only count deref_operand as actual deref, but not all ref-to-place conversions

Author: RalfJung

src/librustc_mir/interpret/intrinsics.rs

@@ -183,18 +183,17 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M>
         } else if Some(def_id) == self.tcx.lang_items().panic_fn() {
             assert!(args.len() == 1);
             // &(&'static str, &'static str, u32, u32)
-            let ptr = self.read_immediate(args[0])?;
-            let place = self.ref_to_mplace(ptr)?;
+            let place = self.deref_operand(args[0])?;
             let (msg, file, line, col) = (
                 self.mplace_field(place, 0)?,
                 self.mplace_field(place, 1)?,
                 self.mplace_field(place, 2)?,
                 self.mplace_field(place, 3)?,
             );
 
-            let msg_place = self.ref_to_mplace(self.read_immediate(msg.into())?)?;
+            let msg_place = self.deref_operand(msg.into())?;
             let msg = Symbol::intern(self.read_str(msg_place)?);
-            let file_place = self.ref_to_mplace(self.read_immediate(file.into())?)?;
+            let file_place = self.deref_operand(file.into())?;
             let file = Symbol::intern(self.read_str(file_place)?);
             let line = self.read_scalar(line.into())?.to_u32()?;
             let col = self.read_scalar(col.into())?.to_u32()?;
@@ -203,17 +202,16 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M>
             assert!(args.len() == 2);
             // &'static str, &(&'static str, u32, u32)
             let msg = args[0];
-            let ptr = self.read_immediate(args[1])?;
-            let place = self.ref_to_mplace(ptr)?;
+            let place = self.deref_operand(args[1])?;
             let (file, line, col) = (
                 self.mplace_field(place, 0)?,
                 self.mplace_field(place, 1)?,
                 self.mplace_field(place, 2)?,
             );
 
-            let msg_place = self.ref_to_mplace(self.read_immediate(msg.into())?)?;
+            let msg_place = self.deref_operand(msg.into())?;
             let msg = Symbol::intern(self.read_str(msg_place)?);
-            let file_place = self.ref_to_mplace(self.read_immediate(file.into())?)?;
+            let file_place = self.deref_operand(file.into())?;
             let file = Symbol::intern(self.read_str(file_place)?);
             let line = self.read_scalar(line.into())?.to_u32()?;
             let col = self.read_scalar(col.into())?.to_u32()?;

src/librustc_mir/interpret/operand.rs

@@ -555,17 +555,6 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M>
         })
     }
 
-    // Take an operand, representing a pointer, and dereference it to a place -- that
-    // will always be a MemPlace.
-    pub(super) fn deref_operand(
-        &self,
-        src: OpTy<'tcx, M::PointerTag>,
-    ) -> EvalResult<'tcx, MPlaceTy<'tcx, M::PointerTag>> {
-        let val = self.read_immediate(src)?;
-        trace!("deref to {} on {:?}", val.layout.ty, *val);
-        Ok(self.ref_to_mplace(val)?)
-    }
-
     pub fn operand_projection(
         &self,
         base: OpTy<'tcx, M::PointerTag>,

src/librustc_mir/interpret/place.rs

@@ -277,18 +277,34 @@ where
 {
     /// Take a value, which represents a (thin or fat) reference, and make it a place.
     /// Alignment is just based on the type.  This is the inverse of `MemPlace::to_ref()`.
+    /// This does NOT call the "deref" machine hook, so it does NOT count as a
+    /// deref as far as Stacked Borrows is concerned.  Use `deref_operand` for that!
     pub fn ref_to_mplace(
         &self,
         val: ImmTy<'tcx, M::PointerTag>,
     ) -> EvalResult<'tcx, MPlaceTy<'tcx, M::PointerTag>> {
         let pointee_type = val.layout.ty.builtin_deref(true).unwrap().ty;
         let layout = self.layout_of(pointee_type)?;
 
-        let align = layout.align;
-        let meta = val.to_meta()?;
-        let ptr = val.to_scalar_ptr()?;
-        let mplace = MemPlace { ptr, align, meta };
-        let mut mplace = MPlaceTy { mplace, layout };
+        let mplace = MemPlace {
+            ptr: val.to_scalar_ptr()?,
+            align: layout.align,
+            meta: val.to_meta()?,
+        };
+        Ok(MPlaceTy { mplace, layout })
+    }
+
+    // Take an operand, representing a pointer, and dereference it to a place -- that
+    // will always be a MemPlace.  Lives in `place.rs` because it creates a place.
+    // This calls the "deref" machine hook, and counts as a deref as far as
+    // Stacked Borrows is concerned.
+    pub fn deref_operand(
+        &self,
+        src: OpTy<'tcx, M::PointerTag>,
+    ) -> EvalResult<'tcx, MPlaceTy<'tcx, M::PointerTag>> {
+        let val = self.read_immediate(src)?;
+        trace!("deref to {} on {:?}", val.layout.ty, *val);
+        let mut place = self.ref_to_mplace(val)?;
         // Pointer tag tracking might want to adjust the tag.
         let mutbl = match val.layout.ty.sty {
             // `builtin_deref` considers boxes immutable, that's useless for our purposes
@@ -297,9 +313,8 @@ where
             ty::RawPtr(_) => None,
             _ => bug!("Unexpected pointer type {}", val.layout.ty.sty),
         };
-        mplace.mplace.ptr = M::tag_dereference(self, mplace, mutbl)?;
-        // Done
-        Ok(mplace)
+        place.mplace.ptr = M::tag_dereference(self, place, mutbl)?;
+        Ok(place)
     }
 
     /// Offset a pointer to project to a field. Unlike place_field, this is always

src/librustc_mir/interpret/terminator.rs

@@ -402,7 +402,7 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M>
             ty::InstanceDef::Virtual(_, idx) => {
                 let ptr_size = self.pointer_size();
                 let ptr_align = self.tcx.data_layout.pointer_align;
-                let ptr = self.ref_to_mplace(self.read_immediate(args[0])?)?;
+                let ptr = self.deref_operand(args[0])?;
                 let vtable = ptr.vtable()?;
                 let fn_ptr = self.memory.read_ptr_sized(
                     vtable.offset(ptr_size * (idx as u64 + 3), self)?,

src/librustc_mir/interpret/validity.rs

@@ -322,13 +322,10 @@ impl<'rt, 'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>>
                         }
                     }
                 }
-                // Turn ptr into place.
-                // `ref_to_mplace` also calls the machine hook for (re)activating the tag,
-                // which in turn will (in full miri) check if the pointer is dereferencable.
-                let place = self.ecx.ref_to_mplace(value)?;
                 // Recursive checking
                 if let Some(ref mut ref_tracking) = self.ref_tracking {
                     assert!(self.const_mode, "We should only do recursie checking in const mode");
+                    let place = self.ecx.ref_to_mplace(value)?;
                     if size != Size::ZERO {
                         // Non-ZST also have to be dereferencable
                         let ptr = try_validation!(place.ptr.to_ptr(),