Add note suggesting to borrow a String argument to find

src/libcore/ops/function.rs

@@ -137,6 +137,10 @@ pub trait Fn<Args> : FnMut<Args> {
 #[rustc_paren_sugar]
 #[rustc_on_unimplemented(
     on(Args="()", note="wrap the `{Self}` in a closure with no arguments: `|| {{ /* code */ }}"),
+    on(
+        all(Args="(char,)", _Self="std::string::String"),
+        note="borrowing the `{Self}` might fix the problem"
+    ),
     message="expected a `{FnMut}<{Args}>` closure, found `{Self}`",
     label="expected an `FnMut<{Args}>` closure, found `{Self}`",
 )]

src/test/ui/suggestions/issue-62843.rs

@@ -0,0 +1,5 @@
+fn main() {
+    let line = String::from("abc");
+    let pattern = String::from("bc");
+    println!("{:?}", line.find(pattern)); //~ ERROR E0277
+}

src/test/ui/suggestions/issue-62843.stderr

@@ -0,0 +1,13 @@
+error[E0277]: expected a `std::ops::FnMut<(char,)>` closure, found `std::string::String`
+  --> $DIR/issue-62843.rs:4:27
+   |
+LL |     println!("{:?}", line.find(pattern));
+   |                           ^^^^ expected an `FnMut<(char,)>` closure, found `std::string::String`
+   |
+   = help: the trait `std::ops::FnMut<(char,)>` is not implemented for `std::string::String`
+   = note: borrowing the `std::string::String` might fix the problem
+   = note: required because of the requirements on the impl of `std::str::pattern::Pattern<'_>` for `std::string::String`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.