Rollup merge of #53296 - estebank:suggest-closure, r=KodrAus

When closure with no arguments was expected, suggest wrapping

Fix #49694.

1 files changed, 15 insertions, 0 deletions

diff --git a/src/libcore/ops/function.rs b/src/libcore/ops/function.rs
index d10fcb86b24..3ebd10a9209 100644
--- a/src/libcore/ops/function.rs
+++ b/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.