diagnostics: give a special note for unsafe fn / Fn/FnOnce/FnMut

Fixes #90073

Author: notriddle

core/src/ops/function.rs

@@ -59,6 +59,10 @@
         Args = "()",
         note = "wrap the `{Self}` in a closure with no arguments: `|| {{ /* code */ }}`"
     ),
+    on(
+        _Self = "unsafe fn",
+        note = "unsafe functions must be wrapped in closures: `|| unsafe {{ /* code */ }}`"
+    ),
     message = "expected a `{Fn}<{Args}>` closure, found `{Self}`",
     label = "expected an `Fn<{Args}>` closure, found `{Self}`"
 )]
@@ -139,6 +143,10 @@ pub trait Fn<Args>: FnMut<Args> {
         Args = "()",
         note = "wrap the `{Self}` in a closure with no arguments: `|| {{ /* code */ }}`"
     ),
+    on(
+        _Self = "unsafe fn",
+        note = "unsafe functions must be wrapped in closures: `|| unsafe {{ /* code */ }}`"
+    ),
     message = "expected a `{FnMut}<{Args}>` closure, found `{Self}`",
     label = "expected an `FnMut<{Args}>` closure, found `{Self}`"
 )]
@@ -211,6 +219,10 @@ pub trait FnMut<Args>: FnOnce<Args> {
         Args = "()",
         note = "wrap the `{Self}` in a closure with no arguments: `|| {{ /* code */ }}`"
     ),
+    on(
+        _Self = "unsafe fn",
+        note = "unsafe functions must be wrapped in closures: `|| unsafe {{ /* code */ }}`"
+    ),
     message = "expected a `{FnOnce}<{Args}>` closure, found `{Self}`",
     label = "expected an `FnOnce<{Args}>` closure, found `{Self}`"
 )]