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, 6 insertions, 0 deletions

diff --git a/compiler/rustc_const_eval/src/interpret/cast.rs b/compiler/rustc_const_eval/src/interpret/cast.rs
index c3d7f2234ed..37bd6d6e530 100644
--- a/compiler/rustc_const_eval/src/interpret/cast.rs
+++ b/compiler/rustc_const_eval/src/interpret/cast.rs
@@ -400,6 +400,12 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
             }
             (ty::Dynamic(data_a, _, ty::Dyn), ty::Dynamic(data_b, _, ty::Dyn)) => {
                 let val = self.read_immediate(src)?;
+                // MIR building generates odd NOP casts, prevent them from causing unexpected trouble.
+                // See <https://github.com/rust-lang/rust/issues/128880>.
+                // FIXME: ideally we wouldn't have to do this.
+                if data_a == data_b {
+                    return self.write_immediate(*val, dest);
+                }
                 // Take apart the old pointer, and find the dynamic type.
                 let (old_data, old_vptr) = val.to_scalar_pair();
                 let old_data = old_data.to_pointer(self)?;