, this often causes the compiler to complain, and the solution is to add a “dummy” use by way of PhantomData.

Examples

Unused lifetime parameters

Perhaps the most common use case for PhantomData is a struct that has an unused lifetime parameter, typically as part of some unsafe code. For example, here is a struct Slice that has two pointers of type *const T, presumably pointing into an array somewhere:

struct Slice<'a, T> {
    start: *const T,
    end: *const T,
}

The intention is that the underlying data is only valid for the lifetime 'a, so Slice should not outlive 'a. However, this intent is not expressed in the code, since there are no uses of the lifetime 'a and hence it is not clear what data it applies to. We can correct this by telling the compiler to act as if the Slice struct contained a reference &'a T:

use std::marker::PhantomData;

struct Slice<'a, T> {
    start: *const T,
    end: *const T,
    phantom: PhantomData<&'a T>,
}

This also in turn infers the lifetime bound T: 'a, indicating that any references in T are valid over the lifetime 'a.

When initializing a Slice you simply provide the value PhantomData for the field phantom:

fn borrow_vec<T>(vec: &Vec<T>) -> Slice<'_, T> {
    let ptr = vec.as_ptr();
    Slice {
        start: ptr,
        end: unsafe { ptr.add(vec.len()) },
        phantom: PhantomData,
    }
}

Unused type parameters

It sometimes happens that you have unused type parameters which indicate what type of data a struct is “tied” to, even though that data is not actually found in the struct itself. Here is an example where this arises with FFI. The foreign interface uses handles of type *mut () to refer to Rust values of different types. We track the Rust type using a phantom type parameter on the struct ExternalResource which wraps a handle.

use std::marker::PhantomData;
use std::mem;

struct ExternalResource<R> {
   resource_handle: *mut (),
   resource_type: PhantomData<R>,
}

impl<R: ResType> ExternalResource<R> {
    fn new() -> Self {
        let size_of_res = mem::size_of::<R>();
        Self {
            resource_handle: foreign_lib::new(size_of_res),
            resource_type: PhantomData,
        }
    }

    fn do_stuff(&self, param: ParamType) {
        let foreign_params = convert_params(param);
        foreign_lib::do_stuff(self.resource_handle, foreign_params);
    }
}

Ownership and the drop check

The exact interaction of PhantomData with drop check may change in the future.

Currently, adding a field of type PhantomData<T> indicates that your type owns data of type T in very rare circumstances. This in turn has effects on the Rust compiler’s drop check analysis. For the exact rules, see the drop check documentation.

Layout

For all T, the following are guaranteed:

• size_of::<PhantomData<T>>() == 0
• align_of::<PhantomData<T>>() == 1

Trait Implementations

impl<T> Archive for PhantomData<T>

where T: ?Sized,

const COPY_OPTIMIZATION: CopyOptimization<PhantomData<T>>

An optimization flag that allows the bytes of this type to be copied directly to a writer instead of calling serialize.

type Archived = PhantomData<T>

The archived representation of this type.

type Resolver = ()

The resolver for this type. It must contain all the additional information from serializing needed to make the archived type from the normal type.

fn resolve( &self, _: <PhantomData<T> as Archive>::Resolver, _: Place<<PhantomData<T> as Archive>::Archived>, )

Creates the archived version of this value at the given position and writes it to the given output.

impl<T> BitSized<0> for PhantomData<T>

const BIT_SIZE: usize = _

The bit size of this type (only the relevant data part, without padding).

const MIN_BYTE_SIZE: usize = _

The rounded up byte size for this type.

fn bit_size(&self) -> usize

Returns the bit size of this type (only the relevant data part, without padding).

fn min_byte_size(&self) -> usize

Returns the rounded up byte size for this type.

impl<T, C> CheckBytes<C> for PhantomData<T>

where C: Fallible + ?Sized, T: ?Sized,

unsafe fn check_bytes( _: *const PhantomData<T>, _: &mut C, ) -> Result<(), <C as Fallible>::Error>

Checks whether the given pointer points to a valid value within the given context.

impl<T> Clone for PhantomData<T>

where T: ?Sized,

fn clone(&self) -> PhantomData<T>

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<T> ConstDefault for PhantomData<T>

const DEFAULT: Self = Self

Returns the compile-time “default value” for a type.

impl<T> Debug for PhantomData<T>

where T: ?Sized,

fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter.

impl<T> Default for PhantomData<T>

where T: ?Sized,

fn default() -> PhantomData<T>

Returns the “default value” for a type.

impl<'de, T> Deserialize<'de> for PhantomData<T>

where T: ?Sized,

fn deserialize<D>( deserializer: D, ) -> Result<PhantomData<T>, <D as Deserializer<'de>>::Error>

where D: Deserializer<'de>,

Deserialize this value from the given Serde deserializer.

impl<T, D> Deserialize<PhantomData<T>, D> for PhantomData<T>

where D: Fallible + ?Sized, T: ?Sized,

fn deserialize( &self, _: &mut D, ) -> Result<PhantomData<T>, <D as Fallible>::Error>

Deserializes using the given deserializer

impl<'de, T> DeserializeSeed<'de> for PhantomData<T>

where T: Deserialize<'de>,

type Value = T

The type produced by using this seed.

fn deserialize<D>( self, deserializer: D, ) -> Result<T, <D as Deserializer<'de>>::Error>

where D: Deserializer<'de>,

Equivalent to the more common Deserialize::deserialize method, except with some initial piece of data (the seed) passed in.

impl<T> Hash for PhantomData<T>

where T: ?Sized,

fn hash<H>(&self, _: &mut H)

where H: Hasher,

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl<T: ?Sized + 'static> MemPod for PhantomData<T>

Available on crate feature unsafe_layout only.

fn zeroed() -> Self

Returns a new instance contrcuted from zeroes.

fn from_bytes(bytes: &[u8]) -> Self

Returns a new instance constructed from the given bytes.

fn as_bytes(&self) -> &[u8]

Returns the instance’s data as a slice of bytes.

fn as_bytes_mut(&mut self) -> &mut [u8]

Returns the instance’s data as a mutable slice of bytes.

impl<T> Ord for PhantomData<T>

where T: ?Sized,

fn cmp(&self, _other: &PhantomData<T>) -> Ordering

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Self

where Self: Sized,

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

where Self: Sized,

Compares and returns the minimum of two values.

fn clamp(self, min: Self, max: Self) -> Self

where Self: Sized,

Restrict a value to a certain interval.

impl<T> PartialEq for PhantomData<T>

where T: ?Sized,

fn eq(&self, _other: &PhantomData<T>) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<T> PartialOrd for PhantomData<T>

where T: ?Sized,

fn partial_cmp(&self, _other: &PhantomData<T>) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

fn lt(&self, other: &Rhs) -> bool

Tests less than (for self and other) and is used by the < operator.

fn le(&self, other: &Rhs) -> bool

Tests less than or equal to (for self and other) and is used by the <= operator.

fn gt(&self, other: &Rhs) -> bool

Tests greater than (for self and other) and is used by the > operator.

fn ge(&self, other: &Rhs) -> bool

Tests greater than or equal to (for self and other) and is used by the >= operator.

impl<T, S> Serialize<S> for PhantomData<T>

where S: Fallible + ?Sized, T: ?Sized,

fn serialize( &self, _: &mut S, ) -> Result<<PhantomData<T> as Archive>::Resolver, <S as Fallible>::Error>

Writes the dependencies for the object and returns a resolver that can create the archived type.

impl<T> Serialize for PhantomData<T>

where T: ?Sized,

fn serialize<S>( &self, serializer: S, ) -> Result<<S as Serializer>::Ok, <S as Serializer>::Error>

where S: Serializer,

Serialize this value into the given Serde serializer.

impl<T> Zeroable for PhantomData<T>

where T: ?Sized,

fn zeroed() -> Self

Calls zeroed.

impl<T> Copy for PhantomData<T>

where T: ?Sized,

impl<T> Eq for PhantomData<T>

where T: ?Sized,

impl<T> Freeze for PhantomData<T>

where T: ?Sized,

impl<T> Pod for PhantomData<T>

where T: 'static + ?Sized,

impl<T> Portable for PhantomData<T>

where T: ?Sized,

impl<T> StructuralPartialEq for PhantomData<T>

where T: ?Sized,