-#![unstable(feature = "ptr_metadata", issue = /* FIXME */ "none")]
+#![unstable(feature = "ptr_metadata", issue = "81513")]
use crate::fmt;
use crate::hash::{Hash, Hasher};
-use crate::ptr::NonNull;
-/// FIXME docs
+/// Provides the pointer metadata type of any pointed-to type.
+///
+/// # Pointer metadata
+///
+/// Raw pointer types and reference types in Rust can be thought of as made of two parts:
+/// a data pointer that contains the memory address of the value, and some metadata.
+///
+/// For statically-sized types (that implement the `Sized` traits)
+/// as well as for `extern` types,
+/// pointers are said to be “thin”: metadata is zero-sized and its type is `()`.
+///
+/// Pointers to [dynamically-sized types][dst] are said to be “wide” or “fat”,
+/// they have non-zero-sized metadata:
+///
+/// * For structs whose last field is a DST, metadata is the metadata for the last field
+/// * For the `str` type, metadata is the length in bytes as `usize`
+/// * For slice types like `[T]`, metadata is the length in items as `usize`
+/// * For trait objects like `dyn SomeTrait`, metadata is [`DynMetadata<Self>`][DynMetadata]
+/// (e.g. `DynMetadata<dyn SomeTrait>`)
+///
+/// In the future, the Rust language may gain new kinds of types
+/// that have different pointer metadata.
+///
+/// [dst]: https://doc.rust-lang.org/nomicon/exotic-sizes.html#dynamically-sized-types-dsts
+///
+///
+/// # The `Pointee` trait
+///
+/// The point of this trait is its `Metadata` associated type,
+/// which is `()` or `usize` or `DynMetadata<_>` as described above.
+/// It is automatically implemented for every type.
+/// It can be assumed to be implemented in a generic context, even without a corresponding bound.
+///
+///
+/// # Usage
+///
+/// Raw pointers can be decomposed into the data address and metadata components
+/// with their [`to_raw_parts`] method.
+///
+/// Alternatively, metadata alone can be extracted with the [`metadata`] function.
+/// A reference can be passed to [`metadata`] and implicitly coerced.
+///
+/// A (possibly-wide) pointer can be put back together from its address and metadata
+/// with [`from_raw_parts`] or [`from_raw_parts_mut`].
+///
+/// [`to_raw_parts`]: *const::to_raw_parts
#[lang = "pointee_trait"]
pub trait Pointee {
/// The type for metadata in pointers and references to `Self`.
type Metadata: Copy + Send + Sync + Ord + Hash + Unpin;
}
-/// Pointers to types implementing this trait alias are “thin”
+/// Pointers to types implementing this trait alias are “thin”.
+///
+/// This includes statically-`Sized` types and `extern` types.
+///
+/// # Example
///
/// ```rust
/// #![feature(ptr_metadata)]
/// assert_eq!(std::mem::size_of::<&T>(), std::mem::size_of::<usize>())
/// }
/// ```
-#[unstable(feature = "ptr_metadata", issue = /* FIXME */ "none")]
+#[unstable(feature = "ptr_metadata", issue = "81513")]
// NOTE: don’t stabilize this before trait aliases are stable in the language?
pub trait Thin = Pointee<Metadata = ()>;
/// Extract the metadata component of a pointer.
+///
+/// Values of type `*mut T`, `&T`, or `&mut T` can be passed directly to this function
+/// as they implicitly coerce to `*const T`.
+///
+/// # Example
+///
+/// ```
+/// #![feature(ptr_metadata)]
+///
+/// assert_eq!(std::ptr::metadata("foo"), 3_usize);
+/// ```
+#[rustc_const_unstable(feature = "ptr_metadata", issue = "81513")]
#[inline]
-pub fn metadata<T: ?Sized>(ptr: *const T) -> <T as Pointee>::Metadata {
+pub const fn metadata<T: ?Sized>(ptr: *const T) -> <T as Pointee>::Metadata {
// SAFETY: Accessing the value from the `PtrRepr` union is safe since *const T
// and PtrComponents<T> have the same memory layouts. Only std can make this
// guarantee.
unsafe { PtrRepr { const_ptr: ptr }.components.metadata }
}
+/// Forms a (possibly-wide) raw pointer from a data address and metadata.
+///
+/// This function is safe but the returned pointer is not necessarily safe to dereference.
+/// For slices, see the documentation of [`slice::from_raw_parts`] for safety requirements.
+/// For trait objects, the metadata must come from a pointer to the same underlying ereased type.
+///
+/// [`slice::from_raw_parts`]: crate::slice::from_raw_parts
+#[unstable(feature = "ptr_metadata", issue = "81513")]
+#[rustc_const_unstable(feature = "ptr_metadata", issue = "81513")]
+#[inline]
+pub const fn from_raw_parts<T: ?Sized>(
+ data_address: *const (),
+ metadata: <T as Pointee>::Metadata,
+) -> *const T {
+ // SAFETY: Accessing the value from the `PtrRepr` union is safe since *const T
+ // and PtrComponents<T> have the same memory layouts. Only std can make this
+ // guarantee.
+ unsafe { PtrRepr { components: PtrComponents { data_address, metadata } }.const_ptr }
+}
+
+/// Performs the same functionality as [`from_raw_parts`], except that a
+/// raw `*mut` pointer is returned, as opposed to a raw `*const` pointer.
+///
+/// See the documentation of [`from_raw_parts`] for more details.
+#[unstable(feature = "ptr_metadata", issue = "81513")]
+#[rustc_const_unstable(feature = "ptr_metadata", issue = "81513")]
+#[inline]
+pub const fn from_raw_parts_mut<T: ?Sized>(
+ data_address: *mut (),
+ metadata: <T as Pointee>::Metadata,
+) -> *mut T {
+ // SAFETY: Accessing the value from the `PtrRepr` union is safe since *const T
+ // and PtrComponents<T> have the same memory layouts. Only std can make this
+ // guarantee.
+ unsafe { PtrRepr { components: PtrComponents { data_address, metadata } }.mut_ptr }
+}
+
#[repr(C)]
-union PtrRepr<T: ?Sized> {
- const_ptr: *const T,
- components: PtrComponents<T>,
+pub(crate) union PtrRepr<T: ?Sized> {
+ pub(crate) const_ptr: *const T,
+ pub(crate) mut_ptr: *mut T,
+ pub(crate) components: PtrComponents<T>,
}
#[repr(C)]
-struct PtrComponents<T: ?Sized> {
- data_address: usize,
- metadata: <T as Pointee>::Metadata,
+pub(crate) struct PtrComponents<T: ?Sized> {
+ pub(crate) data_address: *const (),
+ pub(crate) metadata: <T as Pointee>::Metadata,
}
// Manual impl needed to avoid `T: Copy` bound.
}
}
-/// The metadata for a `dyn SomeTrait` trait object type.
+/// 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 it 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.
#[lang = "dyn_metadata"]
pub struct DynMetadata<Dyn: ?Sized> {
- #[allow(unused)]
- vtable_ptr: NonNull<()>,
+ vtable_ptr: &'static VTable,
phantom: crate::marker::PhantomData<Dyn>,
}
+/// The common prefix of all vtables. It is followed by function pointers for trait methods.
+///
+/// Private implementation detail of `DynMetadata::size_of` etc.
+#[repr(C)]
+struct VTable {
+ drop_in_place: fn(*mut ()),
+ size_of: usize,
+ align_of: usize,
+}
+
+impl<Dyn: ?Sized> DynMetadata<Dyn> {
+ /// Returns the size of the type associated with this vtable.
+ #[inline]
+ pub fn size_of(self) -> usize {
+ self.vtable_ptr.size_of
+ }
+
+ /// Returns the alignment of the type associated with this vtable.
+ #[inline]
+ pub fn align_of(self) -> usize {
+ self.vtable_ptr.align_of
+ }
+
+ /// Returns the size and alignment together as a `Layout`
+ #[inline]
+ pub fn layout(self) -> crate::alloc::Layout {
+ // SAFETY: the compiler emitted this vtable for a concrete Rust type which
+ // is known to have a valid layout. Same rationale as in `Layout::for_value`.
+ unsafe { crate::alloc::Layout::from_size_align_unchecked(self.size_of(), self.align_of()) }
+ }
+}
+
unsafe impl<Dyn: ?Sized> Send for DynMetadata<Dyn> {}
unsafe impl<Dyn: ?Sized> Sync for DynMetadata<Dyn> {}
impl<Dyn: ?Sized> fmt::Debug for DynMetadata<Dyn> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
- f.write_str("DynMetadata { … }")
+ f.debug_tuple("DynMetadata").field(&(self.vtable_ptr as *const VTable)).finish()
}
}
impl<Dyn: ?Sized> PartialEq for DynMetadata<Dyn> {
#[inline]
fn eq(&self, other: &Self) -> bool {
- self.vtable_ptr == other.vtable_ptr
+ crate::ptr::eq::<VTable>(self.vtable_ptr, other.vtable_ptr)
}
}
impl<Dyn: ?Sized> Ord for DynMetadata<Dyn> {
#[inline]
fn cmp(&self, other: &Self) -> crate::cmp::Ordering {
- self.vtable_ptr.cmp(&other.vtable_ptr)
+ (self.vtable_ptr as *const VTable).cmp(&(other.vtable_ptr as *const VTable))
}
}
impl<Dyn: ?Sized> PartialOrd for DynMetadata<Dyn> {
#[inline]
fn partial_cmp(&self, other: &Self) -> Option<crate::cmp::Ordering> {
- Some(self.vtable_ptr.cmp(&other.vtable_ptr))
+ Some(self.cmp(other))
}
}
impl<Dyn: ?Sized> Hash for DynMetadata<Dyn> {
#[inline]
fn hash<H: Hasher>(&self, hasher: &mut H) {
- self.vtable_ptr.hash(hasher)
+ crate::ptr::hash::<VTable, _>(self.vtable_ptr, hasher)
}
}
projects / rust.git / blobdiff