Add an intrinsic that lowers to AggregateKind::RawPtr

Author: scottmcm

core/src/intrinsics.rs

@@ -2779,6 +2779,32 @@ pub unsafe fn vtable_align(_ptr: *const ()) -> usize {
     unreachable!()
 }
 
+/// Lowers in MIR to `Rvalue::Aggregate` with `AggregateKind::RawPtr`.
+///
+/// This is used to implement functions like `slice::from_raw_parts_mut` and
+/// `ptr::from_raw_parts` in a way compatible with the compiler being able to
+/// change the possible layouts of pointers.
+#[rustc_nounwind]
+#[unstable(feature = "core_intrinsics", issue = "none")]
+#[rustc_intrinsic]
+#[rustc_intrinsic_must_be_overridden]
+#[cfg(not(bootstrap))]
+pub const fn aggregate_raw_ptr<P: AggregateRawPtr<D, Metadata = M>, D, M>(_data: D, _meta: M) -> P {
+    // No fallback because `libcore` doesn't want to know the layout
+    unreachable!()
+}
+
+#[unstable(feature = "core_intrinsics", issue = "none")]
+pub trait AggregateRawPtr<D> {
+    type Metadata: Copy;
+}
+impl<P: ?Sized, T: ptr::Thin> AggregateRawPtr<*const T> for *const P {
+    type Metadata = <P as ptr::Pointee>::Metadata;
+}
+impl<P: ?Sized, T: ptr::Thin> AggregateRawPtr<*mut T> for *mut P {
+    type Metadata = <P as ptr::Pointee>::Metadata;
+}
+
 // Some functions are defined here because they accidentally got made
 // available in this module on stable. See <https://github.com/rust-lang/rust/issues/15702>.
 // (`transmute` also falls into this category, but it cannot be wrapped due to the