Sometimes return the same AllocId for a ConstAllocation

compiler/rustc_const_eval/src/interpret/eval_context.rs

@@ -1138,13 +1138,15 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         span: Option<Span>,
         layout: Option<TyAndLayout<'tcx>>,
     ) -> InterpResult<'tcx, OpTy<'tcx, M::Provenance>> {
-        let const_val = val.eval(*self.tcx, self.param_env, span).map_err(|err| {
-            // FIXME: somehow this is reachable even when POST_MONO_CHECKS is on.
-            // Are we not always populating `required_consts`?
-            err.emit_note(*self.tcx);
-            err
-        })?;
-        self.const_val_to_op(const_val, val.ty(), layout)
+        M::eval_mir_constant(self, *val, span, layout, |ecx, val, span, layout| {
+            let const_val = val.eval(*ecx.tcx, ecx.param_env, span).map_err(|err| {
+                // FIXME: somehow this is reachable even when POST_MONO_CHECKS is on.
+                // Are we not always populating `required_consts`?
+                err.emit_note(*ecx.tcx);
+                err
+            })?;
+            ecx.const_val_to_op(const_val, val.ty(), layout)
+        })
     }
 
     #[must_use]

compiler/rustc_const_eval/src/interpret/machine.rs

@@ -13,6 +13,7 @@ use rustc_middle::query::TyCtxtAt;
 use rustc_middle::ty;
 use rustc_middle::ty::layout::TyAndLayout;
 use rustc_span::def_id::DefId;
+use rustc_span::Span;
 use rustc_target::abi::{Align, Size};
 use rustc_target::spec::abi::Abi as CallAbi;
 
@@ -510,6 +511,25 @@ pub trait Machine<'mir, 'tcx: 'mir>: Sized {
     ) -> InterpResult<'tcx> {
         Ok(())
     }
+
+    #[inline(always)]
+    fn eval_mir_constant<F>(
+        ecx: &InterpCx<'mir, 'tcx, Self>,
+        val: mir::Const<'tcx>,
+        span: Option<Span>,
+        layout: Option<TyAndLayout<'tcx>>,
+        eval: F,
+    ) -> InterpResult<'tcx, OpTy<'tcx, Self::Provenance>>
+    where
+        F: Fn(
+            &InterpCx<'mir, 'tcx, Self>,
+            mir::Const<'tcx>,
+            Option<Span>,
+            Option<TyAndLayout<'tcx>>,
+        ) -> InterpResult<'tcx, OpTy<'tcx, Self::Provenance>>,
+    {
+        eval(ecx, val, span, layout)
+    }
 }
 
 /// A lot of the flexibility above is just needed for `Miri`, but all "compile-time" machines