From 21dc49c587c7df282b8e96dc01c288c7d59bbd2c Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Wed, 17 Jul 2024 13:49:32 +0200
Subject: [PATCH 1/2] ptr::metadata: avoid references to extern types

---
 library/core/src/ptr/metadata.rs | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/library/core/src/ptr/metadata.rs b/library/core/src/ptr/metadata.rs
index eb86bf6620652..21d1b7ea0ce77 100644
--- a/library/core/src/ptr/metadata.rs
+++ b/library/core/src/ptr/metadata.rs
@@ -5,6 +5,7 @@ use crate::hash::{Hash, Hasher};
 use crate::intrinsics::aggregate_raw_ptr;
 use crate::intrinsics::ptr_metadata;
 use crate::marker::Freeze;
+use crate::ptr::NonNull;
 
 /// Provides the pointer metadata type of any pointed-to type.
 ///
@@ -153,7 +154,7 @@ pub const fn from_raw_parts_mut<T: ?Sized>(
 /// compare equal (since identical vtables can be deduplicated within a codegen unit).
 #[lang = "dyn_metadata"]
 pub struct DynMetadata<Dyn: ?Sized> {
-    _vtable_ptr: &'static VTable,
+    _vtable_ptr: NonNull<VTable>,
     _phantom: crate::marker::PhantomData<Dyn>,
 }
 
@@ -174,7 +175,7 @@ impl<Dyn: ?Sized> DynMetadata<Dyn> {
     fn vtable_ptr(self) -> *const VTable {
         // SAFETY: this layout assumption is hard-coded into the compiler.
         // If it's somehow not a size match, the transmute will error.
-        unsafe { crate::mem::transmute::<Self, &'static VTable>(self) }
+        unsafe { crate::mem::transmute::<Self, *const VTable>(self) }
     }
 
     /// Returns the size of the type associated with this vtable.

From f9c0d3370f04d08cda2da268a54bc543eb310fc7 Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Wed, 17 Jul 2024 13:50:17 +0200
Subject: [PATCH 2/2] ptr::metadata: update comment on vtable_ptr work-around

---
 library/core/src/ptr/metadata.rs | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/library/core/src/ptr/metadata.rs b/library/core/src/ptr/metadata.rs
index 21d1b7ea0ce77..06f205c0f2670 100644
--- a/library/core/src/ptr/metadata.rs
+++ b/library/core/src/ptr/metadata.rs
@@ -167,10 +167,13 @@ extern "C" {
 }
 
 impl<Dyn: ?Sized> DynMetadata<Dyn> {
-    /// One of the things that rustc_middle does with this being a lang item is
-    /// give it `FieldsShape::Primitive`, which means that as far as codegen can
-    /// tell, it *is* a reference, and thus doesn't have any fields.
-    /// That means we can't use field access, and have to transmute it instead.
+    /// When `DynMetadata` appears as the metadata field of a wide pointer, the rustc_middle layout
+    /// computation does magic and the resulting layout is *not* a `FieldsShape::Aggregate`, instead
+    /// it is a `FieldsShape::Primitive`. This means that the same type can have different layout
+    /// depending on whether it appears as the metadata field of a wide pointer or as a stand-alone
+    /// type, which understandably confuses codegen and leads to ICEs when trying to project to a
+    /// field of `DynMetadata`. To work around that issue, we use `transmute` instead of using a
+    /// field projection.
     #[inline]
     fn vtable_ptr(self) -> *const VTable {
         // SAFETY: this layout assumption is hard-coded into the compiler.