diff --git a/library/core/src/const_closure.rs b/library/core/src/const_closure.rs
index 9e9c02093be..151c8e6d898 100644
--- a/library/core/src/const_closure.rs
+++ b/library/core/src/const_closure.rs
@@ -1,4 +1,6 @@
 use crate::marker::Destruct;
+#[cfg(not(bootstrap))]
+use crate::marker::Tuple;
 
 /// Struct representing a closure with mutably borrowed data.
 ///
@@ -44,6 +46,7 @@ impl<'a, CapturedData: ?Sized, Function> ConstFnMutClosure<&'a mut CapturedData,
 
 macro_rules! impl_fn_mut_tuple {
     ($($var:ident)*) => {
+        #[cfg(bootstrap)]
         #[allow(unused_parens)]
         impl<'a, $($var,)* ClosureArguments, Function, ClosureReturnValue> const
             FnOnce<ClosureArguments> for ConstFnMutClosure<($(&'a mut $var),*), Function>
@@ -56,6 +59,7 @@ macro_rules! impl_fn_mut_tuple {
             self.call_mut(args)
             }
         }
+        #[cfg(bootstrap)]
         #[allow(unused_parens)]
         impl<'a, $($var,)* ClosureArguments, Function, ClosureReturnValue> const
             FnMut<ClosureArguments> for ConstFnMutClosure<($(&'a mut $var),*), Function>
@@ -68,6 +72,32 @@ macro_rules! impl_fn_mut_tuple {
                 (self.func)(($($var),*), args)
             }
         }
+        #[cfg(not(bootstrap))]
+        #[allow(unused_parens)]
+        impl<'a, $($var,)* ClosureArguments: Tuple, Function, ClosureReturnValue> const
+            FnOnce<ClosureArguments> for ConstFnMutClosure<($(&'a mut $var),*), Function>
+        where
+            Function: ~const Fn(($(&mut $var),*), ClosureArguments) -> ClosureReturnValue+ ~const Destruct,
+        {
+            type Output = ClosureReturnValue;
+
+            extern "rust-call" fn call_once(mut self, args: ClosureArguments) -> Self::Output {
+            self.call_mut(args)
+            }
+        }
+        #[cfg(not(bootstrap))]
+        #[allow(unused_parens)]
+        impl<'a, $($var,)* ClosureArguments: Tuple, Function, ClosureReturnValue> const
+            FnMut<ClosureArguments> for ConstFnMutClosure<($(&'a mut $var),*), Function>
+        where
+            Function: ~const Fn(($(&mut $var),*), ClosureArguments)-> ClosureReturnValue,
+        {
+            extern "rust-call" fn call_mut(&mut self, args: ClosureArguments) -> Self::Output {
+                #[allow(non_snake_case)]
+                let ($($var),*) = &mut self.data;
+                (self.func)(($($var),*), args)
+            }
+        }
     };
 }
 impl_fn_mut_tuple!(A);
diff --git a/library/core/src/ops/function.rs b/library/core/src/ops/function.rs
index 7c93fd30d4e..8d4b0a7ccac 100644
--- a/library/core/src/ops/function.rs
+++ b/library/core/src/ops/function.rs
@@ -75,7 +75,6 @@ use crate::marker::Tuple;
 )]
 #[fundamental] // so that regex can rely that `&str: !FnMut`
 #[must_use = "closures are lazy and do nothing unless called"]
-#[cfg_attr(not(bootstrap), const_trait)]
 pub trait Fn<Args>: FnMut<Args> {
     /// Performs the call operation.
     #[unstable(feature = "fn_traits", issue = "29625")]
@@ -245,7 +244,6 @@ pub trait Fn<Args: Tuple>: FnMut<Args> {
 )]
 #[fundamental] // so that regex can rely that `&str: !FnMut`
 #[must_use = "closures are lazy and do nothing unless called"]
-#[cfg_attr(not(bootstrap), const_trait)]
 pub trait FnMut<Args>: FnOnce<Args> {
     /// Performs the call operation.
     #[unstable(feature = "fn_traits", issue = "29625")]
@@ -415,7 +413,6 @@ pub trait FnMut<Args: Tuple>: FnOnce<Args> {
 )]
 #[fundamental] // so that regex can rely that `&str: !FnMut`
 #[must_use = "closures are lazy and do nothing unless called"]
-#[cfg_attr(not(bootstrap), const_trait)]
 pub trait FnOnce<Args> {
     /// The returned type after the call operator is used.
     #[lang = "fn_once_output"]
diff --git a/src/test/ui/function-pointer/unsized-ret.rs b/src/test/ui/function-pointer/unsized-ret.rs
index 60af5769d6d..79009c5cb6c 100644
--- a/src/test/ui/function-pointer/unsized-ret.rs
+++ b/src/test/ui/function-pointer/unsized-ret.rs
@@ -1,7 +1,8 @@
 #![feature(fn_traits)]
 #![feature(unboxed_closures)]
+#![feature(tuple_trait)]
 
-fn foo<F: Fn<T>, T>(f: Option<F>, t: T) {
+fn foo<F: Fn<T>, T:std::marker::Tuple>(f: Option<F>, t: T) {
     let y = (f.unwrap()).call(t);
 }
 
diff --git a/src/test/ui/function-pointer/unsized-ret.stderr b/src/test/ui/function-pointer/unsized-ret.stderr
index bec3e2aa3fe..40bf7a3898a 100644
--- a/src/test/ui/function-pointer/unsized-ret.stderr
+++ b/src/test/ui/function-pointer/unsized-ret.stderr
@@ -1,5 +1,5 @@
 error[E0277]: the size for values of type `str` cannot be known at compilation time
-  --> $DIR/unsized-ret.rs:9:27
+  --> $DIR/unsized-ret.rs:10:27
    |
 LL |     foo::<fn() -> str, _>(None, ());
    |     --------------------- ^^^^ doesn't have a size known at compile-time
@@ -9,13 +9,13 @@ LL |     foo::<fn() -> str, _>(None, ());
    = help: within `fn() -> str`, the trait `Sized` is not implemented for `str`
    = note: required because it appears within the type `fn() -> str`
 note: required by a bound in `foo`
-  --> $DIR/unsized-ret.rs:4:11
+  --> $DIR/unsized-ret.rs:5:11
    |
-LL | fn foo<F: Fn<T>, T>(f: Option<F>, t: T) {
+LL | fn foo<F: Fn<T>, T:std::marker::Tuple>(f: Option<F>, t: T) {
    |           ^^^^^ required by this bound in `foo`
 
 error[E0277]: the size for values of type `(dyn std::fmt::Display + 'a)` cannot be known at compilation time
-  --> $DIR/unsized-ret.rs:12:66
+  --> $DIR/unsized-ret.rs:13:66
    |
 LL |     foo::<for<'a> fn(&'a ()) -> (dyn std::fmt::Display + 'a), _>(None, (&(),));
    |     ------------------------------------------------------------ ^^^^ doesn't have a size known at compile-time
@@ -25,9 +25,9 @@ LL |     foo::<for<'a> fn(&'a ()) -> (dyn std::fmt::Display + 'a), _>(None, (&()
    = help: within `for<'a> fn(&'a ()) -> (dyn std::fmt::Display + 'a)`, the trait `for<'a> Sized` is not implemented for `(dyn std::fmt::Display + 'a)`
    = note: required because it appears within the type `for<'a> fn(&'a ()) -> (dyn std::fmt::Display + 'a)`
 note: required by a bound in `foo`
-  --> $DIR/unsized-ret.rs:4:11
+  --> $DIR/unsized-ret.rs:5:11
    |
-LL | fn foo<F: Fn<T>, T>(f: Option<F>, t: T) {
+LL | fn foo<F: Fn<T>, T:std::marker::Tuple>(f: Option<F>, t: T) {
    |           ^^^^^ required by this bound in `foo`
 
 error: aborting due to 2 previous errors
diff --git a/src/test/ui/parser/kw-in-trait-bounds.stderr b/src/test/ui/parser/kw-in-trait-bounds.stderr
index 28196c7ce2d..546ad84eeee 100644
--- a/src/test/ui/parser/kw-in-trait-bounds.stderr
+++ b/src/test/ui/parser/kw-in-trait-bounds.stderr
@@ -94,8 +94,8 @@ LL | fn _f<F: fn(), G>(_: impl fn(), _: &dyn fn())
    |
   ::: $SRC_DIR/core/src/ops/function.rs:LL:COL
    |
-LL | pub trait Fn<Args>: FnMut<Args> {
-   | ------------------------------- similarly named trait `Fn` defined here
+LL | pub trait Fn<Args: Tuple>: FnMut<Args> {
+   | -------------------------------------- similarly named trait `Fn` defined here
 
 error[E0405]: cannot find trait `r#fn` in this scope
   --> $DIR/kw-in-trait-bounds.rs:17:4
@@ -105,8 +105,8 @@ LL | G: fn(),
    |
   ::: $SRC_DIR/core/src/ops/function.rs:LL:COL
    |
-LL | pub trait Fn<Args>: FnMut<Args> {
-   | ------------------------------- similarly named trait `Fn` defined here
+LL | pub trait Fn<Args: Tuple>: FnMut<Args> {
+   | -------------------------------------- similarly named trait `Fn` defined here
 
 error[E0405]: cannot find trait `r#fn` in this scope
   --> $DIR/kw-in-trait-bounds.rs:3:27
@@ -116,8 +116,8 @@ LL | fn _f<F: fn(), G>(_: impl fn(), _: &dyn fn())
    |
   ::: $SRC_DIR/core/src/ops/function.rs:LL:COL
    |
-LL | pub trait Fn<Args>: FnMut<Args> {
-   | ------------------------------- similarly named trait `Fn` defined here
+LL | pub trait Fn<Args: Tuple>: FnMut<Args> {
+   | -------------------------------------- similarly named trait `Fn` defined here
 
 error[E0405]: cannot find trait `r#fn` in this scope
   --> $DIR/kw-in-trait-bounds.rs:3:41
@@ -127,8 +127,8 @@ LL | fn _f<F: fn(), G>(_: impl fn(), _: &dyn fn())
    |
   ::: $SRC_DIR/core/src/ops/function.rs:LL:COL
    |
-LL | pub trait Fn<Args>: FnMut<Args> {
-   | ------------------------------- similarly named trait `Fn` defined here
+LL | pub trait Fn<Args: Tuple>: FnMut<Args> {
+   | -------------------------------------- similarly named trait `Fn` defined here
 
 error[E0405]: cannot find trait `r#struct` in this scope
   --> $DIR/kw-in-trait-bounds.rs:24:10
diff --git a/src/test/ui/unboxed-closures/non-tupled-arg-mismatch.rs b/src/test/ui/unboxed-closures/non-tupled-arg-mismatch.rs
index 925463d6dee..d2e48600227 100644
--- a/src/test/ui/unboxed-closures/non-tupled-arg-mismatch.rs
+++ b/src/test/ui/unboxed-closures/non-tupled-arg-mismatch.rs
@@ -1,8 +1,8 @@
 #![feature(unboxed_closures)]
 
 fn a<F: Fn<usize>>(f: F) {}
+//~^ ERROR type parameter to bare `Fn` trait must be a tuple
 
 fn main() {
     a(|_: usize| {});
-    //~^ ERROR mismatched types
 }
diff --git a/src/test/ui/unboxed-closures/non-tupled-arg-mismatch.stderr b/src/test/ui/unboxed-closures/non-tupled-arg-mismatch.stderr
index 9a24fb8c2be..1c18eb0fc49 100644
--- a/src/test/ui/unboxed-closures/non-tupled-arg-mismatch.stderr
+++ b/src/test/ui/unboxed-closures/non-tupled-arg-mismatch.stderr
@@ -1,17 +1,15 @@
-error[E0308]: mismatched types
-  --> $DIR/non-tupled-arg-mismatch.rs:6:5
-   |
-LL |     a(|_: usize| {});
-   |     ^ types differ
-   |
-   = note: expected trait `Fn<usize>`
-              found trait `Fn<(usize,)>`
-note: required by a bound in `a`
+error[E0059]: type parameter to bare `Fn` trait must be a tuple
   --> $DIR/non-tupled-arg-mismatch.rs:3:9
    |
 LL | fn a<F: Fn<usize>>(f: F) {}
-   |         ^^^^^^^^^ required by this bound in `a`
+   |         ^^^^^^^^^ the trait `Tuple` is not implemented for `usize`
+   |
+note: required by a bound in `Fn`
+  --> $SRC_DIR/core/src/ops/function.rs:LL:COL
+   |
+LL | pub trait Fn<Args: Tuple>: FnMut<Args> {
+   |                    ^^^^^ required by this bound in `Fn`
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0308`.
+For more information about this error, try `rustc --explain E0059`.