diff --git a/compiler/rustc_middle/src/ty/instance.rs b/compiler/rustc_middle/src/ty/instance.rs
index a753c4ab6ce..41d953216e0 100644
--- a/compiler/rustc_middle/src/ty/instance.rs
+++ b/compiler/rustc_middle/src/ty/instance.rs
@@ -483,6 +483,7 @@ impl<'tcx> Instance<'tcx> {
         if let Some(substs) = self.substs_for_mir_body() { v.subst(tcx, substs) } else { *v }
     }
 
+    #[inline(always)]
     pub fn subst_mir_and_normalize_erasing_regions<T>(
         &self,
         tcx: TyCtxt<'tcx>,
diff --git a/compiler/rustc_middle/src/ty/normalize_erasing_regions.rs b/compiler/rustc_middle/src/ty/normalize_erasing_regions.rs
index 2a8502cab41..071d908655f 100644
--- a/compiler/rustc_middle/src/ty/normalize_erasing_regions.rs
+++ b/compiler/rustc_middle/src/ty/normalize_erasing_regions.rs
@@ -103,6 +103,7 @@ impl TypeFolder<'tcx> for NormalizeAfterErasingRegionsFolder<'tcx> {
         self.tcx.normalize_generic_arg_after_erasing_regions(arg).expect_const()
     }
 
+    #[inline]
     fn fold_mir_const(&mut self, c: mir::ConstantKind<'tcx>) -> mir::ConstantKind<'tcx> {
         let arg = self.param_env.and(c);
         self.tcx.normalize_mir_const_after_erasing_regions(arg)
diff --git a/compiler/rustc_middle/src/ty/subst.rs b/compiler/rustc_middle/src/ty/subst.rs
index 48db2d11f52..0eb379219eb 100644
--- a/compiler/rustc_middle/src/ty/subst.rs
+++ b/compiler/rustc_middle/src/ty/subst.rs
@@ -505,10 +505,8 @@ impl<'a, 'tcx> TypeFolder<'tcx> for SubstFolder<'a, 'tcx> {
         }
     }
 
+    #[inline]
     fn fold_mir_const(&mut self, c: mir::ConstantKind<'tcx>) -> mir::ConstantKind<'tcx> {
-        if !c.needs_subst() {
-            return c;
-        }
         c.super_fold_with(self)
     }
 }