When closure with no arguments was expected, suggest wrapping

src/libcore/ops/function.rs

@@ -66,6 +66,11 @@
 #[lang = "fn"]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_paren_sugar]
+#[rustc_on_unimplemented(
+    on(Args="()", note="wrap the `{Self}` in a closure with no arguments: `|| {{ /* code */ }}"),
+    message="expected a `{Fn}<{Args}>` closure, found `{Self}`",
+    label="expected an `Fn<{Args}>` closure, found `{Self}`",
+)]
 #[fundamental] // so that regex can rely that `&str: !FnMut`
 pub trait Fn<Args> : FnMut<Args> {
     /// Performs the call operation.
@@ -139,6 +144,11 @@ pub trait Fn<Args> : FnMut<Args> {
 #[lang = "fn_mut"]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_paren_sugar]
+#[rustc_on_unimplemented(
+    on(Args="()", note="wrap the `{Self}` in a closure with no arguments: `|| {{ /* code */ }}"),
+    message="expected a `{FnMut}<{Args}>` closure, found `{Self}`",
+    label="expected an `FnMut<{Args}>` closure, found `{Self}`",
+)]
 #[fundamental] // so that regex can rely that `&str: !FnMut`
 pub trait FnMut<Args> : FnOnce<Args> {
     /// Performs the call operation.
@@ -212,6 +222,11 @@ pub trait FnMut<Args> : FnOnce<Args> {
 #[lang = "fn_once"]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_paren_sugar]
+#[rustc_on_unimplemented(
+    on(Args="()", note="wrap the `{Self}` in a closure with no arguments: `|| {{ /* code */ }}"),
+    message="expected a `{FnOnce}<{Args}>` closure, found `{Self}`",
+    label="expected an `FnOnce<{Args}>` closure, found `{Self}`",
+)]
 #[fundamental] // so that regex can rely that `&str: !FnMut`
 pub trait FnOnce<Args> {
     /// The returned type after the call operator is used.

src/test/ui/closure-expected.rs

@@ -0,0 +1,15 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn main() {
+    let x = Some(1);
+    let y = x.or_else(4);
+    //~^ ERROR expected a `std::ops::FnOnce<()>` closure, found `{integer}`
+}

src/test/ui/closure-expected.stderr

@@ -0,0 +1,12 @@
+error[E0277]: expected a `std::ops::FnOnce<()>` closure, found `{integer}`
+  --> $DIR/closure-expected.rs:13:15
+   |
+LL |     let y = x.or_else(4);
+   |               ^^^^^^^ expected an `FnOnce<()>` closure, found `{integer}`
+   |
+   = help: the trait `std::ops::FnOnce<()>` is not implemented for `{integer}`
+   = note: wrap the `{integer}` in a closure with no arguments: `|| { /* code */ }
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.

src/test/ui/extern/extern-wrong-value-type.rs

@@ -17,5 +17,5 @@ fn main() {
     // extern functions are extern "C" fn
     let _x: extern "C" fn() = f; // OK
     is_fn(f);
-    //~^ ERROR `extern "C" fn() {f}: std::ops::Fn<()>` is not satisfied
+    //~^ ERROR expected a `std::ops::Fn<()>` closure, found `extern "C" fn() {f}`
 }

src/test/ui/fn/fn-trait-formatting.rs

@@ -27,5 +27,5 @@ fn main() {
     //~| found type `std::boxed::Box<dyn std::ops::FnMut() -> isize>`
 
     needs_fn(1);
-    //~^ ERROR : std::ops::Fn<(isize,)>`
+    //~^ ERROR expected a `std::ops::Fn<(isize,)>` closure, found `{integer}`
 }

src/test/ui/issues/issue-22034.rs

@@ -16,6 +16,6 @@ fn main() {
     let ptr: *mut () = 0 as *mut _;
     let _: &mut Fn() = unsafe {
         &mut *(ptr as *mut Fn())
-        //~^ ERROR `(): std::ops::Fn<()>` is not satisfied
+        //~^ ERROR expected a `std::ops::Fn<()>` closure, found `()`
     };
 }

src/test/ui/issues/issue-22034.stderr

@@ -1,9 +1,11 @@
-error[E0277]: the trait bound `(): std::ops::Fn<()>` is not satisfied
+error[E0277]: expected a `std::ops::Fn<()>` closure, found `()`
   --> $DIR/issue-22034.rs:18:16
    |
 LL |         &mut *(ptr as *mut Fn())
-   |                ^^^ the trait `std::ops::Fn<()>` is not implemented for `()`
+   |                ^^^ expected an `Fn<()>` closure, found `()`
    |
+   = help: the trait `std::ops::Fn<()>` is not implemented for `()`
+   = note: wrap the `()` in a closure with no arguments: `|| { /* code */ }
    = note: required for the cast to the object type `dyn std::ops::Fn()`
 
 error: aborting due to previous error

src/test/ui/issues/issue-23966.stderr

@@ -1,8 +1,10 @@
-error[E0277]: the trait bound `(): std::ops::FnMut<(_, char)>` is not satisfied
+error[E0277]: expected a `std::ops::FnMut<(_, char)>` closure, found `()`
   --> $DIR/issue-23966.rs:12:16
    |
 LL |     "".chars().fold(|_, _| (), ());
-   |                ^^^^ the trait `std::ops::FnMut<(_, char)>` is not implemented for `()`
+   |                ^^^^ expected an `FnMut<(_, char)>` closure, found `()`
+   |
+   = help: the trait `std::ops::FnMut<(_, char)>` is not implemented for `()`
 
 error: aborting due to previous error