Don't actually create a full MIR stack frame when not needed

src/librustc_mir/const_eval.rs

@@ -43,6 +43,16 @@ pub fn mk_borrowck_eval_cx<'a, 'mir, 'tcx>(
 ) -> EvalResult<'tcx, CompileTimeEvalContext<'a, 'mir, 'tcx>> {
     debug!("mk_borrowck_eval_cx: {:?}", instance);
     let param_env = tcx.param_env(instance.def_id());
+    mk_eval_cx_inner(tcx, instance, mir, span, param_env)
+}
+
+fn mk_eval_cx_inner<'a, 'mir, 'tcx>(
+    tcx: TyCtxt<'a, 'tcx, 'tcx>,
+    instance: Instance<'tcx>,
+    mir: &'mir mir::Mir<'tcx>,
+    span: Span,
+    param_env: ty::ParamEnv<'tcx>,
+) -> EvalResult<'tcx, CompileTimeEvalContext<'a, 'mir, 'tcx>> {
     let mut ecx = EvalContext::new(tcx.at(span), param_env, CompileTimeInterpreter::new());
     // insert a stack frame so any queries have the correct substs
     // cannot use `push_stack_frame`; if we do `const_prop` explodes
@@ -60,24 +70,15 @@ pub fn mk_borrowck_eval_cx<'a, 'mir, 'tcx>(
     Ok(ecx)
 }
 
-pub fn mk_eval_cx<'a, 'tcx>(
+fn mk_eval_cx<'a, 'tcx>(
     tcx: TyCtxt<'a, 'tcx, 'tcx>,
     instance: Instance<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
 ) -> EvalResult<'tcx, CompileTimeEvalContext<'a, 'tcx, 'tcx>> {
     debug!("mk_eval_cx: {:?}, {:?}", instance, param_env);
     let span = tcx.def_span(instance.def_id());
-    let mut ecx = EvalContext::new(tcx.at(span), param_env, CompileTimeInterpreter::new());
-    let mir = ecx.load_mir(instance.def)?;
-    // insert a stack frame so any queries have the correct substs
-    ecx.push_stack_frame(
-        instance,
-        mir.span,
-        mir,
-        None,
-        StackPopCleanup::Goto(None), // never pop
-    )?;
-    Ok(ecx)
+    let mir = tcx.optimized_mir(instance.def.def_id());
+    mk_eval_cx_inner(tcx, instance, mir, span, param_env)
 }
 
 pub(crate) fn eval_promoted<'a, 'mir, 'tcx>(