Auto merge of #128742 - RalfJung:miri-vtable-uniqueness, r=saethlin

miri: make vtable addresses not globally unique

Miri currently gives vtables a unique global address. That's not actually matching reality though. So this PR enables Miri to generate different addresses for the same type-trait pair.

To avoid generating an unbounded number of `AllocId` (and consuming unbounded amounts of memory), we use the "salt" technique that we also already use for giving constants non-unique addresses: the cache is keyed on a "salt" value n top of the actually relevant key, and Miri picks a random salt (currently in the range `0..16`) each time it needs to choose an `AllocId` for one of these globals -- that means we'll get up to 16 different addresses for each vtable. The salt scheme is integrated into the global allocation deduplication logic in `tcx`, and also used for functions and string literals. (So this also fixes the problem that casting the same function to a fn ptr over and over will consume unbounded memory.)

r? `@saethlin`
Fixes https://github.com/rust-lang/miri/issues/3737

1 files changed, 3 insertions, 1 deletions

diff --git a/compiler/rustc_const_eval/src/interpret/traits.rs b/compiler/rustc_const_eval/src/interpret/traits.rs
index fb50661b826..cd4faf06655 100644
--- a/compiler/rustc_const_eval/src/interpret/traits.rs
+++ b/compiler/rustc_const_eval/src/interpret/traits.rs
@@ -28,7 +28,9 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
         ensure_monomorphic_enough(*self.tcx, ty)?;
         ensure_monomorphic_enough(*self.tcx, poly_trait_ref)?;
 
-        let vtable_symbolic_allocation = self.tcx.reserve_and_set_vtable_alloc(ty, poly_trait_ref);
+        let salt = M::get_global_alloc_salt(self, None);
+        let vtable_symbolic_allocation =
+            self.tcx.reserve_and_set_vtable_alloc(ty, poly_trait_ref, salt);
         let vtable_ptr = self.global_root_pointer(Pointer::from(vtable_symbolic_allocation))?;
         Ok(vtable_ptr.into())
     }