rust-lang
diff --git a/‎compiler/rustc_const_eval/src/const_eval/machine.rs
+13-3 b/‎compiler/rustc_const_eval/src/const_eval/machine.rs
+13-3
diff --git a/‎compiler/rustc_const_eval/src/interpret/eval_context.rs
+23-2 b/‎compiler/rustc_const_eval/src/interpret/eval_context.rs
+23-2
diff --git a/‎compiler/rustc_const_eval/src/interpret/machine.rs
+1-1 b/‎compiler/rustc_const_eval/src/interpret/machine.rs
+1-1
@@ -260,7 +260,7 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
         args: &[OpTy<'tcx>],
         _ret: Option<(&PlaceTy<'tcx>, mir::BasicBlock)>,
         _unwind: StackPopUnwind, // unwinding is not supported in consts
-    ) -> InterpResult<'tcx, Option<&'mir mir::Body<'tcx>>> {
+    ) -> InterpResult<'tcx, Option<(&'mir mir::Body<'tcx>, ty::Instance<'tcx>)>> {
         debug!("find_mir_or_eval_fn: {:?}", instance);
 
         // Only check non-glue functions
@@ -279,11 +279,21 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
 
             if let Some(new_instance) = ecx.hook_special_const_fn(instance, args)? {
                 // We call another const fn instead.
-                return Self::find_mir_or_eval_fn(ecx, new_instance, _abi, args, _ret, _unwind);
+                // However, we return the *original* instance to make backtraces work out
+                // (and we hope this does not confuse the FnAbi checks too much).
+                return Ok(Self::find_mir_or_eval_fn(
+                    ecx,
+                    new_instance,
+                    _abi,
+                    args,
+                    _ret,
+                    _unwind,
+                )?
+                .map(|(body, _instance)| (body, instance)));
             }
         }
         // This is a const fn. Call it.
-        Ok(Some(ecx.load_mir(instance.def, None)?))
+        Ok(Some((ecx.load_mir(instance.def, None)?, instance)))
     }
 
     fn call_intrinsic(
 
@@ -8,15 +8,18 @@ use rustc_index::vec::IndexVec;
 use rustc_macros::HashStable;
 use rustc_middle::mir;
 use rustc_middle::mir::interpret::{InterpError, InvalidProgramInfo};
-use rustc_middle::ty::layout::{self, LayoutError, LayoutOf, LayoutOfHelpers, TyAndLayout};
+use rustc_middle::ty::layout::{
+    self, FnAbiError, FnAbiOfHelpers, FnAbiRequest, LayoutError, LayoutOf, LayoutOfHelpers,
+    TyAndLayout,
+};
 use rustc_middle::ty::{
     self, query::TyCtxtAt, subst::SubstsRef, ParamEnv, Ty, TyCtxt, TypeFoldable,
 };
 use rustc_mir_dataflow::storage::AlwaysLiveLocals;
 use rustc_query_system::ich::StableHashingContext;
 use rustc_session::Limit;
 use rustc_span::{Pos, Span};
-use rustc_target::abi::{Align, HasDataLayout, Size, TargetDataLayout};
+use rustc_target::abi::{call::FnAbi, Align, HasDataLayout, Size, TargetDataLayout};
 
 use super::{
     AllocId, GlobalId, Immediate, InterpErrorInfo, InterpResult, MPlaceTy, Machine, MemPlace,
@@ -333,6 +336,24 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> LayoutOfHelpers<'tcx> for InterpC
     }
 }
 
+impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> FnAbiOfHelpers<'tcx> for InterpCx<'mir, 'tcx, M> {
+    type FnAbiOfResult = InterpResult<'tcx, &'tcx FnAbi<'tcx, Ty<'tcx>>>;
+
+    fn handle_fn_abi_err(
+        &self,
+        err: FnAbiError<'tcx>,
+        _span: Span,
+        _fn_abi_request: FnAbiRequest<'tcx>,
+    ) -> InterpErrorInfo<'tcx> {
+        match err {
+            FnAbiError::Layout(err) => err_inval!(Layout(err)).into(),
+            FnAbiError::AdjustForForeignAbi(err) => {
+                err_inval!(FnAbiAdjustForForeignAbi(err)).into()
+            }
+        }
+    }
+}
+
 /// Test if it is valid for a MIR assignment to assign `src`-typed place to `dest`-typed value.
 /// This test should be symmetric, as it is primarily about layout compatibility.
 pub(super) fn mir_assign_valid_types<'tcx>(
 
@@ -167,7 +167,7 @@ pub trait Machine<'mir, 'tcx>: Sized {
         args: &[OpTy<'tcx, Self::PointerTag>],
         ret: Option<(&PlaceTy<'tcx, Self::PointerTag>, mir::BasicBlock)>,
         unwind: StackPopUnwind,
-    ) -> InterpResult<'tcx, Option<&'mir mir::Body<'tcx>>>;
+    ) -> InterpResult<'tcx, Option<(&'mir mir::Body<'tcx>, ty::Instance<'tcx>)>>;
 
     /// Execute `fn_val`.  It is the hook's responsibility to advance the instruction
     /// pointer as appropriate.