Struct DynMetadata

pub struct DynMetadata<Dyn: ?Sized> {
    _vtable_ptr: NonNull<VTable>,
    _phantom: PhantomData<Dyn>,
}

🔬This is a nightly-only experimental API. (ptr_metadata #81513)

The metadata for a Dyn = dyn SomeTrait trait object type.

It is a pointer to a vtable (virtual call table) that represents all the necessary information to manipulate the concrete type stored inside a trait object. The vtable notably contains:

• type size
• type alignment
• a pointer to the type’s drop_in_place impl (may be a no-op for plain-old-data)
• pointers to all the methods for the type’s implementation of the trait

Note that the first three are special because they’re necessary to allocate, drop, and deallocate any trait object.

It is possible to name this struct with a type parameter that is not a dyn trait object (for example DynMetadata<u64>) but not to obtain a meaningful value of that struct.

Note that while this type implements PartialEq, comparing vtable pointers is unreliable: pointers to vtables of the same type for the same trait can compare inequal (because vtables are duplicated in multiple codegen units), and pointers to vtables of different types/traits can compare equal (since identical vtables can be deduplicated within a codegen unit).

Fields

_vtable_ptr: NonNull<VTable>

🔬This is a nightly-only experimental API. (ptr_metadata #81513)

_phantom: PhantomData<Dyn>

🔬This is a nightly-only experimental API. (ptr_metadata #81513)

Implementations

impl<Dyn: ?Sized> DynMetadata<Dyn>

fn vtable_ptr(self) -> *const VTable

🔬This is a nightly-only experimental API. (ptr_metadata #81513)

When DynMetadata appears as the metadata field of a wide pointer, the rustc_middle layout computation does magic and the resulting layout is not a FieldsShape::Aggregate, instead it is a FieldsShape::Primitive. This means that the same type can have different layout depending on whether it appears as the metadata field of a wide pointer or as a stand-alone type, which understandably confuses codegen and leads to ICEs when trying to project to a field of DynMetadata. To work around that issue, we use transmute instead of using a field projection.

pub fn size_of(self) -> usize

🔬This is a nightly-only experimental API. (ptr_metadata #81513)

Returns the size of the type associated with this vtable.

pub fn align_of(self) -> usize

🔬This is a nightly-only experimental API. (ptr_metadata #81513)

Returns the alignment of the type associated with this vtable.

pub fn layout(self) -> Layout

🔬This is a nightly-only experimental API. (ptr_metadata #81513)

Returns the size and alignment together as a Layout

Trait Implementations

impl<Dyn: ?Sized> Clone for DynMetadata<Dyn>

fn clone(&self) -> Self

Returns a copy of the value.

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

Performs copy-assignment from source.

impl<Dyn: ?Sized> Debug for DynMetadata<Dyn>

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

Formats the value using the given formatter.

impl<Dyn: ?Sized> Eq for DynMetadata<Dyn>

#[doc(hidden)] fn assert_receiver_is_total_eq(&self)

impl<Dyn: ?Sized> Hash for DynMetadata<Dyn>

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

Feeds this value into the given Hasher.

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

where Self: Sized,

Feeds a slice of this type into the given Hasher.

impl<Dyn: ?Sized> Ord for DynMetadata<Dyn>

fn cmp(&self, other: &Self) -> 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<Dyn: ?Sized> PartialEq for DynMetadata<Dyn>

fn eq(&self, other: &Self) -> 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<Dyn: ?Sized> PartialOrd for DynMetadata<Dyn>

fn partial_cmp(&self, other: &Self) -> 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.

#[doc(hidden)] fn __chaining_lt(&self, other: &Rhs) -> ControlFlow<bool>

🔬This is a nightly-only experimental API. (partial_ord_chaining_methods)

If self == other, returns ControlFlow::Continue(()). Otherwise, returns ControlFlow::Break(self < other).

#[doc(hidden)] fn __chaining_le(&self, other: &Rhs) -> ControlFlow<bool>

🔬This is a nightly-only experimental API. (partial_ord_chaining_methods)

Same as __chaining_lt, but for <= instead of <.

#[doc(hidden)] fn __chaining_gt(&self, other: &Rhs) -> ControlFlow<bool>

🔬This is a nightly-only experimental API. (partial_ord_chaining_methods)

Same as __chaining_lt, but for > instead of <.

#[doc(hidden)] fn __chaining_ge(&self, other: &Rhs) -> ControlFlow<bool>

🔬This is a nightly-only experimental API. (partial_ord_chaining_methods)

Same as __chaining_lt, but for >= instead of <.

impl<Dyn: ?Sized> Copy for DynMetadata<Dyn>

impl<Dyn: ?Sized> Send for DynMetadata<Dyn>

impl<Dyn: ?Sized> Sync for DynMetadata<Dyn>

impl<Dyn: ?Sized> Unpin for DynMetadata<Dyn>