diff --git a/src/librustc/ty/instance.rs b/src/librustc/ty/instance.rs
index 06fc168bd5740..f270d3d3a2a59 100644
--- a/src/librustc/ty/instance.rs
+++ b/src/librustc/ty/instance.rs
@@ -1,8 +1,7 @@
 use crate::hir::CodegenFnAttrFlags;
-use crate::hir::Unsafety;
 use crate::hir::def::Namespace;
 use crate::hir::def_id::DefId;
-use crate::ty::{self, Ty, PolyFnSig, TypeFoldable, SubstsRef, TyCtxt};
+use crate::ty::{self, Ty, TypeFoldable, SubstsRef, TyCtxt};
 use crate::ty::print::{FmtPrinter, Printer};
 use crate::traits;
 use crate::middle::lang_items::DropInPlaceFnLangItem;
@@ -10,7 +9,6 @@ use rustc_target::spec::abi::Abi;
 use rustc_macros::HashStable;
 
 use std::fmt;
-use std::iter;
 
 #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug, RustcEncodable, RustcDecodable)]
 #[derive(HashStable, Lift)]
@@ -66,68 +64,6 @@ impl<'tcx> Instance<'tcx> {
             &ty,
         )
     }
-
-    pub fn fn_sig(&self, tcx: TyCtxt<'tcx>) -> PolyFnSig<'tcx> {
-        let ty = self.ty(tcx);
-        match ty.kind {
-            ty::FnDef(..) |
-            // Shims currently have type FnPtr. Not sure this should remain.
-            ty::FnPtr(_) => {
-                let mut sig = ty.fn_sig(tcx);
-                if let InstanceDef::VtableShim(..) = self.def {
-                    // Modify `fn(self, ...)` to `fn(self: *mut Self, ...)`.
-                    sig = sig.map_bound(|mut sig| {
-                        let mut inputs_and_output = sig.inputs_and_output.to_vec();
-                        inputs_and_output[0] = tcx.mk_mut_ptr(inputs_and_output[0]);
-                        sig.inputs_and_output = tcx.intern_type_list(&inputs_and_output);
-                        sig
-                    });
-                }
-                sig
-            }
-            ty::Closure(def_id, substs) => {
-                let sig = substs.as_closure().sig(def_id, tcx);
-
-                let env_ty = tcx.closure_env_ty(def_id, substs).unwrap();
-                sig.map_bound(|sig| tcx.mk_fn_sig(
-                    iter::once(*env_ty.skip_binder()).chain(sig.inputs().iter().cloned()),
-                    sig.output(),
-                    sig.c_variadic,
-                    sig.unsafety,
-                    sig.abi
-                ))
-            }
-            ty::Generator(def_id, substs, _) => {
-                let sig = substs.as_generator().poly_sig(def_id, tcx);
-
-                let env_region = ty::ReLateBound(ty::INNERMOST, ty::BrEnv);
-                let env_ty = tcx.mk_mut_ref(tcx.mk_region(env_region), ty);
-
-                let pin_did = tcx.lang_items().pin_type().unwrap();
-                let pin_adt_ref = tcx.adt_def(pin_did);
-                let pin_substs = tcx.intern_substs(&[env_ty.into()]);
-                let env_ty = tcx.mk_adt(pin_adt_ref, pin_substs);
-
-                sig.map_bound(|sig| {
-                    let state_did = tcx.lang_items().gen_state().unwrap();
-                    let state_adt_ref = tcx.adt_def(state_did);
-                    let state_substs = tcx.intern_substs(&[
-                        sig.yield_ty.into(),
-                        sig.return_ty.into(),
-                    ]);
-                    let ret_ty = tcx.mk_adt(state_adt_ref, state_substs);
-
-                    tcx.mk_fn_sig(iter::once(env_ty),
-                        ret_ty,
-                        false,
-                        Unsafety::Normal,
-                        Abi::Rust
-                    )
-                })
-            }
-            _ => bug!("unexpected type {:?} in Instance::fn_sig", ty)
-        }
-    }
 }
 
 impl<'tcx> InstanceDef<'tcx> {
diff --git a/src/librustc/ty/layout.rs b/src/librustc/ty/layout.rs
index 1662776adf50f..4d48c7841451f 100644
--- a/src/librustc/ty/layout.rs
+++ b/src/librustc/ty/layout.rs
@@ -2339,6 +2339,76 @@ impl<'a, 'tcx> HashStable<StableHashingContext<'a>> for LayoutError<'tcx> {
     }
 }
 
+
+impl<'tcx> ty::Instance<'tcx> {
+    // NOTE(eddyb) this is private to avoid using it from outside of
+    // `FnAbi::of_instance` - any other uses are either too high-level
+    // for `Instance` (e.g. typeck would use `Ty::fn_sig` instead),
+    // or should go through `FnAbi` instead, to avoid losing any
+    // adjustments `FnAbi::of_instance` might be performing.
+    fn fn_sig_for_fn_abi(&self, tcx: TyCtxt<'tcx>) -> ty::PolyFnSig<'tcx> {
+        let ty = self.ty(tcx);
+        match ty.kind {
+            ty::FnDef(..) |
+            // Shims currently have type FnPtr. Not sure this should remain.
+            ty::FnPtr(_) => {
+                let mut sig = ty.fn_sig(tcx);
+                if let ty::InstanceDef::VtableShim(..) = self.def {
+                    // Modify `fn(self, ...)` to `fn(self: *mut Self, ...)`.
+                    sig = sig.map_bound(|mut sig| {
+                        let mut inputs_and_output = sig.inputs_and_output.to_vec();
+                        inputs_and_output[0] = tcx.mk_mut_ptr(inputs_and_output[0]);
+                        sig.inputs_and_output = tcx.intern_type_list(&inputs_and_output);
+                        sig
+                    });
+                }
+                sig
+            }
+            ty::Closure(def_id, substs) => {
+                let sig = substs.as_closure().sig(def_id, tcx);
+
+                let env_ty = tcx.closure_env_ty(def_id, substs).unwrap();
+                sig.map_bound(|sig| tcx.mk_fn_sig(
+                    iter::once(*env_ty.skip_binder()).chain(sig.inputs().iter().cloned()),
+                    sig.output(),
+                    sig.c_variadic,
+                    sig.unsafety,
+                    sig.abi
+                ))
+            }
+            ty::Generator(def_id, substs, _) => {
+                let sig = substs.as_generator().poly_sig(def_id, tcx);
+
+                let env_region = ty::ReLateBound(ty::INNERMOST, ty::BrEnv);
+                let env_ty = tcx.mk_mut_ref(tcx.mk_region(env_region), ty);
+
+                let pin_did = tcx.lang_items().pin_type().unwrap();
+                let pin_adt_ref = tcx.adt_def(pin_did);
+                let pin_substs = tcx.intern_substs(&[env_ty.into()]);
+                let env_ty = tcx.mk_adt(pin_adt_ref, pin_substs);
+
+                sig.map_bound(|sig| {
+                    let state_did = tcx.lang_items().gen_state().unwrap();
+                    let state_adt_ref = tcx.adt_def(state_did);
+                    let state_substs = tcx.intern_substs(&[
+                        sig.yield_ty.into(),
+                        sig.return_ty.into(),
+                    ]);
+                    let ret_ty = tcx.mk_adt(state_adt_ref, state_substs);
+
+                    tcx.mk_fn_sig(iter::once(env_ty),
+                        ret_ty,
+                        false,
+                        hir::Unsafety::Normal,
+                        rustc_target::spec::abi::Abi::Rust
+                    )
+                })
+            }
+            _ => bug!("unexpected type {:?} in Instance::fn_sig", ty)
+        }
+    }
+}
+
 pub trait FnAbiExt<'tcx, C>
 where
     C: LayoutOf<Ty = Ty<'tcx>, TyLayout = TyLayout<'tcx>>
@@ -2371,7 +2441,7 @@ where
     }
 
     fn of_instance(cx: &C, instance: ty::Instance<'tcx>, extra_args: &[Ty<'tcx>]) -> Self {
-        let sig = instance.fn_sig(cx.tcx());
+        let sig = instance.fn_sig_for_fn_abi(cx.tcx());
 
         call::FnAbi::new_internal(cx, sig, extra_args, |ty, arg_idx| {
             let mut layout = cx.layout_of(ty);