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, 4 insertions, 2 deletions

diff --git a/compiler/rustc_mir_build/src/build/expr/as_constant.rs b/compiler/rustc_mir_build/src/build/expr/as_constant.rs
index 10cf545f1b7..4430aab73a8 100644
--- a/compiler/rustc_mir_build/src/build/expr/as_constant.rs
+++ b/compiler/rustc_mir_build/src/build/expr/as_constant.rs
@@ -2,7 +2,9 @@
 
 use rustc_ast as ast;
 use rustc_hir::LangItem;
-use rustc_middle::mir::interpret::{Allocation, LitToConstError, LitToConstInput, Scalar};
+use rustc_middle::mir::interpret::{
+    Allocation, LitToConstError, LitToConstInput, Scalar, CTFE_ALLOC_SALT,
+};
 use rustc_middle::mir::*;
 use rustc_middle::thir::*;
 use rustc_middle::ty::{
@@ -140,7 +142,7 @@ fn lit_to_mir_constant<'tcx>(
             ConstValue::Slice { data: allocation, meta: allocation.inner().size().bytes() }
         }
         (ast::LitKind::ByteStr(data, _), ty::Ref(_, inner_ty, _)) if inner_ty.is_array() => {
-            let id = tcx.allocate_bytes_dedup(data);
+            let id = tcx.allocate_bytes_dedup(data, CTFE_ALLOC_SALT);
             ConstValue::Scalar(Scalar::from_pointer(id.into(), &tcx))
         }
         (ast::LitKind::CStr(data, _), ty::Ref(_, inner_ty, _)) if matches!(inner_ty.kind(), ty::Adt(def, _) if tcx.is_lang_item(def.did(), LangItem::CStr)) =>