More dynamic content representation

1 files changed, 64 insertions, 0 deletions

diff --git a/src/util/fat.rs b/src/util/fat.rs
new file mode 100644
index 00000000..bb557bc9
--- /dev/null
+++ b/src/util/fat.rs
@@ -0,0 +1,64 @@
+//! Fat pointer handling.
+//!
+//! This assumes the memory representation of fat pointers. Although it is not
+//! guaranteed by Rust, it's improbable that it will change. Still, when the
+//! pointer metadata APIs are stable, we should definitely move to them:
+//! <https://github.com/rust-lang/rust/issues/81513>
+
+use std::alloc;
+use std::mem;
+
+/// Create a fat pointer from a data address and a vtable address.
+///
+/// # Safety
+/// Must only be called when `T` is a `dyn Trait`. The data address must point
+/// to a value whose type implements the trait of `T` and the `vtable` must have
+/// been extracted with [`vtable`].
+pub unsafe fn from_raw_parts<T: ?Sized>(data: *const (), vtable: *const ()) -> *const T {
+    debug_assert_eq!(
+        alloc::Layout::new::<*const T>(),
+        alloc::Layout::new::<FatPointer>(),
+    );
+
+    let fat = FatPointer { data, vtable };
+    mem::transmute_copy::<FatPointer, *const T>(&fat)
+}
+
+/// Create a mutable fat pointer from a data address and a vtable address.
+///
+/// # Safety
+/// Must only be called when `T` is a `dyn Trait`. The data address must point
+/// to a value whose type implements the trait of `T` and the `vtable` must have
+/// been extracted with [`vtable`].
+pub unsafe fn from_raw_parts_mut<T: ?Sized>(data: *mut (), vtable: *const ()) -> *mut T {
+    debug_assert_eq!(
+        alloc::Layout::new::<*mut T>(),
+        alloc::Layout::new::<FatPointer>(),
+    );
+
+    let fat = FatPointer { data, vtable };
+    mem::transmute_copy::<FatPointer, *mut T>(&fat)
+}
+
+/// Extract the address to a trait object's vtable.
+///
+/// # Safety
+/// Must only be called when `T` is a `dyn Trait`.
+pub unsafe fn vtable<T: ?Sized>(ptr: *const T) -> *const () {
+    debug_assert_eq!(
+        alloc::Layout::new::<*const T>(),
+        alloc::Layout::new::<FatPointer>(),
+    );
+
+    mem::transmute_copy::<*const T, FatPointer>(&ptr).vtable
+}
+
+/// The memory representation of a trait object pointer.
+///
+/// Although this is not guaranteed by Rust, it's improbable that it will
+/// change.
+#[repr(C)]
+struct FatPointer {
+    data: *const (),
+    vtable: *const (),
+}