Auto merge of rust-lang#2319 - RalfJung:dont-touch-vtables, r=RalfJung

fix retagging of vtable ptrs

Fixes a problem reported by `@saethlin` on [Zulip](https://rust-lang.zulipchat.com/#narrow/stream/269128-miri/topic/Stacked.20Borrows.20field.20retagging/near/288040048).

Author: bors

src/stacked_borrows.rs

@@ -1039,6 +1039,11 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                     let val = self.ecx.read_immediate(&place.into())?;
                     let val = self.ecx.retag_reference(&val, mutbl, protector)?;
                     self.ecx.write_immediate(*val, &place.into())?;
+                } else if matches!(place.layout.ty.kind(), ty::RawPtr(..)) {
+                    // Wide raw pointers *do* have fields and their types are strange.
+                    // vtables have a type like `&[*const (); 3]` or so!
+                    // Do *not* recurse into them.
+                    // (No need to worry about wide references or boxes, those always "qualify".)
                 } else {
                     // Maybe we need to go deeper.
                     self.walk_value(place)?;

tests/pass/stacked-borrows/stacked-borrows.rs

@@ -1,4 +1,5 @@
 // compile-flags: -Zmiri-retag-fields
+#![feature(allocator_api)]
 use std::ptr;
 
 // Test various stacked-borrows-related things.
@@ -17,6 +18,7 @@ fn main() {
     raw_ref_to_part();
     array_casts();
     mut_below_shr();
+    wide_raw_ptr_in_tuple();
 }
 
 // Make sure that reading from an `&mut` does, like reborrowing to `&`,
@@ -205,3 +207,15 @@ fn mut_below_shr() {
     let r = &**p;
     let _val = *r;
 }
+
+fn wide_raw_ptr_in_tuple() {
+    let mut x: Box<dyn std::any::Any> = Box::new("ouch");
+    let r = &mut *x as *mut dyn std::any::Any;
+    // This triggers the visitor-based recursive retagging. It is *not* supposed to retag raw
+    // pointers, but then the visitor might recurse into the "fields" of a wide raw pointer and
+    // finds a reference (to a vtable) there that it wants to retag... and that would be Wrong.
+    let pair = (r, &0);
+    let r = unsafe { &mut *pair.0 };
+    // Make sure the fn ptr part of the vtable is still fine.
+    r.type_id();
+}