Auto merge of #99943 - compiler-errors:tuple-trait, r=jackh726

Implement `std::marker::Tuple`, use it in `extern "rust-call"` and `Fn`-family traits

Implements rust-lang/compiler-team#537

I made a few opinionated decisions in this implementation, specifically:
1. Enforcing `extern "rust-call"` on fn items during wfcheck,
2. Enforcing this for all functions (not just ones that have bodies),
3. Gating this `Tuple` marker trait behind its own feature, instead of grouping it into (e.g.) `unboxed_closures`.

Still needing to be done:
1. Enforce that `extern "rust-call"` `fn`-ptrs are well-formed only if they have 1/2 args and the second one implements `Tuple`. (Doing this would fix ICE in #66696.)
2. Deny all explicit/user `impl`s of the `Tuple` trait, kinda like `Sized`.
3. Fixing `Tuple` trait built-in impl for chalk, so that chalkification tests are un-broken.

Open questions:
1. Does this need t-lang or t-libs signoff?

Fixes #99820

36 files changed, 309 insertions, 178 deletions

diff --git a/src/test/ui/abi/issues/issue-22565-rust-call.rs b/src/test/ui/abi/issues/issue-22565-rust-call.rs
index a08e0bfb5e5..a572666c888 100644
--- a/src/test/ui/abi/issues/issue-22565-rust-call.rs
+++ b/src/test/ui/abi/issues/issue-22565-rust-call.rs
@@ -1,32 +1,31 @@
 #![feature(unboxed_closures)]
 
 extern "rust-call" fn b(_i: i32) {}
-//~^ ERROR functions with the "rust-call" ABI must take a single non-self argument that is a tuple
+//~^ ERROR functions with the "rust-call" ABI must take a single non-self tuple argument
 
 trait Tr {
     extern "rust-call" fn a();
+    //~^ ERROR functions with the "rust-call" ABI must take a single non-self tuple argument
 
     extern "rust-call" fn b() {}
-    //~^ ERROR functions with the "rust-call" ABI must take a single non-self argument
+    //~^ ERROR functions with the "rust-call" ABI must take a single non-self tuple argument
 }
 
 struct Foo;
 
 impl Foo {
     extern "rust-call" fn bar() {}
-    //~^ ERROR functions with the "rust-call" ABI must take a single non-self argument
+    //~^ ERROR functions with the "rust-call" ABI must take a single non-self tuple argument
 }
 
 impl Tr for Foo {
     extern "rust-call" fn a() {}
-    //~^ ERROR functions with the "rust-call" ABI must take a single non-self argument
+    //~^ ERROR functions with the "rust-call" ABI must take a single non-self tuple argument
 }
 
-fn main () {
+fn main() {
     b(10);
-
     Foo::bar();
-
     <Foo as Tr>::a();
     <Foo as Tr>::b();
 }
diff --git a/src/test/ui/abi/issues/issue-22565-rust-call.stderr b/src/test/ui/abi/issues/issue-22565-rust-call.stderr
index 3eee10bc5e9..9d205b444fa 100644
--- a/src/test/ui/abi/issues/issue-22565-rust-call.stderr
+++ b/src/test/ui/abi/issues/issue-22565-rust-call.stderr
@@ -1,26 +1,33 @@
-error: functions with the "rust-call" ABI must take a single non-self argument that is a tuple
+error[E0277]: functions with the "rust-call" ABI must take a single non-self tuple argument
   --> $DIR/issue-22565-rust-call.rs:3:1
    |
 LL | extern "rust-call" fn b(_i: i32) {}
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Tuple` is not implemented for `i32`
 
-error: functions with the "rust-call" ABI must take a single non-self argument that is a tuple
-  --> $DIR/issue-22565-rust-call.rs:9:5
-   |
-LL |     extern "rust-call" fn b() {}
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^
-
-error: functions with the "rust-call" ABI must take a single non-self argument that is a tuple
-  --> $DIR/issue-22565-rust-call.rs:16:5
+error: functions with the "rust-call" ABI must take a single non-self tuple argument
+  --> $DIR/issue-22565-rust-call.rs:17:5
    |
 LL |     extern "rust-call" fn bar() {}
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: functions with the "rust-call" ABI must take a single non-self argument that is a tuple
-  --> $DIR/issue-22565-rust-call.rs:21:5
+error: functions with the "rust-call" ABI must take a single non-self tuple argument
+  --> $DIR/issue-22565-rust-call.rs:22:5
    |
 LL |     extern "rust-call" fn a() {}
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 4 previous errors
+error: functions with the "rust-call" ABI must take a single non-self tuple argument
+  --> $DIR/issue-22565-rust-call.rs:7:5
+   |
+LL |     extern "rust-call" fn a();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: functions with the "rust-call" ABI must take a single non-self tuple argument
+  --> $DIR/issue-22565-rust-call.rs:10:5
+   |
+LL |     extern "rust-call" fn b() {}
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 5 previous errors
 
+For more information about this error, try `rustc --explain E0277`.
diff --git a/src/test/ui/abi/rustcall-generic.rs b/src/test/ui/abi/rustcall-generic.rs
index 411c98e1031..6eaccc436b6 100644
--- a/src/test/ui/abi/rustcall-generic.rs
+++ b/src/test/ui/abi/rustcall-generic.rs
@@ -2,9 +2,9 @@
 // check-pass
 //[opt] compile-flags: -Zmir-opt-level=3
 
-#![feature(unboxed_closures)]
+#![feature(unboxed_closures, tuple_trait)]
 
-extern "rust-call" fn foo<T>(_: T) {}
+extern "rust-call" fn foo<T: std::marker::Tuple>(_: T) {}
 
 fn main() {
     foo(());
diff --git a/src/test/ui/borrowck/borrow-immutable-upvar-mutation.rs b/src/test/ui/borrowck/borrow-immutable-upvar-mutation.rs
index e2f016614bf..a3350024e75 100644
--- a/src/test/ui/borrowck/borrow-immutable-upvar-mutation.rs
+++ b/src/test/ui/borrowck/borrow-immutable-upvar-mutation.rs
@@ -1,4 +1,4 @@
-#![feature(unboxed_closures)]
+#![feature(unboxed_closures, tuple_trait)]
 
 // Tests that we can't assign to or mutably borrow upvars from `Fn`
 // closures (issue #17780)
@@ -7,10 +7,10 @@ fn set(x: &mut usize) {
     *x = 5;
 }
 
-fn to_fn<A, F: Fn<A>>(f: F) -> F {
+fn to_fn<A: std::marker::Tuple, F: Fn<A>>(f: F) -> F {
     f
 }
-fn to_fn_mut<A, F: FnMut<A>>(f: F) -> F {
+fn to_fn_mut<A: std::marker::Tuple, F: FnMut<A>>(f: F) -> F {
     f
 }
 
diff --git a/src/test/ui/borrowck/borrow-immutable-upvar-mutation.stderr b/src/test/ui/borrowck/borrow-immutable-upvar-mutation.stderr
index 093589ed092..a0eaf1f163b 100644
--- a/src/test/ui/borrowck/borrow-immutable-upvar-mutation.stderr
+++ b/src/test/ui/borrowck/borrow-immutable-upvar-mutation.stderr
@@ -1,8 +1,8 @@
 error[E0594]: cannot assign to `x`, as it is a captured variable in a `Fn` closure
   --> $DIR/borrow-immutable-upvar-mutation.rs:21:27
    |
-LL | fn to_fn<A, F: Fn<A>>(f: F) -> F {
-   |                          - change this to accept `FnMut` instead of `Fn`
+LL | fn to_fn<A: std::marker::Tuple, F: Fn<A>>(f: F) -> F {
+   |                                              - change this to accept `FnMut` instead of `Fn`
 ...
 LL |         let _f = to_fn(|| x = 42);
    |                  ----- -- ^^^^^^ cannot assign
@@ -13,8 +13,8 @@ LL |         let _f = to_fn(|| x = 42);
 error[E0596]: cannot borrow `y` as mutable, as it is a captured variable in a `Fn` closure
   --> $DIR/borrow-immutable-upvar-mutation.rs:24:31
    |
-LL | fn to_fn<A, F: Fn<A>>(f: F) -> F {
-   |                          - change this to accept `FnMut` instead of `Fn`
+LL | fn to_fn<A: std::marker::Tuple, F: Fn<A>>(f: F) -> F {
+   |                                              - change this to accept `FnMut` instead of `Fn`
 ...
 LL |         let _g = to_fn(|| set(&mut y));
    |                  ----- --     ^^^^^^ cannot borrow as mutable
@@ -25,8 +25,8 @@ LL |         let _g = to_fn(|| set(&mut y));
 error[E0594]: cannot assign to `z`, as it is a captured variable in a `Fn` closure
   --> $DIR/borrow-immutable-upvar-mutation.rs:29:22
    |
-LL | fn to_fn<A, F: Fn<A>>(f: F) -> F {
-   |                          - change this to accept `FnMut` instead of `Fn`
+LL | fn to_fn<A: std::marker::Tuple, F: Fn<A>>(f: F) -> F {
+   |                                              - change this to accept `FnMut` instead of `Fn`
 ...
 LL |             to_fn(|| z = 42);
    |             ----- -- ^^^^^^ cannot assign
@@ -37,8 +37,8 @@ LL |             to_fn(|| z = 42);
 error[E0594]: cannot assign to `x`, as it is a captured variable in a `Fn` closure
   --> $DIR/borrow-immutable-upvar-mutation.rs:36:32
    |
-LL | fn to_fn<A, F: Fn<A>>(f: F) -> F {
-   |                          - change this to accept `FnMut` instead of `Fn`
+LL | fn to_fn<A: std::marker::Tuple, F: Fn<A>>(f: F) -> F {
+   |                                              - change this to accept `FnMut` instead of `Fn`
 ...
 LL |         let _f = to_fn(move || x = 42);
    |                  ----- ------- ^^^^^^ cannot assign
@@ -49,8 +49,8 @@ LL |         let _f = to_fn(move || x = 42);
 error[E0596]: cannot borrow `y` as mutable, as it is a captured variable in a `Fn` closure
   --> $DIR/borrow-immutable-upvar-mutation.rs:39:36
    |
-LL | fn to_fn<A, F: Fn<A>>(f: F) -> F {
-   |                          - change this to accept `FnMut` instead of `Fn`
+LL | fn to_fn<A: std::marker::Tuple, F: Fn<A>>(f: F) -> F {
+   |                                              - change this to accept `FnMut` instead of `Fn`
 ...
 LL |         let _g = to_fn(move || set(&mut y));
    |                  ----- -------     ^^^^^^ cannot borrow as mutable
@@ -61,8 +61,8 @@ LL |         let _g = to_fn(move || set(&mut y));
 error[E0594]: cannot assign to `z`, as it is a captured variable in a `Fn` closure
   --> $DIR/borrow-immutable-upvar-mutation.rs:44:27
    |
-LL | fn to_fn<A, F: Fn<A>>(f: F) -> F {
-   |                          - change this to accept `FnMut` instead of `Fn`
+LL | fn to_fn<A: std::marker::Tuple, F: Fn<A>>(f: F) -> F {
+   |                                              - change this to accept `FnMut` instead of `Fn`
 ...
 LL |             to_fn(move || z = 42);
    |             ----- ------- ^^^^^^ cannot assign
diff --git a/src/test/ui/borrowck/borrowck-move-by-capture.rs b/src/test/ui/borrowck/borrowck-move-by-capture.rs
index f26edef17f3..6f0eb1870f3 100644
--- a/src/test/ui/borrowck/borrowck-move-by-capture.rs
+++ b/src/test/ui/borrowck/borrowck-move-by-capture.rs
@@ -1,7 +1,7 @@
-#![feature(unboxed_closures)]
+#![feature(unboxed_closures, tuple_trait)]
 
-fn to_fn_mut<A,F:FnMut<A>>(f: F) -> F { f }
-fn to_fn_once<A,F:FnOnce<A>>(f: F) -> F { f }
+fn to_fn_mut<A:std::marker::Tuple,F:FnMut<A>>(f: F) -> F { f }
+fn to_fn_once<A:std::marker::Tuple,F:FnOnce<A>>(f: F) -> F { f }
 
 pub fn main() {
     let bar: Box<_> = Box::new(3);
diff --git a/src/test/ui/c-variadic/issue-86053-1.stderr b/src/test/ui/c-variadic/issue-86053-1.stderr
index 60b379faf4e..075bd1fc488 100644
--- a/src/test/ui/c-variadic/issue-86053-1.stderr
+++ b/src/test/ui/c-variadic/issue-86053-1.stderr
@@ -66,8 +66,8 @@ LL |     self , ... ,   self ,   self , ... ) where F : FnOnce ( & 'a & 'b usize
    |
   ::: $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
    |
 help: a trait with a similar name exists
    |
diff --git a/src/test/ui/cannot-mutate-captured-non-mut-var.rs b/src/test/ui/cannot-mutate-captured-non-mut-var.rs
index a83884acb1d..952dab25bf9 100644
--- a/src/test/ui/cannot-mutate-captured-non-mut-var.rs
+++ b/src/test/ui/cannot-mutate-captured-non-mut-var.rs
@@ -1,8 +1,8 @@
-#![feature(unboxed_closures)]
+#![feature(unboxed_closures, tuple_trait)]
 
 use std::io::Read;
 
-fn to_fn_once<A,F:FnOnce<A>>(f: F) -> F { f }
+fn to_fn_once<A:std::marker::Tuple,F:FnOnce<A>>(f: F) -> F { f }
 
 fn main() {
     let x = 1;
diff --git a/src/test/ui/chalkify/closure.rs b/src/test/ui/chalkify/closure.rs
index 81114d491d7..408e8802d86 100644
--- a/src/test/ui/chalkify/closure.rs
+++ b/src/test/ui/chalkify/closure.rs
@@ -1,4 +1,5 @@
-// check-fail
+// known-bug: unknown
+// FIXME(chalk): Chalk needs support for the Tuple trait
 // compile-flags: -Z chalk
 
 fn main() -> () {
@@ -25,7 +26,7 @@ fn main() -> () {
     let mut c = b;
 
     c();
-    b(); //~ ERROR
+    b(); // FIXME: reenable when this is fixed ~ ERROR
 
     // FIXME(chalk): this doesn't quite work
     /*
diff --git a/src/test/ui/chalkify/closure.stderr b/src/test/ui/chalkify/closure.stderr
index 515e0cf0142..bcee0cab96a 100644
--- a/src/test/ui/chalkify/closure.stderr
+++ b/src/test/ui/chalkify/closure.stderr
@@ -1,22 +1,80 @@
-error[E0382]: borrow of moved value: `b`
-  --> $DIR/closure.rs:28:5
+error[E0277]: `()` is not a tuple
+  --> $DIR/closure.rs:7:5
+   |
+LL |     t();
+   |     ^^^ the trait `Tuple` is not implemented for `()`
+   |
+help: consider introducing a `where` clause, but there might be an alternative better way to express this requirement
+   |
+LL | fn main() -> () where (): Tuple {
+   |                 +++++++++++++++
+
+error[E0277]: `()` is not a tuple
+  --> $DIR/closure.rs:13:5
+   |
+LL |     b();
+   |     ^^^ the trait `Tuple` is not implemented for `()`
+   |
+help: consider introducing a `where` clause, but there might be an alternative better way to express this requirement
+   |
+LL | fn main() -> () where (): Tuple {
+   |                 +++++++++++++++
+
+error[E0277]: `()` is not a tuple
+  --> $DIR/closure.rs:17:5
+   |
+LL |     c();
+   |     ^^^ the trait `Tuple` is not implemented for `()`
+   |
+help: consider introducing a `where` clause, but there might be an alternative better way to express this requirement
+   |
+LL | fn main() -> () where (): Tuple {
+   |                 +++++++++++++++
+
+error[E0277]: `()` is not a tuple
+  --> $DIR/closure.rs:18:5
    |
-LL |     let mut c = b;
-   |                 - value moved here
-...
 LL |     b();
-   |     ^ value borrowed here after move
+   |     ^^^ the trait `Tuple` is not implemented for `()`
+   |
+help: consider introducing a `where` clause, but there might be an alternative better way to express this requirement
+   |
+LL | fn main() -> () where (): Tuple {
+   |                 +++++++++++++++
+
+error[E0277]: `()` is not a tuple
+  --> $DIR/closure.rs:24:5
+   |
+LL |     b();
+   |     ^^^ the trait `Tuple` is not implemented for `()`
+   |
+help: consider introducing a `where` clause, but there might be an alternative better way to express this requirement
+   |
+LL | fn main() -> () where (): Tuple {
+   |                 +++++++++++++++
+
+error[E0277]: `()` is not a tuple
+  --> $DIR/closure.rs:28:5
+   |
+LL |     c();
+   |     ^^^ the trait `Tuple` is not implemented for `()`
+   |
+help: consider introducing a `where` clause, but there might be an alternative better way to express this requirement
+   |
+LL | fn main() -> () where (): Tuple {
+   |                 +++++++++++++++
+
+error[E0277]: `()` is not a tuple
+  --> $DIR/closure.rs:29:5
    |
-note: closure cannot be moved more than once as it is not `Copy` due to moving the variable `a` out of its environment
-  --> $DIR/closure.rs:21:9
+LL |     b(); // FIXME: reenable when this is fixed ~ ERROR
+   |     ^^^ the trait `Tuple` is not implemented for `()`
    |
-LL |         a = 1;
-   |         ^
-help: consider mutably borrowing `b`
+help: consider introducing a `where` clause, but there might be an alternative better way to express this requirement
    |
-LL |     let mut c = &mut b;
-   |                 ++++
+LL | fn main() -> () where (): Tuple {
+   |                 +++++++++++++++
 
-error: aborting due to previous error
+error: aborting due to 7 previous errors
 
-For more information about this error, try `rustc --explain E0382`.
+For more information about this error, try `rustc --explain E0277`.
diff --git a/src/test/ui/chalkify/trait-objects.rs b/src/test/ui/chalkify/trait-objects.rs
index d56abc42bf5..30929e943bd 100644
--- a/src/test/ui/chalkify/trait-objects.rs
+++ b/src/test/ui/chalkify/trait-objects.rs
@@ -1,4 +1,5 @@
-// check-pass
+// known-bug: unknown
+// FIXME(chalk): Chalk needs support for the Tuple trait
 // compile-flags: -Z chalk
 
 use std::fmt::Display;
diff --git a/src/test/ui/chalkify/trait-objects.stderr b/src/test/ui/chalkify/trait-objects.stderr
new file mode 100644
index 00000000000..098bd2d3226
--- /dev/null
+++ b/src/test/ui/chalkify/trait-objects.stderr
@@ -0,0 +1,28 @@
+error: the type `&dyn Fn(i32) -> _` is not well-formed (chalk)
+  --> $DIR/trait-objects.rs:11:12
+   |
+LL |     let f: &dyn Fn(i32) -> _ = &|x| x + x;
+   |            ^^^^^^^^^^^^^^^^^
+
+error[E0277]: `(i32,)` is not a tuple
+  --> $DIR/trait-objects.rs:12:5
+   |
+LL |     f(2);
+   |     ^^^^ the trait `Tuple` is not implemented for `(i32,)`
+   |
+help: consider introducing a `where` clause, but there might be an alternative better way to express this requirement
+   |
+LL | fn main() where (i32,): Tuple {
+   |           +++++++++++++++++++
+
+error[E0277]: expected a `Fn<(i32,)>` closure, found `dyn Fn(i32) -> i32`
+  --> $DIR/trait-objects.rs:12:5
+   |
+LL |     f(2);
+   |     ^^^^ expected an `Fn<(i32,)>` closure, found `dyn Fn(i32) -> i32`
+   |
+   = help: the trait `Fn<(i32,)>` is not implemented for `dyn Fn(i32) -> i32`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0277`.
diff --git a/src/test/ui/closures/issue-78720.stderr b/src/test/ui/closures/issue-78720.stderr
index 3dd13877298..da3f539a007 100644
--- a/src/test/ui/closures/issue-78720.stderr
+++ b/src/test/ui/closures/issue-78720.stderr
@@ -12,8 +12,8 @@ LL |     _func: F,
    |
   ::: $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
    |
 help: a trait with a similar name exists
    |
diff --git a/src/test/ui/error-codes/E0059.stderr b/src/test/ui/error-codes/E0059.stderr
index a1b8aeaedbb..f331d014226 100644
--- a/src/test/ui/error-codes/E0059.stderr
+++ b/src/test/ui/error-codes/E0059.stderr
@@ -1,8 +1,14 @@
-error[E0059]: cannot use call notation; the first type parameter for the function trait is neither a tuple nor unit
-  --> $DIR/E0059.rs:3:41
+error[E0059]: type parameter to bare `Fn` trait must be a tuple
+  --> $DIR/E0059.rs:3:11
    |
 LL | fn foo<F: Fn<i32>>(f: F) -> F::Output { f(3) }
-   |                                         ^^^^
+   |           ^^^^^^^ the trait `Tuple` is not implemented for `i32`
+   |
+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
 
diff --git a/src/test/ui/feature-gates/feature-gate-abi.rs b/src/test/ui/feature-gates/feature-gate-abi.rs
index 855263595d0..15b674c62e4 100644
--- a/src/test/ui/feature-gates/feature-gate-abi.rs
+++ b/src/test/ui/feature-gates/feature-gate-abi.rs
@@ -9,6 +9,9 @@
 #[lang="sized"]
 trait Sized { }
 
+#[lang="tuple_trait"]
+trait Tuple { }
+
 // Functions
 extern "rust-intrinsic" fn f1() {} //~ ERROR intrinsics are subject to change
                                    //~^ ERROR intrinsic must be in
diff --git a/src/test/ui/feature-gates/feature-gate-abi.stderr b/src/test/ui/feature-gates/feature-gate-abi.stderr
index bcca39c8fb8..33ec250f090 100644
--- a/src/test/ui/feature-gates/feature-gate-abi.stderr
+++ b/src/test/ui/feature-gates/feature-gate-abi.stderr
@@ -1,5 +1,5 @@
 error[E0658]: intrinsics are subject to change
-  --> $DIR/feature-gate-abi.rs:13:8
+  --> $DIR/feature-gate-abi.rs:16:8
    |
 LL | extern "rust-intrinsic" fn f1() {}
    |        ^^^^^^^^^^^^^^^^
@@ -7,7 +7,7 @@ LL | extern "rust-intrinsic" fn f1() {}
    = help: add `#![feature(intrinsics)]` to the crate attributes to enable
 
 error[E0658]: platform intrinsics are experimental and possibly buggy
-  --> $DIR/feature-gate-abi.rs:15:8
+  --> $DIR/feature-gate-abi.rs:18:8
    |
 LL | extern "platform-intrinsic" fn f2() {}
    |        ^^^^^^^^^^^^^^^^^^^^
@@ -16,7 +16,7 @@ LL | extern "platform-intrinsic" fn f2() {}
    = help: add `#![feature(platform_intrinsics)]` to the crate attributes to enable
 
 error[E0658]: rust-call ABI is subject to change
-  --> $DIR/feature-gate-abi.rs:17:8
+  --> $DIR/feature-gate-abi.rs:20:8
    |
 LL | extern "rust-call" fn f4(_: ()) {}
    |        ^^^^^^^^^^^
@@ -25,7 +25,7 @@ LL | extern "rust-call" fn f4(_: ()) {}
    = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
 
 error[E0658]: efiapi ABI is experimental and subject to change
-  --> $DIR/feature-gate-abi.rs:18:8
+  --> $DIR/feature-gate-abi.rs:21:8
    |
 LL | extern "efiapi" fn f10() {}
    |        ^^^^^^^^
@@ -34,7 +34,7 @@ LL | extern "efiapi" fn f10() {}
    = help: add `#![feature(abi_efiapi)]` to the crate attributes to enable
 
 error[E0658]: intrinsics are subject to change
-  --> $DIR/feature-gate-abi.rs:22:12
+  --> $DIR/feature-gate-abi.rs:25:12
    |
 LL |     extern "rust-intrinsic" fn m1();
    |            ^^^^^^^^^^^^^^^^
@@ -42,7 +42,7 @@ LL |     extern "rust-intrinsic" fn m1();
    = help: add `#![feature(intrinsics)]` to the crate attributes to enable
 
 error[E0658]: platform intrinsics are experimental and possibly buggy
-  --> $DIR/feature-gate-abi.rs:24:12
+  --> $DIR/feature-gate-abi.rs:27:12
    |
 LL |     extern "platform-intrinsic" fn m2();
    |            ^^^^^^^^^^^^^^^^^^^^
@@ -51,7 +51,7 @@ LL |     extern "platform-intrinsic" fn m2();
    = help: add `#![feature(platform_intrinsics)]` to the crate attributes to enable
 
 error[E0658]: rust-call ABI is subject to change
-  --> $DIR/feature-gate-abi.rs:26:12
+  --> $DIR/feature-gate-abi.rs:29:12
    |
 LL |     extern "rust-call" fn m4(_: ());
    |            ^^^^^^^^^^^
@@ -60,7 +60,7 @@ LL |     extern "rust-call" fn m4(_: ());
    = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
 
 error[E0658]: efiapi ABI is experimental and subject to change
-  --> $DIR/feature-gate-abi.rs:27:12
+  --> $DIR/feature-gate-abi.rs:30:12
    |
 LL |     extern "efiapi" fn m10();
    |            ^^^^^^^^
@@ -69,7 +69,7 @@ LL |     extern "efiapi" fn m10();
    = help: add `#![feature(abi_efiapi)]` to the crate attributes to enable
 
 error[E0658]: rust-call ABI is subject to change
-  --> $DIR/feature-gate-abi.rs:29:12
+  --> $DIR/feature-gate-abi.rs:32:12
    |
 LL |     extern "rust-call" fn dm4(_: ()) {}
    |            ^^^^^^^^^^^
@@ -78,7 +78,7 @@ LL |     extern "rust-call" fn dm4(_: ()) {}
    = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
 
 error[E0658]: efiapi ABI is experimental and subject to change
-  --> $DIR/feature-gate-abi.rs:30:12
+  --> $DIR/feature-gate-abi.rs:33:12
    |
 LL |     extern "efiapi" fn dm10() {}
    |            ^^^^^^^^
@@ -87,7 +87,7 @@ LL |     extern "efiapi" fn dm10() {}
    = help: add `#![feature(abi_efiapi)]` to the crate attributes to enable
 
 error[E0658]: intrinsics are subject to change
-  --> $DIR/feature-gate-abi.rs:37:12
+  --> $DIR/feature-gate-abi.rs:40:12
    |
 LL |     extern "rust-intrinsic" fn m1() {}
    |            ^^^^^^^^^^^^^^^^
@@ -95,7 +95,7 @@ LL |     extern "rust-intrinsic" fn m1() {}
    = help: add `#![feature(intrinsics)]` to the crate attributes to enable
 
 error[E0658]: platform intrinsics are experimental and possibly buggy
-  --> $DIR/feature-gate-abi.rs:39:12
+  --> $DIR/feature-gate-abi.rs:42:12
    |
 LL |     extern "platform-intrinsic" fn m2() {}
    |            ^^^^^^^^^^^^^^^^^^^^
@@ -104,7 +104,7 @@ LL |     extern "platform-intrinsic" fn m2() {}
    = help: add `#![feature(platform_intrinsics)]` to the crate attributes to enable
 
 error[E0658]: rust-call ABI is subject to change
-  --> $DIR/feature-gate-abi.rs:41:12
+  --> $DIR/feature-gate-abi.rs:44:12
    |
 LL |     extern "rust-call" fn m4(_: ()) {}
    |            ^^^^^^^^^^^
@@ -113,7 +113,7 @@ LL |     extern "rust-call" fn m4(_: ()) {}
    = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
 
 error[E0658]: efiapi ABI is experimental and subject to change
-  --> $DIR/feature-gate-abi.rs:42:12
+  --> $DIR/feature-gate-abi.rs:45:12
    |
 LL |     extern "efiapi" fn m10() {}
    |            ^^^^^^^^
@@ -122,7 +122,7 @@ LL |     extern "efiapi" fn m10() {}
    = help: add `#![feature(abi_efiapi)]` to the crate attributes to enable
 
 error[E0658]: intrinsics are subject to change
-  --> $DIR/feature-gate-abi.rs:47:12
+  --> $DIR/feature-gate-abi.rs:50:12
    |
 LL |     extern "rust-intrinsic" fn im1() {}
    |            ^^^^^^^^^^^^^^^^
@@ -130,7 +130,7 @@ LL |     extern "rust-intrinsic" fn im1() {}
    = help: add `#![feature(intrinsics)]` to the crate attributes to enable
 
 error[E0658]: platform intrinsics are experimental and possibly buggy
-  --> $DIR/feature-gate-abi.rs:49:12
+  --> $DIR/feature-gate-abi.rs:52:12
    |
 LL |     extern "platform-intrinsic" fn im2() {}
    |            ^^^^^^^^^^^^^^^^^^^^
@@ -139,7 +139,7 @@ LL |     extern "platform-intrinsic" fn im2() {}
    = help: add `#![feature(platform_intrinsics)]` to the crate attributes to enable
 
 error[E0658]: rust-call ABI is subject to change
-  --> $DIR/feature-gate-abi.rs:51:12
+  --> $DIR/feature-gate-abi.rs:54:12
    |
 LL |     extern "rust-call" fn im4(_: ()) {}
    |            ^^^^^^^^^^^
@@ -148,7 +148,7 @@ LL |     extern "rust-call" fn im4(_: ()) {}
    = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
 
 error[E0658]: efiapi ABI is experimental and subject to change
-  --> $DIR/feature-gate-abi.rs:52:12
+  --> $DIR/feature-gate-abi.rs:55:12
    |
 LL |     extern "efiapi" fn im10() {}
    |            ^^^^^^^^
@@ -157,7 +157,7 @@ LL |     extern "efiapi" fn im10() {}
    = help: add `#![feature(abi_efiapi)]` to the crate attributes to enable
 
 error[E0658]: intrinsics are subject to change
-  --> $DIR/feature-gate-abi.rs:56:18
+  --> $DIR/feature-gate-abi.rs:59:18
    |
 LL | type A1 = extern "rust-intrinsic" fn();
    |                  ^^^^^^^^^^^^^^^^
@@ -165,7 +165,7 @@ LL | type A1 = extern "rust-intrinsic" fn();
    = help: add `#![feature(intrinsics)]` to the crate attributes to enable
 
 error[E0658]: platform intrinsics are experimental and possibly buggy
-  --> $DIR/feature-gate-abi.rs:57:18
+  --> $DIR/feature-gate-abi.rs:60:18
    |
 LL | type A2 = extern "platform-intrinsic" fn();
    |                  ^^^^^^^^^^^^^^^^^^^^
@@ -174,7 +174,7 @@ LL | type A2 = extern "platform-intrinsic" fn();
    = help: add `#![feature(platform_intrinsics)]` to the crate attributes to enable
 
 error[E0658]: rust-call ABI is subject to change
-  --> $DIR/feature-gate-abi.rs:58:18
+  --> $DIR/feature-gate-abi.rs:61:18
    |
 LL | type A4 = extern "rust-call" fn(_: ());
    |                  ^^^^^^^^^^^
@@ -183,7 +183,7 @@ LL | type A4 = extern "rust-call" fn(_: ());
    = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
 
 error[E0658]: efiapi ABI is experimental and subject to change
-  --> $DIR/feature-gate-abi.rs:59:19
+  --> $DIR/feature-gate-abi.rs:62:19
    |
 LL | type A10 = extern "efiapi" fn();
    |                   ^^^^^^^^
@@ -192,7 +192,7 @@ LL | type A10 = extern "efiapi" fn();
    = help: add `#![feature(abi_efiapi)]` to the crate attributes to enable
 
 error[E0658]: intrinsics are subject to change
-  --> $DIR/feature-gate-abi.rs:62:8
+  --> $DIR/feature-gate-abi.rs:65:8
    |
 LL | extern "rust-intrinsic" {}
    |        ^^^^^^^^^^^^^^^^
@@ -200,7 +200,7 @@ LL | extern "rust-intrinsic" {}
    = help: add `#![feature(intrinsics)]` to the crate attributes to enable
 
 error[E0658]: platform intrinsics are experimental and possibly buggy
-  --> $DIR/feature-gate-abi.rs:63:8
+  --> $DIR/feature-gate-abi.rs:66:8
    |
 LL | extern "platform-intrinsic" {}
    |        ^^^^^^^^^^^^^^^^^^^^
@@ -209,7 +209,7 @@ LL | extern "platform-intrinsic" {}
    = help: add `#![feature(platform_intrinsics)]` to the crate attributes to enable
 
 error[E0658]: rust-call ABI is subject to change
-  --> $DIR/feature-gate-abi.rs:64:8
+  --> $DIR/feature-gate-abi.rs:67:8
    |
 LL | extern "rust-call" {}
    |        ^^^^^^^^^^^
@@ -218,7 +218,7 @@ LL | extern "rust-call" {}
    = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
 
 error[E0658]: efiapi ABI is experimental and subject to change
-  --> $DIR/feature-gate-abi.rs:65:8
+  --> $DIR/feature-gate-abi.rs:68:8
    |
 LL | extern "efiapi" {}
    |        ^^^^^^^^
@@ -227,49 +227,49 @@ LL | extern "efiapi" {}
    = help: add `#![feature(abi_efiapi)]` to the crate attributes to enable
 
 error: intrinsic must be in `extern "rust-intrinsic" { ... }` block
-  --> $DIR/feature-gate-abi.rs:22:32
+  --> $DIR/feature-gate-abi.rs:25:32
    |
 LL |     extern "rust-intrinsic" fn m1();
    |                                ^^
 
 error: intrinsic must be in `extern "rust-intrinsic" { ... }` block
-  --> $DIR/feature-gate-abi.rs:24:36
+  --> $DIR/feature-gate-abi.rs:27:36
    |
 LL |     extern "platform-intrinsic" fn m2();
    |                                    ^^
 
 error: intrinsic must be in `extern "rust-intrinsic" { ... }` block
-  --> $DIR/feature-gate-abi.rs:13:33
+  --> $DIR/feature-gate-abi.rs:16:33
    |
 LL | extern "rust-intrinsic" fn f1() {}
    |                                 ^^
 
 error: intrinsic must be in `extern "rust-intrinsic" { ... }` block
-  --> $DIR/feature-gate-abi.rs:15:37
+  --> $DIR/feature-gate-abi.rs:18:37
    |
 LL | extern "platform-intrinsic" fn f2() {}
    |                                     ^^
 
 error: intrinsic must be in `extern "rust-intrinsic" { ... }` block
-  --> $DIR/feature-gate-abi.rs:37:37
+  --> $DIR/feature-gate-abi.rs:40:37
    |
 LL |     extern "rust-intrinsic" fn m1() {}
    |                                     ^^
 
 error: intrinsic must be in `extern "rust-intrinsic" { ... }` block
-  --> $DIR/feature-gate-abi.rs:39:41
+  --> $DIR/feature-gate-abi.rs:42:41
    |
 LL |     extern "platform-intrinsic" fn m2() {}
    |                                         ^^
 
 error: intrinsic must be in `extern "rust-intrinsic" { ... }` block
-  --> $DIR/feature-gate-abi.rs:47:38
+  --> $DIR/feature-gate-abi.rs:50:38
    |
 LL |     extern "rust-intrinsic" fn im1() {}
    |                                      ^^
 
 error: intrinsic must be in `extern "rust-intrinsic" { ... }` block
-  --> $DIR/feature-gate-abi.rs:49:42
+  --> $DIR/feature-gate-abi.rs:52:42
    |
 LL |     extern "platform-intrinsic" fn im2() {}
    |                                          ^^
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/issues/issue-12127.rs b/src/test/ui/issues/issue-12127.rs
index 8b30ddc2de6..199d542e816 100644
--- a/src/test/ui/issues/issue-12127.rs
+++ b/src/test/ui/issues/issue-12127.rs
@@ -1,6 +1,6 @@
-#![feature(unboxed_closures)]
+#![feature(unboxed_closures, tuple_trait)]
 
-fn to_fn_once<A,F:FnOnce<A>>(f: F) -> F { f }
+fn to_fn_once<A:std::marker::Tuple,F:FnOnce<A>>(f: F) -> F { f }
 fn do_it(x: &isize) { }
 
 fn main() {
diff --git a/src/test/ui/issues/issue-23024.stderr b/src/test/ui/issues/issue-23024.stderr
index 73f93c51d34..dc8b34a70c3 100644
--- a/src/test/ui/issues/issue-23024.stderr
+++ b/src/test/ui/issues/issue-23024.stderr
@@ -16,7 +16,7 @@ LL |     println!("{:?}",(vfnfer[0] as dyn Fn)(3));
 note: trait defined here, with 1 generic parameter: `Args`
   --> $SRC_DIR/core/src/ops/function.rs:LL:COL
    |
-LL | pub trait Fn<Args>: FnMut<Args> {
+LL | pub trait Fn<Args: Tuple>: FnMut<Args> {
    |           ^^ ----
 help: add missing generic argument
    |
diff --git a/src/test/ui/issues/issue-7607-1.stderr b/src/test/ui/issues/issue-7607-1.stderr
index ecff8b42b0e..f1ab0ad26d7 100644
--- a/src/test/ui/issues/issue-7607-1.stderr
+++ b/src/test/ui/issues/issue-7607-1.stderr
@@ -6,8 +6,8 @@ LL | impl Fo {
    |
   ::: $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: aborting due to previous error
 
diff --git a/src/test/ui/lang-items/lang-item-missing-generator.rs b/src/test/ui/lang-items/lang-item-missing-generator.rs
index 0c329542928..9b9aff38e52 100644
--- a/src/test/ui/lang-items/lang-item-missing-generator.rs
+++ b/src/test/ui/lang-items/lang-item-missing-generator.rs
@@ -1,12 +1,14 @@
 // error-pattern: requires `generator` lang_item
-#![feature(no_core, lang_items, unboxed_closures)]
+#![feature(no_core, lang_items, unboxed_closures, tuple_trait)]
 #![no_core]
 
 #[lang = "sized"] pub trait Sized { }
 
+#[lang = "tuple_trait"] pub trait Tuple { }
+
 #[lang = "fn_once"]
 #[rustc_paren_sugar]
-pub trait FnOnce<Args> {
+pub trait FnOnce<Args: Tuple> {
     type Output;
 
     extern "rust-call" fn call_once(self, args: Args) -> Self::Output;
diff --git a/src/test/ui/lang-items/lang-item-missing-generator.stderr b/src/test/ui/lang-items/lang-item-missing-generator.stderr
index fa13bf0b127..a24fdb5fb65 100644
--- a/src/test/ui/lang-items/lang-item-missing-generator.stderr
+++ b/src/test/ui/lang-items/lang-item-missing-generator.stderr
@@ -1,8 +1,15 @@
+error[E0635]: unknown feature `tuple_trait`
+  --> $DIR/lang-item-missing-generator.rs:2:51
+   |
+LL | #![feature(no_core, lang_items, unboxed_closures, tuple_trait)]
+   |                                                   ^^^^^^^^^^^
+
 error: requires `generator` lang_item
-  --> $DIR/lang-item-missing-generator.rs:15:17
+  --> $DIR/lang-item-missing-generator.rs:17:17
    |
 LL | pub fn abc() -> impl FnOnce(f32) {
    |                 ^^^^^^^^^^^^^^^^
 
-error: aborting due to previous error
+error: aborting due to 2 previous errors
 
+For more information about this error, try `rustc --explain E0635`.
diff --git a/src/test/ui/mismatched_types/unboxed-closures-vtable-mismatch.rs b/src/test/ui/mismatched_types/unboxed-closures-vtable-mismatch.rs
index 8dbe3472ea8..307104e47a1 100644
--- a/src/test/ui/mismatched_types/unboxed-closures-vtable-mismatch.rs
+++ b/src/test/ui/mismatched_types/unboxed-closures-vtable-mismatch.rs
@@ -1,8 +1,8 @@
-#![feature(unboxed_closures)]
+#![feature(unboxed_closures,tuple_trait)]
 
 use std::ops::FnMut;
 
-fn to_fn_mut<A, F: FnMut<A>>(f: F) -> F { f }
+fn to_fn_mut<A:std::marker::Tuple, F:FnMut<A>>(f: F) -> F { f }
 
 fn call_it<F: FnMut(isize, isize) -> isize>(y: isize, mut f: F) -> isize {
     //~^ NOTE required by this bound in `call_it`
diff --git a/src/test/ui/moves/moves-based-on-type-move-out-of-closure-env-issue-1965.rs b/src/test/ui/moves/moves-based-on-type-move-out-of-closure-env-issue-1965.rs
index 76b7aab542d..490d91ac118 100644
--- a/src/test/ui/moves/moves-based-on-type-move-out-of-closure-env-issue-1965.rs
+++ b/src/test/ui/moves/moves-based-on-type-move-out-of-closure-env-issue-1965.rs
@@ -1,6 +1,6 @@
-#![feature(unboxed_closures)]
+#![feature(unboxed_closures, tuple_trait)]
 
-fn to_fn<A,F:Fn<A>>(f: F) -> F { f }
+fn to_fn<A:std::marker::Tuple,F:Fn<A>>(f: F) -> F { f }
 
 fn test(_x: Box<usize>) {}
 
diff --git a/src/test/ui/overloaded/overloaded-calls-nontuple.rs b/src/test/ui/overloaded/overloaded-calls-nontuple.rs
index 07d44ff82b1..32a3b93e0a1 100644
--- a/src/test/ui/overloaded/overloaded-calls-nontuple.rs
+++ b/src/test/ui/overloaded/overloaded-calls-nontuple.rs
@@ -8,22 +8,23 @@ struct S {
 }
 
 impl FnMut<isize> for S {
+    //~^ ERROR type parameter to bare `FnMut` trait must be a tuple
     extern "rust-call" fn call_mut(&mut self, z: isize) -> isize {
+        //~^ ERROR functions with the "rust-call" ABI must take a single non-self tuple argument
         self.x + self.y + z
     }
-    //~^^^ ERROR functions with the "rust-call" ABI must take a single non-self argument
 }
 
 impl FnOnce<isize> for S {
+    //~^ ERROR type parameter to bare `FnOnce` trait must be a tuple
     type Output = isize;
-    extern "rust-call" fn call_once(mut self, z: isize) -> isize { self.call_mut(z) }
-    //~^ ERROR functions with the "rust-call" ABI must take a single non-self argument
+    extern "rust-call" fn call_once(mut self, z: isize) -> isize {
+        //~^ ERROR functions with the "rust-call" ABI must take a single non-self tuple argument
+        self.call_mut(z)
+    }
 }
 
 fn main() {
-    let mut s = S {
-        x: 1,
-        y: 2,
-    };
-    drop(s(3))  //~ ERROR cannot use call notation
+    let mut s = S { x: 1, y: 2 };
+    drop(s(3))
 }
diff --git a/src/test/ui/overloaded/overloaded-calls-nontuple.stderr b/src/test/ui/overloaded/overloaded-calls-nontuple.stderr
index 8f299bc9434..794535aeb11 100644
--- a/src/test/ui/overloaded/overloaded-calls-nontuple.stderr
+++ b/src/test/ui/overloaded/overloaded-calls-nontuple.stderr
@@ -1,21 +1,40 @@
-error: functions with the "rust-call" ABI must take a single non-self argument that is a tuple
-  --> $DIR/overloaded-calls-nontuple.rs:11:5
+error[E0059]: type parameter to bare `FnMut` trait must be a tuple
+  --> $DIR/overloaded-calls-nontuple.rs:10:6
    |
-LL |     extern "rust-call" fn call_mut(&mut self, z: isize) -> isize {
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | impl FnMut<isize> for S {
+   |      ^^^^^^^^^^^^ the trait `Tuple` is not implemented for `isize`
+   |
+note: required by a bound in `FnMut`
+  --> $SRC_DIR/core/src/ops/function.rs:LL:COL
+   |
+LL | pub trait FnMut<Args: Tuple>: FnOnce<Args> {
+   |                       ^^^^^ required by this bound in `FnMut`
+
+error[E0059]: type parameter to bare `FnOnce` trait must be a tuple
+  --> $DIR/overloaded-calls-nontuple.rs:18:6
+   |
+LL | impl FnOnce<isize> for S {
+   |      ^^^^^^^^^^^^^ the trait `Tuple` is not implemented for `isize`
+   |
+note: required by a bound in `FnOnce`
+  --> $SRC_DIR/core/src/ops/function.rs:LL:COL
+   |
+LL | pub trait FnOnce<Args: Tuple> {
+   |                        ^^^^^ required by this bound in `FnOnce`
 
-error: functions with the "rust-call" ABI must take a single non-self argument that is a tuple
-  --> $DIR/overloaded-calls-nontuple.rs:19:5
+error[E0277]: functions with the "rust-call" ABI must take a single non-self tuple argument
+  --> $DIR/overloaded-calls-nontuple.rs:12:5
    |
-LL |     extern "rust-call" fn call_once(mut self, z: isize) -> isize { self.call_mut(z) }
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |     extern "rust-call" fn call_mut(&mut self, z: isize) -> isize {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Tuple` is not implemented for `isize`
 
-error[E0059]: cannot use call notation; the first type parameter for the function trait is neither a tuple nor unit
-  --> $DIR/overloaded-calls-nontuple.rs:28:10
+error[E0277]: functions with the "rust-call" ABI must take a single non-self tuple argument
+  --> $DIR/overloaded-calls-nontuple.rs:21:5
    |
-LL |     drop(s(3))
-   |          ^^^^
+LL |     extern "rust-call" fn call_once(mut self, z: isize) -> isize {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Tuple` is not implemented for `isize`
 
-error: aborting due to 3 previous errors
+error: aborting due to 4 previous errors
 
-For more information about this error, try `rustc --explain E0059`.
+Some errors have detailed explanations: E0059, E0277.
+For more information about an error, try `rustc --explain E0059`.
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/span/issue-11925.rs b/src/test/ui/span/issue-11925.rs
index d9c08fbdd0f..cac9fd5bfb6 100644
--- a/src/test/ui/span/issue-11925.rs
+++ b/src/test/ui/span/issue-11925.rs
@@ -1,6 +1,6 @@
-#![feature(unboxed_closures)]
+#![feature(unboxed_closures, tuple_trait)]
 
-fn to_fn_once<A,F:FnOnce<A>>(f: F) -> F { f }
+fn to_fn_once<A:std::marker::Tuple,F:FnOnce<A>>(f: F) -> F { f }
 
 fn main() {
     let r = {
diff --git a/src/test/ui/typeck/issue-83693.stderr b/src/test/ui/typeck/issue-83693.stderr
index 0d8bbf1ce98..1e45c2d35df 100644
--- a/src/test/ui/typeck/issue-83693.stderr
+++ b/src/test/ui/typeck/issue-83693.stderr
@@ -6,8 +6,8 @@ LL | impl F {
    |
   ::: $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[E0412]: cannot find type `TestResult` in this scope
   --> $DIR/issue-83693.rs:9:22
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`.
diff --git a/src/test/ui/unboxed-closures/unboxed-closure-illegal-move.rs b/src/test/ui/unboxed-closures/unboxed-closure-illegal-move.rs
index ed8d7211461..7377359b6b0 100644
--- a/src/test/ui/unboxed-closures/unboxed-closure-illegal-move.rs
+++ b/src/test/ui/unboxed-closures/unboxed-closure-illegal-move.rs
@@ -1,12 +1,12 @@
-#![feature(unboxed_closures)]
+#![feature(unboxed_closures, tuple_trait)]
 
 // Tests that we can't move out of an unboxed closure environment
 // if the upvar is captured by ref or the closure takes self by
 // reference.
 
-fn to_fn<A,F:Fn<A>>(f: F) -> F { f }
-fn to_fn_mut<A,F:FnMut<A>>(f: F) -> F { f }
-fn to_fn_once<A,F:FnOnce<A>>(f: F) -> F { f }
+fn to_fn<A:std::marker::Tuple,F:Fn<A>>(f: F) -> F { f }
+fn to_fn_mut<A:std::marker::Tuple,F:FnMut<A>>(f: F) -> F { f }
+fn to_fn_once<A:std::marker::Tuple,F:FnOnce<A>>(f: F) -> F { f }
 
 fn main() {
     // By-ref cases
diff --git a/src/test/ui/unboxed-closures/unboxed-closures-mutate-upvar.rs b/src/test/ui/unboxed-closures/unboxed-closures-mutate-upvar.rs
index 57e6d30658c..c57312b4387 100644
--- a/src/test/ui/unboxed-closures/unboxed-closures-mutate-upvar.rs
+++ b/src/test/ui/unboxed-closures/unboxed-closures-mutate-upvar.rs
@@ -2,12 +2,12 @@
 // as `mut` through a closure. Also test that we CAN mutate a moved copy,
 // unless this is a `Fn` closure. Issue #16749.
 
-#![feature(unboxed_closures)]
+#![feature(unboxed_closures, tuple_trait)]
 
 use std::mem;
 
-fn to_fn<A,F:Fn<A>>(f: F) -> F { f }
-fn to_fn_mut<A,F:FnMut<A>>(f: F) -> F { f }
+fn to_fn<A:std::marker::Tuple,F:Fn<A>>(f: F) -> F { f }
+fn to_fn_mut<A:std::marker::Tuple,F:FnMut<A>>(f: F) -> F { f }
 
 fn a() {
     let n = 0;
diff --git a/src/test/ui/unboxed-closures/unboxed-closures-mutate-upvar.stderr b/src/test/ui/unboxed-closures/unboxed-closures-mutate-upvar.stderr
index d6e74b5b8b9..26f97b51913 100644
--- a/src/test/ui/unboxed-closures/unboxed-closures-mutate-upvar.stderr
+++ b/src/test/ui/unboxed-closures/unboxed-closures-mutate-upvar.stderr
@@ -28,8 +28,8 @@ LL |         n += 1;
 error[E0594]: cannot assign to `n`, as it is a captured variable in a `Fn` closure
   --> $DIR/unboxed-closures-mutate-upvar.rs:53:9
    |
-LL | fn to_fn<A,F:Fn<A>>(f: F) -> F { f }
-   |                        - change this to accept `FnMut` instead of `Fn`
+LL | fn to_fn<A:std::marker::Tuple,F:Fn<A>>(f: F) -> F { f }
+   |                                           - change this to accept `FnMut` instead of `Fn`
 ...
 LL |     let mut f = to_fn(move || {
    |                 ----- ------- in this closure
diff --git a/src/test/ui/unboxed-closures/unboxed-closures-static-call-wrong-trait.rs b/src/test/ui/unboxed-closures/unboxed-closures-static-call-wrong-trait.rs
index 0e727b11cd2..7289d9322d0 100644
--- a/src/test/ui/unboxed-closures/unboxed-closures-static-call-wrong-trait.rs
+++ b/src/test/ui/unboxed-closures/unboxed-closures-static-call-wrong-trait.rs
@@ -1,6 +1,6 @@
-#![feature(unboxed_closures)]
+#![feature(unboxed_closures, tuple_trait)]
 
-fn to_fn_mut<A,F:FnMut<A>>(f: F) -> F { f }
+fn to_fn_mut<A:std::marker::Tuple,F:FnMut<A>>(f: F) -> F { f }
 
 fn main() {
     let mut_ = to_fn_mut(|x| x);