const-eval: do not make UbChecks behavior depend on current crate's flags

compiler/rustc_const_eval/src/interpret/machine.rs

@@ -280,6 +280,9 @@ pub trait Machine<'tcx>: Sized {
         Ok(())
     }
 
+    /// Determines the result of a `NullaryOp::UbChecks` invocation.
+    fn ub_checks(_ecx: &InterpCx<'tcx, Self>) -> InterpResult<'tcx, bool>;
+
     /// Called when the interpreter encounters a `StatementKind::ConstEvalCounter` instruction.
     /// You can use this to detect long or endlessly running programs.
     #[inline]
@@ -627,6 +630,13 @@ pub macro compile_time_machine(<$tcx: lifetime>) {
         match fn_val {}
     }
 
+    #[inline(always)]
+    fn ub_checks(_ecx: &InterpCx<$tcx, Self>) -> InterpResult<$tcx, bool> {
+        // We can't look at `tcx.sess` here as that can differ across crates, which can lead to
+        // unsound differences in evaluating the same constant at different instantiation sites.
+        Ok(true)
+    }
+
     #[inline(always)]
     fn adjust_global_allocation<'b>(
         _ecx: &InterpCx<$tcx, Self>,

compiler/rustc_const_eval/src/interpret/operator.rs

@@ -512,7 +512,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                     self.tcx.offset_of_subfield(self.param_env, layout, fields.iter()).bytes();
                 ImmTy::from_uint(val, usize_layout())
             }
-            UbChecks => ImmTy::from_bool(self.tcx.sess.ub_checks(), *self.tcx),
+            UbChecks => ImmTy::from_bool(M::ub_checks(self)?, *self.tcx),
         })
     }
 }

src/tools/miri/src/machine.rs

@@ -1060,6 +1060,10 @@ impl<'tcx> Machine<'tcx> for MiriMachine<'tcx> {
         ecx.generate_nan(inputs)
     }
 
+    fn ub_checks(ecx: &InterpCx<'tcx, Self>) -> InterpResult<'tcx, bool> {
+        Ok(ecx.tcx.sess.ub_checks())
+    }
+
     fn thread_local_static_pointer(
         ecx: &mut MiriInterpCx<'tcx>,
         def_id: DefId,