make alloc_extra machine hook a bit nicer

src/librustc_mir/const_eval.rs

@@ -457,14 +457,14 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
     }
 
     #[inline(always)]
-    fn tag_allocation<'b>(
+    fn init_allocation_extra<'b>(
         _memory_extra: &(),
         _id: AllocId,
         alloc: Cow<'b, Allocation>,
         _kind: Option<MemoryKind<!>>,
-    ) -> (Cow<'b, Allocation<Self::PointerTag>>, Self::PointerTag) {
+    ) -> Cow<'b, Allocation<Self::PointerTag>> {
         // We do not use a tag so we can just cheaply forward the allocation
-        (alloc, ())
+        alloc
     }
 
     #[inline(always)]

src/librustc_mir/interpret/eval_context.rs

@@ -250,6 +250,10 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         self.memory.force_bits(scalar, size)
     }
 
+    /// Call this to turn untagged "global" pointers (obtained via `tcx`) into
+    /// the *canonical* machine pointer to the allocation. This represents a *direct*
+    /// access to that memory, as opposed to access through a pointer that was created
+    /// by the program. Must never be used for derived (program-created) pointers!
     #[inline(always)]
     pub fn tag_static_base_pointer(&self, ptr: Pointer) -> Pointer<M::PointerTag> {
         self.memory.tag_static_base_pointer(ptr)

src/librustc_mir/interpret/machine.rs

@@ -240,15 +240,12 @@ pub trait Machine<'mir, 'tcx>: Sized {
     /// allocation (because a copy had to be done to add tags or metadata), machine memory will
     /// cache the result. (This relies on `AllocMap::get_or` being able to add the
     /// owned allocation to the map even when the map is shared.)
-    ///
+    fn init_allocation_extra<'b>(
-    /// For static allocations, the tag returned must be the same as the one returned by
-    /// `tag_static_base_pointer`.
-    fn tag_allocation<'b>(
         memory_extra: &Self::MemoryExtra,
         id: AllocId,
         alloc: Cow<'b, Allocation>,
         kind: Option<MemoryKind<Self::MemoryKinds>>,
-    ) -> (Cow<'b, Allocation<Self::PointerTag, Self::AllocExtra>>, Self::PointerTag);
+    ) -> Cow<'b, Allocation<Self::PointerTag, Self::AllocExtra>>;
 
     /// Return the "base" tag for the given static allocation: the one that is used for direct
     /// accesses to this static/const/fn allocation.

src/librustc_mir/interpret/memory.rs

@@ -143,6 +143,10 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> Memory<'mir, 'tcx, M> {
         }
     }
 
+    /// Call this to turn untagged "global" pointers (obtained via `tcx`) into
+    /// the *canonical* machine pointer to the allocation. This represents a *direct*
+    /// access to that memory, as opposed to access through a pointer that was created
+    /// by the program. Must never be used for derived (program-created) pointers!
     #[inline]
     pub fn tag_static_base_pointer(&self, ptr: Pointer) -> Pointer<M::PointerTag> {
         ptr.with_tag(M::tag_static_base_pointer(&self.extra, ptr.alloc_id))
@@ -191,9 +195,9 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> Memory<'mir, 'tcx, M> {
         kind: MemoryKind<M::MemoryKinds>,
     ) -> Pointer<M::PointerTag> {
         let id = self.tcx.alloc_map.lock().reserve();
-        let (alloc, tag) = M::tag_allocation(&self.extra, id, Cow::Owned(alloc), Some(kind));
+        let alloc = M::init_allocation_extra(&self.extra, id, Cow::Owned(alloc), Some(kind));
         self.alloc_map.insert(id, (kind, alloc.into_owned()));
-        Pointer::from(id).with_tag(tag)
+        self.tag_static_base_pointer(Pointer::from(id))
     }
 
     pub fn reallocate(
@@ -473,14 +477,13 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> Memory<'mir, 'tcx, M> {
                 }
             }
         };
-        // We got tcx memory. Let the machine figure out whether and how to
+        // We got tcx memory. Let the machine initialize its "extra" stuff.
-        // turn that into memory with the right pointer tag.
+        Ok(M::init_allocation_extra(
-        Ok(M::tag_allocation(
             memory_extra,
             id, // always use the ID we got as input, not the "hidden" one.
             alloc,
             M::STATIC_KIND.map(MemoryKind::Machine),
-        ).0)
+        ))
     }
 
     /// Gives raw access to the `Allocation`, without bounds or alignment checks.

src/librustc_mir/transform/const_prop.rs

@@ -191,14 +191,14 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine {
     }
 
     #[inline(always)]
-    fn tag_allocation<'b>(
+    fn init_allocation_extra<'b>(
         _memory_extra: &(),
         _id: AllocId,
         alloc: Cow<'b, Allocation>,
         _kind: Option<MemoryKind<!>>,
-    ) -> (Cow<'b, Allocation<Self::PointerTag>>, Self::PointerTag) {
+    ) -> Cow<'b, Allocation<Self::PointerTag>> {
         // We do not use a tag so we can just cheaply forward the allocation
-        (alloc, ())
+        alloc
     }
 
     #[inline(always)]