Struct PhantomData 

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

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;

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

impl<R: ResType> ExternalResource<R> {
    fn new() -> Self {
        let size_of_res = 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> AsBytes for PhantomData<T>

where T: ?Sized,

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

Gets the bytes of this value.

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

where Self: FromBytes,

Gets the bytes of this value mutably.

fn write_to(&self, bytes: &mut [u8]) -> Option<()>

Writes a copy of self to bytes.

fn write_to_prefix(&self, bytes: &mut [u8]) -> Option<()>

Writes a copy of self to the prefix of bytes.

fn write_to_suffix(&self, bytes: &mut [u8]) -> Option<()>

Writes a copy of self to the suffix of bytes.

impl<T> CanonicalDeserialize for PhantomData<T>

where T: Send + Sync,

fn deserialize_with_mode<R>( _reader: R, _compress: Compress, _validate: Validate, ) -> Result<PhantomData<T>, SerializationError>

where R: Read,

The general deserialize method that takes in customization flags.

fn deserialize_compressed<R>(reader: R) -> Result<Self, SerializationError>

where R: Read,

fn deserialize_compressed_unchecked<R>( reader: R, ) -> Result<Self, SerializationError>

where R: Read,

fn deserialize_uncompressed<R>(reader: R) -> Result<Self, SerializationError>

where R: Read,

fn deserialize_uncompressed_unchecked<R>( reader: R, ) -> Result<Self, SerializationError>

where R: Read,

impl<T> CanonicalSerialize for PhantomData<T>

fn serialize_with_mode<W>( &self, _writer: W, _compress: Compress, ) -> Result<(), SerializationError>

where W: Write,

The general serialize method that takes in customization flags.

fn serialized_size(&self, _compress: Compress) -> usize

fn serialize_compressed<W>(&self, writer: W) -> Result<(), SerializationError>

where W: Write,

fn compressed_size(&self) -> usize

fn serialize_uncompressed<W>(&self, writer: W) -> Result<(), SerializationError>

where W: Write,

fn uncompressed_size(&self) -> usize

impl<T> Clone for PhantomData<T>

where T: ?Sized,

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

Returns a duplicate of the value.

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

Performs copy-assignment from source.

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> Decodable for PhantomData<T>

where T: ?Sized,

fn decode(_buf: &mut &[u8]) -> Result<PhantomData<T>, Error>

Decodes the blob into the appropriate type. buf must be advanced past the decoded object.

impl<'a, T> Decode<'a> for PhantomData<T>

where T: ?Sized,

Dummy implementation for PhantomData which allows deriving implementations on structs with phantom fields.

fn decode<R>(_reader: &mut R) -> Result<PhantomData<T>, Error>

where R: Reader<'a>,

Attempt to decode this message using the provided decoder.

fn from_der(bytes: &'a [u8]) -> Result<Self, Error>

Parse Self from the provided DER-encoded byte slice.

impl<T> Decode for PhantomData<T>

fn decode<I>(_input: &mut I) -> Result<PhantomData<T>, Error>

where I: Input,

Attempt to deserialise the value from input.

fn decode_into<I>( input: &mut I, dst: &mut MaybeUninit<Self>, ) -> Result<DecodeFinished, Error>

where I: Input,

Attempt to deserialize the value from input into a pre-allocated piece of memory.

fn skip<I>(input: &mut I) -> Result<(), Error>

where I: Input,

Attempt to skip the encoded value from input.

fn encoded_fixed_size() -> Option<usize>

Returns the fixed encoded size of the type.

impl<T> Decode for PhantomData<T>

fn decode<I>(_input: &mut I) -> Result<PhantomData<T>, Error>

where I: Input,

Attempt to deserialise the value from input.

fn decode_into<I>( input: &mut I, dst: &mut MaybeUninit<Self>, ) -> Result<DecodeFinished, Error>

where I: Input,

Attempt to deserialize the value from input into a pre-allocated piece of memory.

fn skip<I>(input: &mut I) -> Result<(), Error>

where I: Input,

Attempt to skip the encoded value from input.

fn encoded_fixed_size() -> Option<usize>

Returns the fixed encoded size of the type.

impl<T> Default for PhantomData<T>

where T: ?Sized,

fn default() -> PhantomData<T>

Returns the “default value” for a type.

impl<T> DerOrd for PhantomData<T>

Provide a no-op implementation for PhantomData

fn der_cmp(&self, _other: &PhantomData<T>) -> Result<Ordering, Error>

Return an Ordering between self and other when serialized as ASN.1 DER.

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<'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> Encodable for PhantomData<T>

where T: ?Sized,

fn length(&self) -> usize

Returns the length of the encoding of this type in bytes.

fn encode(&self, _out: &mut dyn BufMut)

Encodes the type into the out buffer.

impl<T> Encode for PhantomData<T>

fn encode_to<W>(&self, _dest: &mut W)

where W: Output + ?Sized,

Convert self to a slice and append it to the destination.

fn size_hint(&self) -> usize

If possible give a hint of expected size of the encoding.

fn encode(&self) -> Vec<u8>

Convert self to an owned vector.

fn using_encoded<R, F>(&self, f: F) -> R

where F: FnOnce(&[u8]) -> R,

Convert self to a slice and then invoke the given closure with it.

fn encoded_size(&self) -> usize

Calculates the encoded size.

impl<T> Encode for PhantomData<T>

fn encode_to<W>(&self, _dest: &mut W)

where W: Output + ?Sized,

Convert self to a slice and append it to the destination.

fn size_hint(&self) -> usize

If possible give a hint of expected size of the encoding.

fn encode(&self) -> Vec<u8>

Convert self to an owned vector.

fn using_encoded<R, F>(&self, f: F) -> R

where F: FnOnce(&[u8]) -> R,

Convert self to a slice and then invoke the given closure with it.

fn encoded_size(&self) -> usize

Calculates the encoded size.

impl<T> Encode for PhantomData<T>

where T: ?Sized,

Dummy implementation for PhantomData which allows deriving implementations on structs with phantom fields.

fn encoded_len(&self) -> Result<Length, Error>

Compute the length of this value in bytes when encoded as ASN.1 DER.

fn encode(&self, _writer: &mut impl Writer) -> Result<(), Error>

Encode this value as ASN.1 DER using the provided Writer.

fn encode_to_slice<'a>(&self, buf: &'a mut [u8]) -> Result<&'a [u8], Error>

Encode this value to the provided byte slice, returning a sub-slice containing the encoded message.

fn encode_to_vec(&self, buf: &mut Vec<u8>) -> Result<Length, Error>

Encode this message as ASN.1 DER, appending it to the provided byte vector.

fn to_der(&self) -> Result<Vec<u8>, Error>

Encode this type as DER, returning a byte vector.

impl<T> EncodeAsType for PhantomData<T>

fn encode_as_type_to<R>( &self, type_id: <R as TypeResolver>::TypeId, types: &R, out: &mut Vec<u8>, ) -> Result<(), Error>

where R: TypeResolver,

Given some type_id, types, a context and some output target for the SCALE encoded bytes, attempt to SCALE encode the current value into the type given by type_id.

fn encode_as_type<R>( &self, type_id: <R as TypeResolver>::TypeId, types: &R, ) -> Result<Vec<u8>, Error>

where R: TypeResolver,

This is a helper function which internally calls EncodeAsType::encode_as_type_to. Prefer to implement that instead.

impl<T> FromBytes for PhantomData<T>

where T: ?Sized,

fn ref_from(bytes: &[u8]) -> Option<&Self>

where Self: Sized,

Interprets the given bytes as a &Self without copying.

fn ref_from_prefix(bytes: &[u8]) -> Option<&Self>

where Self: Sized,

Interprets the prefix of the given bytes as a &Self without copying.

fn ref_from_suffix(bytes: &[u8]) -> Option<&Self>

where Self: Sized,

Interprets the suffix of the given bytes as a &Self without copying.

fn mut_from(bytes: &mut [u8]) -> Option<&mut Self>

where Self: Sized + AsBytes,

Interprets the given bytes as a &mut Self without copying.

fn mut_from_prefix(bytes: &mut [u8]) -> Option<&mut Self>

where Self: Sized + AsBytes,

Interprets the prefix of the given bytes as a &mut Self without copying.

fn mut_from_suffix(bytes: &mut [u8]) -> Option<&mut Self>

where Self: Sized + AsBytes,

Interprets the suffix of the given bytes as a &mut Self without copying.

fn slice_from(bytes: &[u8]) -> Option<&[Self]>

where Self: Sized,

Interprets the given bytes as a &[Self] without copying.

fn slice_from_prefix(bytes: &[u8], count: usize) -> Option<(&[Self], &[u8])>

where Self: Sized,

Interprets the prefix of the given bytes as a &[Self] with length equal to count without copying.

fn slice_from_suffix(bytes: &[u8], count: usize) -> Option<(&[u8], &[Self])>

where Self: Sized,

Interprets the suffix of the given bytes as a &[Self] with length equal to count without copying.

fn mut_slice_from(bytes: &mut [u8]) -> Option<&mut [Self]>

where Self: Sized + AsBytes,

Interprets the given bytes as a &mut [Self] without copying.

fn mut_slice_from_prefix( bytes: &mut [u8], count: usize, ) -> Option<(&mut [Self], &mut [u8])>

where Self: Sized + AsBytes,

Interprets the prefix of the given bytes as a &mut [Self] with length equal to count without copying.

fn mut_slice_from_suffix( bytes: &mut [u8], count: usize, ) -> Option<(&mut [u8], &mut [Self])>

where Self: Sized + AsBytes,

Interprets the suffix of the given bytes as a &mut [Self] with length equal to count without copying.

fn read_from(bytes: &[u8]) -> Option<Self>

where Self: Sized,

Reads a copy of Self from bytes.

fn read_from_prefix(bytes: &[u8]) -> Option<Self>

where Self: Sized,

Reads a copy of Self from the prefix of bytes.

fn read_from_suffix(bytes: &[u8]) -> Option<Self>

where Self: Sized,

Reads a copy of Self from the suffix of bytes.

impl<T> FromZeroes for PhantomData<T>

where T: ?Sized,

fn zero(&mut self)

Overwrites self with zeroes.

fn new_zeroed() -> Self

where Self: Sized,

Creates an instance of Self from zeroed bytes.

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> IntoVisitor for PhantomData<T>

type AnyVisitor<R: TypeResolver> = BasicVisitor<PhantomData<T>, R>

The visitor type used to decode SCALE encoded bytes to Self.

fn into_visitor<R>() -> <PhantomData<T> as IntoVisitor>::AnyVisitor<R>

where R: TypeResolver,

A means of obtaining this visitor.

impl<T> MaxEncodedLen for PhantomData<T>

fn max_encoded_len() -> usize

Upper bound, in bytes, of the maximum encoded size of this item.

impl<T> MaxEncodedLen for PhantomData<T>

fn max_encoded_len() -> usize

Upper bound, in bytes, of the maximum encoded size of this item.

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> PalletError for PhantomData<T>

const MAX_ENCODED_SIZE: usize = 0usize

The maximum encoded size for the implementing type.

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> 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> TypeInfo for PhantomData<T>

type Identity = PhantomData<()>

The type identifying for which type info is provided.

fn type_info() -> Type

Returns the static type identifier for Self.

impl<T> Valid for PhantomData<T>

where T: Sync,

fn check(&self) -> Result<(), SerializationError>

fn batch_check<'a>( batch: impl Iterator<Item = &'a Self> + Send, ) -> Result<(), SerializationError>

where Self: 'a,

impl<T> ValueOrd for PhantomData<T>

Provide a no-op implementation for PhantomData

fn value_cmp(&self, _other: &PhantomData<T>) -> Result<Ordering, Error>

Return an Ordering between value portion of TLV-encoded self and other when serialized as ASN.1 DER.

impl<Z> Zeroize for PhantomData<Z>

PhantomData is always zero sized so provide a Zeroize implementation.

fn zeroize(&mut self)

Zero out this object from memory using Rust intrinsics which ensure the zeroization operation is not “optimized away” by the compiler.

impl<T> ConstEncodedLen for PhantomData<T>

where T: ConstEncodedLen,

impl<T> ConstEncodedLen for PhantomData<T>

where T: ConstEncodedLen,

impl<T> Copy for PhantomData<T>

where T: ?Sized,

impl<T> DecodeWithMemTracking for PhantomData<T>

where PhantomData<T>: Decode,

impl<T> DecodeWithMemTracking for PhantomData<T>

where PhantomData<T>: Decode,

impl<T> EncodeLike for PhantomData<T>

impl<T> EncodeLike for PhantomData<T>

impl<T> Eq for PhantomData<T>

where T: ?Sized,

impl<T> Freeze for PhantomData<T>

where T: ?Sized,

impl<T> StructuralPartialEq for PhantomData<T>

where T: ?Sized,

impl<T> Unaligned for PhantomData<T>

where T: ?Sized,

impl<Z> ZeroizeOnDrop for PhantomData<Z>

[PhantomData is always zero sized so provide a ZeroizeOnDrop implementation.