diff --git a/gcc/rust/typecheck/rust-tyty.cc b/gcc/rust/typecheck/rust-tyty.cc
index ca88c4c7b03e..4d4746e9c3d0 100644
--- a/gcc/rust/typecheck/rust-tyty.cc
+++ b/gcc/rust/typecheck/rust-tyty.cc
@@ -2222,6 +2222,56 @@ FnPtr::clone () const
 		    get_combined_refs ());
 }
 
+void
+FnPtr::setup_fn_once_output () const
+{
+  // lookup the lang items
+  auto fn_once_lookup = mappings.lookup_lang_item (LangItem::Kind::FN_ONCE);
+  auto fn_once_output_lookup
+    = mappings.lookup_lang_item (LangItem::Kind::FN_ONCE_OUTPUT);
+  if (!fn_once_lookup)
+    {
+      rust_fatal_error (UNKNOWN_LOCATION,
+			"Missing required %<fn_once%> lang item");
+      return;
+    }
+  if (!fn_once_output_lookup)
+    {
+      rust_fatal_error (UNKNOWN_LOCATION,
+			"Missing required %<fn_once_ouput%> lang item");
+      return;
+    }
+
+  DefId &trait_id = fn_once_lookup.value ();
+  DefId &trait_item_id = fn_once_output_lookup.value ();
+
+  // resolve to the trait
+  HIR::Item *item = mappings.lookup_defid (trait_id).value ();
+  rust_assert (item->get_item_kind () == HIR::Item::ItemKind::Trait);
+  HIR::Trait *trait = static_cast<HIR::Trait *> (item);
+
+  Resolver::TraitReference *trait_ref
+    = Resolver::TraitResolver::Resolve (*trait);
+  rust_assert (!trait_ref->is_error ());
+
+  // resolve to trait item
+  HIR::TraitItem *trait_item
+    = mappings.lookup_trait_item_defid (trait_item_id).value ();
+  rust_assert (trait_item->get_item_kind ()
+	       == HIR::TraitItem::TraitItemKind::TYPE);
+  std::string item_identifier = trait_item->trait_identifier ();
+
+  // setup associated types  #[lang = "fn_once_output"]
+  Resolver::TraitItemReference *item_reference = nullptr;
+  bool found = trait_ref->lookup_trait_item_by_type (
+    item_identifier, Resolver::TraitItemReference::TraitItemType::TYPE,
+    &item_reference);
+  rust_assert (found);
+
+  // setup
+  item_reference->associated_type_set (&get_result_type ());
+}
+
 void
 ClosureType::accept_vis (TyVisitor &vis)
 {
diff --git a/gcc/rust/typecheck/rust-tyty.h b/gcc/rust/typecheck/rust-tyty.h
index 163d383ac728..02d064d83bab 100644
--- a/gcc/rust/typecheck/rust-tyty.h
+++ b/gcc/rust/typecheck/rust-tyty.h
@@ -971,7 +971,9 @@ class FnPtr : public CallableTypeInterface
 			     {Resolver::CanonicalPath::create_empty (), locus},
 			     refs),
       params (std::move (params)), result_type (result_type)
-  {}
+  {
+    setup_fn_once_output ();
+  }
 
   FnPtr (HirId ref, HirId ty_ref, location_t locus, std::vector<TyVar> params,
 	 TyVar result_type, std::set<HirId> refs = std::set<HirId> ())
@@ -979,7 +981,9 @@ class FnPtr : public CallableTypeInterface
 			     {Resolver::CanonicalPath::create_empty (), locus},
 			     refs),
       params (params), result_type (result_type)
-  {}
+  {
+    setup_fn_once_output ();
+  }
 
   std::string get_name () const override final { return as_string (); }
 
@@ -1000,6 +1004,7 @@ class FnPtr : public CallableTypeInterface
 
   const TyVar &get_var_return_type () const { return result_type; }
 
+  TyTy::BaseType &get_result_type () const { return *result_type.get_tyty (); }
   size_t num_params () const { return params.size (); }
 
   void accept_vis (TyVisitor &vis) override;
@@ -1016,6 +1021,8 @@ class FnPtr : public CallableTypeInterface
   std::vector<TyVar> &get_params () { return params; }
   const std::vector<TyVar> &get_params () const { return params; }
 
+  void setup_fn_once_output () const;
+
 private:
   std::vector<TyVar> params;
   TyVar result_type;
diff --git a/gcc/testsuite/rust/compile/format_args_basic_expansion.rs b/gcc/testsuite/rust/compile/format_args_basic_expansion.rs
index 40bcd3c1433e..97c3831660b6 100644
--- a/gcc/testsuite/rust/compile/format_args_basic_expansion.rs
+++ b/gcc/testsuite/rust/compile/format_args_basic_expansion.rs
@@ -8,6 +8,14 @@ macro_rules! format_args {
 #[lang = "sized"]
 trait Sized {}
 
+#[lang = "fn_once"]
+pub trait FnOnce<Args> {
+    #[lang = "fn_once_output"]
+    type Output;
+
+    extern "rust-call" fn call_once(self, args: Args) -> Self::Output;
+}
+
 pub mod core {
     pub mod fmt {
         pub struct Formatter;
diff --git a/gcc/testsuite/rust/compile/issue-2042.rs b/gcc/testsuite/rust/compile/issue-2042.rs
index 9fee13de2f93..f5ed45e7e0c5 100644
--- a/gcc/testsuite/rust/compile/issue-2042.rs
+++ b/gcc/testsuite/rust/compile/issue-2042.rs
@@ -1,3 +1,15 @@
+#[lang = "sized"]
+pub trait Sized {}
+
+#[lang = "fn_once"]
+pub trait FnOnce<Args> {
+    #[lang = "fn_once_output"]
+    type Output;
+
+    extern "rust-call" fn call_once(self, args: Args) -> Self::Output;
+}
+
+
 fn f<'r>(p: &'r mut fn(p: &mut ())) {
     (*p)(())
     // { dg-error "expected .&mut ()." "" { target *-*-* } .-1 }
diff --git a/gcc/testsuite/rust/compile/privacy6.rs b/gcc/testsuite/rust/compile/privacy6.rs
index e1e0fac282b4..43a905a79282 100644
--- a/gcc/testsuite/rust/compile/privacy6.rs
+++ b/gcc/testsuite/rust/compile/privacy6.rs
@@ -1,3 +1,13 @@
+
+#[lang = "fn_once"]
+pub trait FnOnce<Args> {
+    #[lang = "fn_once_output"]
+    type Output;
+
+    extern "rust-call" fn call_once(self, args: Args) -> Self::Output;
+}
+
+
 // { dg-additional-options "-w" }
 
 #[lang = "sized"]
diff --git a/gcc/testsuite/rust/compile/rust_abi.rs b/gcc/testsuite/rust/compile/rust_abi.rs
index ec9dc0d419e4..3cc42e2fcba5 100644
--- a/gcc/testsuite/rust/compile/rust_abi.rs
+++ b/gcc/testsuite/rust/compile/rust_abi.rs
@@ -1 +1,12 @@
+#[lang = "sized"]
+pub trait Sized {}
+
+#[lang = "fn_once"]
+pub trait FnOnce<Args> {
+    #[lang = "fn_once_output"]
+    type Output;
+
+    extern "rust-call" fn call_once(self, args: Args) -> Self::Output;
+}
+
 pub fn f(_: extern "Rust" fn()) {}
diff --git a/gcc/testsuite/rust/compile/torture/function_reference2.rs b/gcc/testsuite/rust/compile/torture/function_reference2.rs
index 3c3e7c10910c..55706e679279 100644
--- a/gcc/testsuite/rust/compile/torture/function_reference2.rs
+++ b/gcc/testsuite/rust/compile/torture/function_reference2.rs
@@ -1,3 +1,14 @@
+#[lang = "sized"]
+pub trait Sized {}
+
+#[lang = "fn_once"]
+pub trait FnOnce<Args> {
+    #[lang = "fn_once_output"]
+    type Output;
+
+    extern "rust-call" fn call_once(self, args: Args) -> Self::Output;
+}
+
 fn test(a: i32) -> i32 {
     a + 1
 }
diff --git a/gcc/testsuite/rust/compile/torture/function_reference3.rs b/gcc/testsuite/rust/compile/torture/function_reference3.rs
index 0cb3181f4a16..2a92277cd0b3 100644
--- a/gcc/testsuite/rust/compile/torture/function_reference3.rs
+++ b/gcc/testsuite/rust/compile/torture/function_reference3.rs
@@ -1,3 +1,15 @@
+#[lang = "sized"]
+pub trait Sized {}
+
+#[lang = "fn_once"]
+pub trait FnOnce<Args> {
+    #[lang = "fn_once_output"]
+    type Output;
+
+    extern "rust-call" fn call_once(self, args: Args) -> Self::Output;
+}
+
+
 struct Foo {
     a: fn(i32) -> i32,
     b: i32,
diff --git a/gcc/testsuite/rust/compile/torture/function_reference4.rs b/gcc/testsuite/rust/compile/torture/function_reference4.rs
index 977e4c972159..70682b922aaa 100644
--- a/gcc/testsuite/rust/compile/torture/function_reference4.rs
+++ b/gcc/testsuite/rust/compile/torture/function_reference4.rs
@@ -1,3 +1,14 @@
+#[lang = "sized"]
+pub trait Sized {}
+
+#[lang = "fn_once"]
+pub trait FnOnce<Args> {
+    #[lang = "fn_once_output"]
+    type Output;
+
+    extern "rust-call" fn call_once(self, args: Args) -> Self::Output;
+}
+
 fn test(a: i32) -> i32 {
     a + 1
 }
diff --git a/gcc/testsuite/rust/compile/try-catch-unwind-new.rs b/gcc/testsuite/rust/compile/try-catch-unwind-new.rs
index b176f7a100ba..93909bf0c49c 100644
--- a/gcc/testsuite/rust/compile/try-catch-unwind-new.rs
+++ b/gcc/testsuite/rust/compile/try-catch-unwind-new.rs
@@ -1,6 +1,17 @@
 // { dg-options "-O2 -w -fdump-tree-optimized" }
 #![feature(intrinsics)]
 
+#[lang = "sized"]
+pub trait Sized {}
+
+#[lang = "fn_once"]
+pub trait FnOnce<Args> {
+    #[lang = "fn_once_output"]
+    type Output;
+
+    extern "rust-call" fn call_once(self, args: Args) -> Self::Output;
+}
+
 extern "rust-intrinsic" {
     // { dg-final { scan-tree-dump-times "__builtin_eh_pointer" 1 "optimized" } }
     fn catch_unwind(try_fn: fn(_: *mut u8), data: *mut u8, catch_fn: fn(_: *mut u8, _: *mut u8));
diff --git a/gcc/testsuite/rust/compile/try-catch-unwind-old.rs b/gcc/testsuite/rust/compile/try-catch-unwind-old.rs
index e97d52cd64e3..39c1271f5dfc 100644
--- a/gcc/testsuite/rust/compile/try-catch-unwind-old.rs
+++ b/gcc/testsuite/rust/compile/try-catch-unwind-old.rs
@@ -1,6 +1,17 @@
 // { dg-options "-O2 -w -fdump-tree-optimized" }
 #![feature(intrinsics)]
 
+#[lang = "sized"]
+pub trait Sized {}
+
+#[lang = "fn_once"]
+pub trait FnOnce<Args> {
+    #[lang = "fn_once_output"]
+    type Output;
+
+    extern "rust-call" fn call_once(self, args: Args) -> Self::Output;
+}
+
 extern "rust-intrinsic" {
     // { dg-final { scan-tree-dump-times "__builtin_eh_pointer" 1 "optimized" } }
     fn r#try(try_fn: fn(_: *mut u8), data: *mut u8, catch_fn: fn(_: *mut u8, _: *mut u8)) -> i32;