Handle reporting invariance of fn pointer

Author: compiler-errors

Diff for: compiler/rustc_borrowck/src/diagnostics/region_errors.rs

@@ -358,6 +358,17 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                         );
                         (desc, note)
                     }
+                    ty::FnDef(def_id, _) => {
+                        let name = self.infcx.tcx.item_name(*def_id);
+                        let identity_substs =
+                            InternalSubsts::identity_for_item(self.infcx.tcx, *def_id);
+                        let desc = format!("a function pointer to `{name}`");
+                        let note = format!(
+                            "the function `{name}` is invariant over the parameter `{}`",
+                            identity_substs[param_index as usize]
+                        );
+                        (desc, note)
+                    }
                     _ => panic!("Unexpected type {:?}", ty),
                 };
                 diag.note(&format!("requirement occurs because of {desc}",));

Diff for: src/test/ui/nll/issue-95272.rs

@@ -0,0 +1,17 @@
+#![feature(nll)]
+
+use std::cell::Cell;
+
+fn check<'a, 'b>(x: Cell<&'a ()>, y: Cell<&'b ()>)
+where
+    'a: 'b,
+{
+}
+
+fn test<'a, 'b>(x: Cell<&'a ()>, y: Cell<&'b ()>) {
+    let f = check;
+    //~^ ERROR lifetime may not live long enough
+    f(x, y);
+}
+
+fn main() {}

Diff for: src/test/ui/nll/issue-95272.stderr

@@ -0,0 +1,17 @@
+error: lifetime may not live long enough
+  --> $DIR/issue-95272.rs:12:13
+   |
+LL | fn test<'a, 'b>(x: Cell<&'a ()>, y: Cell<&'b ()>) {
+   |         --  -- lifetime `'b` defined here
+   |         |
+   |         lifetime `'a` defined here
+LL |     let f = check;
+   |             ^^^^^ assignment requires that `'a` must outlive `'b`
+   |
+   = help: consider adding the following bound: `'a: 'b`
+   = note: requirement occurs because of a function pointer to `check`
+   = note: the function `check` is invariant over the parameter `'a`
+   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
+
+error: aborting due to previous error
+