RFC: Pointer metadata & VTable

text/0000-ptr-meta.md

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+- Feature Name: `ptr-meta`
+- Start Date: 2018-10-26
+- RFC PR:
+- Rust Issue:
+
+# Summary
+[summary]: #summary
+
+Add generic APIs that allow manipulating the metadata of fat pointers:
+
+* Naming the metadata’s type  (as an associated type)
+* Extracting metadata from a pointer
+* Reconstructing a pointer from a data pointer and metadata
+* Representing vtables, the metadata for trait objects, as a type with some limited API
+
+This RFC does *not* propose a mechanism for defining custom dynamically-sized types,
+but tries to stay compatible with future proposals that do.
+
+
+# Background
+[background]: #background
+
+Typical high-level code doesn’t need to worry about fat pointers,
+a reference `&Foo` “just works” wether or not `Foo` is a DST.
+But unsafe code such as a custom collection library may want to access a fat pointer’s
+components separately.
+
+In Rust 1.11 we *removed* a [`std::raw::Repr`] trait and a [`std::raw::Slice`] type
+from the standard library.
+`Slice` could be `transmute`d to a `&[U]` or `&mut [U]` reference to a slice
+as it was guaranteed to have the same memory layout.
+This was replaced with more specific and less wildly unsafe
+`std::slice::from_raw_parts` and `std::slice::from_raw_parts_mut` functions,
+together with `as_ptr` and `len` methods that extract each fat pointer component separatly.
+
+For trait objects, where we still have an unstable `std::raw::TraitObject` type
+that can only be used with `transmute`:
+
+```rust
+#[repr(C)]
+pub struct TraitObject {
+    pub data: *mut (),
+    pub vtable: *mut (),
+}
+```
+
+[`std::raw::Repr`]: https://doc.rust-lang.org/1.10.0/std/raw/trait.Repr.html
+[`std::raw::Slice`]: https://doc.rust-lang.org/1.10.0/std/raw/struct.Slice.html
+[`std::raw::TraitObjet`]: https://doc.rust-lang.org/1.30.0/std/raw/struct.TraitObject.html
+
+
+# Motivation
+[motivation]: #motivation
+
+We now have APIs in Stable Rust to let unsafe code freely and reliably manipulate slices,
+accessing the separate components of a fat pointers and then re-assembling them.
+However `std::raw::TraitObject` is still unstable,
+but it’s probably not the style of API that we’ll want to stabilize
+as it encourages dangerous `transmute` calls.
+This is a “hole” in available APIs to manipulate existing Rust types.
+
+For example [this library][lib] stores multiple trait objects of varying size
+in contiguous memory together with their vtable pointers,
+and during iteration recreates fat pointers from separate data and vtable pointers.
+
+The new `Thin` trait alias also expanding to [extern types] some APIs
+that were unnecessarily restricted to `Sized` types
+because there was previously no way to express pointer-thinness in generic code.
+
+[lib]: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2015&gist=bbeecccc025f5a7a0ad06086678e13f3
+
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+
+Let’s build generic type similar to `Box<dyn Trait>`,
+but where the vtable pointer is stored in heap memory next to the value
+so that the pointer is thin.
+First, let’s get some boilerplate out of the way:
+
+```rust
+use std::marker::{PhantomData, Unsize};
+use std::ptr::{self, DynMetadata};
+
+trait DynTrait = Pointee<Metadata=DynMetadata>;
+
+pub struct ThinBox<Dyn: ?Sized + DynTrait> {
+    ptr: ptr::NonNull<WithMeta<()>>,
+    phantom: PhantomData<Dyn>,
+}
+
+#[repr(C)]
+struct WithMeta<T: ?Sized> {
+    vtable: DynMetadata,
+    value: T,
+}
+```
+
+Since [unsized rvalues] are not implemented yet,
+our constructor is going to “unsize” from a concrete type that implements our trait.
+The `Unsize` bound ensures we can cast from `&S` to a `&Dyn` trait object
+and construct the appopriate metadata.
+
+[unsized rvalues]: https://github.com/rust-lang/rust/issues/48055
+
+We let `Box` do the memory layout computation and allocation:
+
+```rust
+impl<Dyn: ?Sized + DynTrait> ThinBox<Dyn> {
+    pub fn new_unsize<S>(value: S) -> Self where S: Unsize<Dyn> {
+        let vtable = ptr::metadata(&value as &Dyn);
+        let ptr = NonNull::from(Box::leak(Box::new(WithMeta { vtable, value }))).cast();
+        ThinBox { ptr, phantom: PhantomData }
+    }
+}
+```
+
+(Another possible constructor is `pub fn new_copy(value: &Dyn) where Dyn: Copy`,
+but it would involve slightly more code.)
+
+Accessing the value requires knowing its alignment:
+
+```rust
+impl<Dyn: ?Sized + DynTrait> ThinBox<Dyn> {
+    fn data_ptr(&self) -> *mut () {
+        unsafe {
+            let offset = std::mem::size_of::<DynMetadata>();
+            let value_align = self.ptr.as_ref().vtable.align();
+            let offset = align_up_to(offset, value_align);
+            (self.ptr.as_ptr() as *mut u8).add(offset) as *mut ()
+        }
+    }
+}
+
+/// <https://github.com/rust-lang/rust/blob/1.30.0/src/libcore/alloc.rs#L199-L219>
+fn align_up_to(offset: usize, align: usize) -> usize {
+    offset.wrapping_add(align).wrapping_sub(1) & !align.wrapping_sub(1)
+}
+
+// Similarly Deref
+impl<Dyn: ?Sized + DynTrait> DerefMut for ThinBox<Dyn> {
+    fn deref_mut(&mut self) -> &mut Dyn {
+        unsafe {
+            &mut *<*mut Dyn>::from_raw_parts(self.data_ptr(), *self.ptr.as_ref().vtable)
+        }
+    }
+}
+```
+
+Finally, in `Drop` we may not be able to take advantage of `Box` again
+since the original `Sized` type `S` is not statically known at this point.
+
+```rust
+impl<Dyn: ?Sized + DynTrait> Drop for ThinBox<Dyn> {
+    fn drop(&mut self) {
+        unsafe {
+            let layout = /* left as an exercise for the reader */;
+            ptr::drop_in_place::<Dyn>(&mut **self);
+            alloc::dealloc(self.ptr.cast(), layout);
+        }
+    }
+}
+```
+
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+The APIs whose full definition is found below
+are added to `core::ptr` and re-exported in `std::ptr`:
+
+* A `Pointee` trait,
+  implemented automatically for all types
+  (similar to how `Sized` and `Unsize` are implemented automatically).
+* A `Thin` [trait alias].
+  If this RFC is implemented before type aliases are,
+  uses of `Thin` should be replaced with its definition.
+* A `metadata` free function
+* A `DynMetadata` struct
+* A `from_raw_parts` constructor for each of `*const T`, `*mut T`, and `NonNull<T>`.
+
+The bounds on `null()` and `null_mut()` function in that same module
+as well as the `NonNull::dangling` constructor
+are changed from (implicit) `T: Sized` to `T: ?Sized + Thin`.
+Similarly for the `U` type parameter of the `NonNull::cast` method.
+This enables using those functions with [extern types].
+
+The `Pointee` trait is implemented for all types.
+This can be relied on in generic code,
+even if a type parameter `T` does not have an explicit `T: Pointee` bound.
+This is similar to how the `Any` trait can be used without an explicit `T: Any` bound,
+only `T: 'static`, because a blanket `impl<T: 'static> Any for T {…}` exists.
+(Except that `Pointee` is not restricted to `'static`.)
+
+For the purpose of pointer casts being allowed by the `as` operator,
+a pointer to `T` is considered to be thin if `T: Thin` instead of `T: Sized`.
+This similarly includes extern types.
+
+`std::raw::TraitObject` and `std::raw` are deprecated and eventually removed.
+
+[trait alias]: https://github.com/rust-lang/rust/issues/41517
+[extern types]: https://github.com/rust-lang/rust/issues/43467
+
+```rust
+/// This trait is automatically implement for every type.
+///
+/// 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 “fat”
+/// and 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`].
+///
+/// In the future, the Rust language may gain new kinds of types
+/// that have different pointer metadata.
+///
+/// Pointer metadata can be extracted from a pointer or reference with the [`metadata`] function.
+/// The data pointer can be extracted by casting a (fat) pointer
+/// to a (thin) pointer to a `Sized` type the `as` operator,
+/// for example `(x: &dyn SomeTrait) as *const SomeTrait as *const ()`.
+///
+/// [dst]: https://doc.rust-lang.org/nomicon/exotic-sizes.html#dynamically-sized-types-dsts
+#[lang = "pointee"]
+pub trait Pointee {
+    /// The type for metadata in pointers and references to `Self`.
+    type Metadata: Copy + Send + Sync + Ord + Hash + Unpin + 'static;
+}
+
+/// Pointers to types implementing this trait alias are “thin”:
+///
+/// ```rust
+/// fn this_never_panics<T: std::ptr::Thin>() {
+///     assert_eq!(std::mem::size_of::<&T>(), std::mem::size_of::<usize>())
+/// }
+/// ```
+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`.
+/// For example:
+///
+/// ```
+/// assert_eq(std::ptr::metadata("foo"), 3_usize);
+/// ```
+///
+/// Note that the data component of a (fat) pointer can be extracted by casting
+/// to a (thin) pointer to any `Sized` type:
+///
+/// ```
+/// # trait SomeTrait {}
+/// # fn example(something: &SomeTrait) {
+/// let object: &SomeTrait = something;
+/// let data_ptr = object as *const SomeTrait as *const ();
+/// # }
+/// ```
+pub fn metadata<T: ?Sized>(ptr: *const T) -> <T as Pointee>::Metadata {…}
+
+impl<T: ?Sized> *const T {
+    pub fn from_raw_parts(data: *const (), meta: <T as Pointee>::Metadata) -> Self {…}
+}
+
+impl<T: ?Sized> *mut T {
+    pub fn from_raw_parts(data: *mut (), meta: <T as Pointee>::Metadata) -> Self {…}
+}
+
+impl<T: ?Sized> NonNull<T> {
+    pub fn from_raw_parts(data: NonNull<()>, meta: <T as Pointee>::Metadata) -> Self {
+        unsafe {
+            NonNull::new_unchecked(<*mut _>::from_raw_parts(data.as_ptr(), meta))
+        }
+    }
+}
+
+/// The metadata for a `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.
+///
+/// The layout of vtables is still unspecified, so this type is a more-type-safe
+/// convenience for accessing those 3 special values. Note however that `DynMetadata` does
+/// not actually know the trait it’s associated with, indicating that, at very least,
+/// the location of `size`, `align`, and `drop_in_place` is identical for all
+/// trait object vtables in a single program.
+#[derive(Copy, Clone)]
+pub struct DynMetadata {
+    vtable_ptr: ptr::NonNull<()>,
+}
+
+impl DynMetadata {
+    /// Returns the size of the type associated with this vtable.
+    pub fn size(self) -> usize { ... }
+
+    /// Returns the alignment of the type associated with this vtable.
+    pub fn align(self) -> usize { ... }
+
+    /// Returns the size and alignment together as a `Layout`
+    pub fn layout(self) -> alloc::Layout {
+        unsafe {
+            alloc::Layout::from_size_align_unchecked(self.size(), self.align())
+        }
+    }
+}
+```
+
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+The status quo is that code (such as linked in [Motivation]) that requires this functionality
+needs to transmute to and from `std::raw::TraitObject`
+or a copy of it (to be compatible with Stable Rust).
+Additionally, in cases where constructing the data pointer
+requires knowing the alignment of the concrete type,
+a dangling pointer such as `0x8000_0000_usize as *mut ()` needs to be created.
+It is not clear whether `std::mem::align_of(&*ptr)` with `ptr: *const dyn SomeTrait`
+is Undefined Behavior with a dangling data pointer.
+
+A [previous iteration][2579] of this RFC proposed a `DynTrait`
+that would only be implemented for trait objects like `dyn SomeTrait`.
+There would be no `Metadata` associated type, `DynMetadata` was hard-coded in the trait.
+In addition to being more general
+and (hopefully) more compatible with future custom DSTs proposals,
+this RFC resolves the question of what happens
+if trait objects with super-fat pointers with multiple vtable pointers are ever added.
+(Answer: they can use a different metadata type like `[DynMetadata; N]`.)
+
+`DynMetadata` could be made generic with a type parameter for the trait object type that it describes.
+This would avoid forcing that the size, alignment, and destruction pointers
+be in the same location (offset) for every vtable.
+But keeping them in the same location is probaly desirable anyway to keep code size small.
+
+[2579]: https://github.com/rust-lang/rfcs/pull/2579
+
+
+# Prior art
+[prior-art]: #prior-art
+
+A previous [Custom Dynamically-Sized Types][cdst] RFC was postponed.
+[Internals thread #6663][6663] took the same ideas
+and was even more ambitious in being very general.
+Except for `DynMetadata`’s methods, this RFC proposes a subset of what that thread did.
+
+[cdst]: https://github.com/rust-lang/rfcs/pull/1524
+[6663]: https://internals.rust-lang.org/t/pre-erfc-lets-fix-dsts/6663
+
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+* The name of `Pointee`. [Internals thread #6663][6663] used `Referent`.
+
+* The location of `DynMetadata`. Is another module more appropriate than `std::ptr`?
+
+* Should `DynMetadata` have a type parameter for what trait object type it is a metadata of?
+  Such that `<dyn SomeTrait as Pointee>::Metadata` is `DynMetadata<dyn SomTrait>`.
+  This would allow the memory layout to change based on the trait object type
+  (potentially super-wide pointers with multiple vtable pointers for multi-trait objects?)
+  or to have different methods.
+
+* The name of `Thin`.
+  This name is short and sweet but `T: Thin` suggests that `T` itself is thin,
+  rather than pointers and references to `T`.
+
+* The location of `Thin`. Better in `std::marker`?
+
+* Should `Thin` be added as a supertrait of `Sized`?
+  Or could it ever make sense to have fat pointers to statically-sized types?
+
+* Are there other generic standard library APIs like `ptr::null()`
+  that have an (implicit) `T: Sized` bound that unneccesarily excludes extern types?
+
+* Should `<*mut _>::from_raw_parts` and friends be `unsafe fn`s?
+
+* API design: free functions v.s. methods/constructors on `*mut _` and `*const _`?