diff --git a/compiler/rustc_smir/src/rustc_internal/mod.rs b/compiler/rustc_smir/src/rustc_internal/mod.rs
index 2ac3046ce6d..e4f129bc1d6 100644
--- a/compiler/rustc_smir/src/rustc_internal/mod.rs
+++ b/compiler/rustc_smir/src/rustc_internal/mod.rs
@@ -35,6 +35,10 @@ pub fn foreign_def(did: DefId) -> stable_mir::ty::ForeignDef {
     with_tables(|t| t.foreign_def(did))
 }
 
+pub fn fn_def(did: DefId) -> stable_mir::ty::FnDef {
+    with_tables(|t| t.fn_def(did))
+}
+
 impl<'tcx> Tables<'tcx> {
     pub fn item_def_id(&self, item: &stable_mir::CrateItem) -> DefId {
         self.def_ids[item.0]
@@ -52,6 +56,10 @@ impl<'tcx> Tables<'tcx> {
         stable_mir::ty::ForeignDef(self.create_def_id(did))
     }
 
+    pub fn fn_def(&mut self, did: DefId) -> stable_mir::ty::FnDef {
+        stable_mir::ty::FnDef(self.create_def_id(did))
+    }
+
     fn create_def_id(&mut self, did: DefId) -> stable_mir::DefId {
         // FIXME: this becomes inefficient when we have too many ids
         for (i, &d) in self.def_ids.iter().enumerate() {
diff --git a/compiler/rustc_smir/src/rustc_smir/mod.rs b/compiler/rustc_smir/src/rustc_smir/mod.rs
index f624c4dc8ff..81ca0f985cc 100644
--- a/compiler/rustc_smir/src/rustc_smir/mod.rs
+++ b/compiler/rustc_smir/src/rustc_smir/mod.rs
@@ -8,7 +8,7 @@
 //! For now, we are developing everything inside `rustc`, thus, we keep this module private.
 
 use crate::rustc_internal::{self, opaque};
-use crate::stable_mir::ty::{FloatTy, GenericArgs, GenericArgKind, IntTy, RigidTy, TyKind, UintTy};
+use crate::stable_mir::ty::{FloatTy, GenericArgKind, GenericArgs, IntTy, RigidTy, TyKind, UintTy};
 use crate::stable_mir::{self, Context};
 use rustc_middle::mir;
 use rustc_middle::ty::{self, Ty, TyCtxt};
@@ -127,7 +127,25 @@ impl<'tcx> Tables<'tcx> {
             ty::Ref(region, ty, mutbl) => {
                 TyKind::RigidTy(RigidTy::Ref(opaque(region), self.intern_ty(*ty), mutbl.stable()))
             }
-            ty::FnDef(_, _) => todo!(),
+            ty::FnDef(def_id, generic_args) => TyKind::RigidTy(RigidTy::FnDef(
+                rustc_internal::fn_def(*def_id),
+                GenericArgs(
+                    generic_args
+                        .iter()
+                        .map(|arg| match arg.unpack() {
+                            ty::GenericArgKind::Lifetime(region) => {
+                                GenericArgKind::Lifetime(opaque(&region))
+                            }
+                            ty::GenericArgKind::Type(ty) => {
+                                GenericArgKind::Type(self.intern_ty(ty))
+                            }
+                            ty::GenericArgKind::Const(const_) => {
+                                GenericArgKind::Const(opaque(&const_))
+                            }
+                        })
+                        .collect(),
+                ),
+            )),
             ty::FnPtr(_) => todo!(),
             ty::Dynamic(_, _, _) => todo!(),
             ty::Closure(_, _) => todo!(),
diff --git a/compiler/rustc_smir/src/stable_mir/ty.rs b/compiler/rustc_smir/src/stable_mir/ty.rs
index 36b562850e9..f3dbde907a0 100644
--- a/compiler/rustc_smir/src/stable_mir/ty.rs
+++ b/compiler/rustc_smir/src/stable_mir/ty.rs
@@ -32,6 +32,7 @@ pub enum RigidTy {
     Slice(Ty),
     RawPtr(Ty, Mutability),
     Ref(Region, Ty, Mutability),
+    FnDef(FnDef, GenericArgs),
     Never,
     Tuple(Vec<Ty>),
 }
@@ -65,6 +66,9 @@ pub enum FloatTy {
 #[derive(Clone, PartialEq, Eq, Debug)]
 pub struct ForeignDef(pub(crate) DefId);
 
+#[derive(Clone, PartialEq, Eq, Debug)]
+pub struct FnDef(pub(crate) DefId);
+
 #[derive(Clone, PartialEq, Eq, Debug)]
 pub struct AdtDef(pub(crate) DefId);