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+In a `fn` type, a lifetime appears only in the return type
+and not in the arguments types.
+
+Erroneous code example:
+
+```compile_fail,E0581
+fn main() {
+    // Here, `'a` appears only in the return type:
+    let x: for<'a> fn() -> &'a i32;
+}
+```
+
+The problem here is that the lifetime isn't constrained by any of the arguments,
+making it impossible to determine how long it's supposed to live.
+
+To fix this issue, either use the lifetime in the arguments, or use the
+`'static` lifetime. Example:
+
+```
+fn main() {
+    // Here, `'a` appears only in the return type:
+    let x: for<'a> fn(&'a i32) -> &'a i32;
+    let y: fn() -> &'static i32;
+}
+```
+
+Note: The examples above used to be (erroneously) accepted by the
+compiler, but this was since corrected. See [issue #33685] for more
+details.
+
+[issue #33685]: https://github.com/rust-lang/rust/issues/33685