Add ptr::from_raw_parts, ptr::from_raw_parts_mut, and NonNull::from_raw_parts

The use of module-level functions instead of associated functions
on `<*const T>` or `<*mut T>` follows the precedent of
`ptr::slice_from_raw_parts` and `ptr::slice_from_raw_parts_mut`.

Author: SimonSapin

library/core/src/lib.rs

@@ -123,6 +123,7 @@
 #![feature(auto_traits)]
 #![feature(or_patterns)]
 #![feature(prelude_import)]
+#![cfg_attr(not(bootstrap), feature(ptr_metadata))]
 #![feature(repr_simd, platform_intrinsics)]
 #![feature(rustc_attrs)]
 #![feature(simd_ffi)]

library/core/src/ptr/metadata.rs

@@ -36,16 +36,48 @@ pub fn metadata<T: ?Sized>(ptr: *const T) -> <T as Pointee>::Metadata {
     unsafe { PtrRepr { const_ptr: ptr }.components.metadata }
 }
 
+/// Forms a raw pointer from a data address and metadata.
+#[unstable(feature = "ptr_metadata", issue = /* FIXME */ "none")]
+#[rustc_const_unstable(feature = "ptr_metadata", issue = /* FIXME */ "none")]
+#[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 = /* FIXME */ "none")]
+#[rustc_const_unstable(feature = "ptr_metadata", issue = /* FIXME */ "none")]
+#[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.

library/core/src/ptr/mod.rs

@@ -86,7 +86,7 @@ pub use crate::intrinsics::write_bytes;
 mod metadata;
 #[cfg(not(bootstrap))]
 #[unstable(feature = "ptr_metadata", issue = /* FIXME */ "none")]
-pub use metadata::{metadata, DynMetadata, Pointee, Thin};
+pub use metadata::{from_raw_parts, from_raw_parts_mut, metadata, DynMetadata, Pointee, Thin};
 
 mod non_null;
 #[stable(feature = "nonnull", since = "1.25.0")]

library/core/src/ptr/non_null.rs

@@ -175,6 +175,24 @@ impl<T: ?Sized> NonNull<T> {
         }
     }
 
+    /// Performs the same functionality as [`std::ptr::from_raw_parts`], except that a
+    /// `NonNull` pointer is returned, as opposed to a raw `*const` pointer.
+    ///
+    /// See the documentation of [`std::ptr::from_raw_parts`] for more details.
+    #[cfg(not(bootstrap))]
+    #[unstable(feature = "ptr_metadata", issue = /* FIXME */ "none")]
+    #[rustc_const_unstable(feature = "ptr_metadata", issue = /* FIXME */ "none")]
+    #[inline]
+    pub const fn from_raw_parts(
+        data_address: NonNull<()>,
+        metadata: <T as super::Pointee>::Metadata,
+    ) -> NonNull<T> {
+        // SAFETY: The result of `ptr::from::raw_parts_mut` is non-null because `data_address` is.
+        unsafe {
+            NonNull::new_unchecked(super::from_raw_parts_mut(data_address.as_ptr(), metadata))
+        }
+    }
+
     /// Acquires the underlying `*mut` pointer.
     #[stable(feature = "nonnull", since = "1.25.0")]
     #[rustc_const_stable(feature = "const_nonnull_as_ptr", since = "1.32.0")]

library/core/tests/ptr.rs

@@ -1,5 +1,5 @@
 use core::cell::RefCell;
-use core::ptr::*;
+use core::ptr::{self, *};
 use std::fmt::{Debug, Display};
 
 #[test]
@@ -525,3 +525,30 @@ fn dyn_metadata() {
 
     assert!(format!("{:?}", meta).starts_with("DynMetadata(0x"));
 }
+
+#[test]
+#[cfg(not(bootstrap))]
+fn from_raw_parts() {
+    let mut value = 5_u32;
+    let address = &mut value as *mut _ as *mut ();
+    let trait_object: &dyn Display = &mut value;
+    let vtable = metadata(trait_object);
+    let trait_object = NonNull::from(trait_object);
+
+    assert_eq!(ptr::from_raw_parts(address, vtable), trait_object.as_ptr());
+    assert_eq!(ptr::from_raw_parts_mut(address, vtable), trait_object.as_ptr());
+    assert_eq!(NonNull::from_raw_parts(NonNull::new(address).unwrap(), vtable), trait_object);
+
+    let mut array = [5_u32, 5, 5, 5, 5];
+    let address = &mut array as *mut _ as *mut ();
+    let array_ptr = NonNull::from(&mut array);
+    let slice_ptr = NonNull::from(&mut array[..]);
+
+    assert_eq!(ptr::from_raw_parts(address, ()), array_ptr.as_ptr());
+    assert_eq!(ptr::from_raw_parts_mut(address, ()), array_ptr.as_ptr());
+    assert_eq!(NonNull::from_raw_parts(NonNull::new(address).unwrap(), ()), array_ptr);
+
+    assert_eq!(ptr::from_raw_parts(address, 5), slice_ptr.as_ptr());
+    assert_eq!(ptr::from_raw_parts_mut(address, 5), slice_ptr.as_ptr());
+    assert_eq!(NonNull::from_raw_parts(NonNull::new(address).unwrap(), 5), slice_ptr);
+}