--- /dev/null
+// Test to show what happens if we were not careful and allowed invariant
+// lifetimes to escape though an impl trait.
+//
+// Specifically we swap a long lived and short lived reference, giving us a
+// dangling pointer.
+
+use std::cell::RefCell;
+use std::rc::Rc;
+
+trait Swap: Sized {
+ fn swap(self, other: Self);
+}
+
+impl<T> Swap for &mut T {
+ fn swap(self, other: Self) {
+ std::mem::swap(self, other);
+ }
+}
+
+impl<T> Swap for Rc<RefCell<T>> {
+ fn swap(self, other: Self) {
+ <RefCell<T>>::swap(&self, &other);
+ }
+}
+
+// Here we are hiding `'b` making the caller believe that `&'a mut &'s T` and
+// `&'a mut &'l T` are the same type.
+fn hide_ref<'a, 'b, T: 'static>(x: &'a mut &'b T) -> impl Swap + 'a {
+ //~^ ERROR hidden type
+ x
+}
+
+fn dangle_ref() -> &'static [i32; 3] {
+ let mut res = &[4, 5, 6];
+ let x = [1, 2, 3];
+ hide_ref(&mut res).swap(hide_ref(&mut &x));
+ res
+}
+
+// Here we are hiding `'b` making the caller believe that `Rc<RefCell<&'s T>>`
+// and `Rc<RefCell<&'l T>>` are the same type.
+//
+// This is different to the previous example because the concrete return type
+// only has a single lifetime.
+fn hide_rc_refcell<'a, 'b: 'a, T: 'static>(x: Rc<RefCell<&'b T>>) -> impl Swap + 'a {
+ //~^ ERROR hidden type
+ x
+}
+
+fn dangle_rc_refcell() -> &'static [i32; 3] {
+ let long = Rc::new(RefCell::new(&[4, 5, 6]));
+ let x = [1, 2, 3];
+ let short = Rc::new(RefCell::new(&x));
+ hide_rc_refcell(long.clone()).swap(hide_rc_refcell(short));
+ let res: &'static [i32; 3] = *long.borrow();
+ res
+}
+
+fn main() {
+ // both will print nonsense values.
+ println!("{:?}", dangle_ref());
+ println!("{:?}", dangle_rc_refcell())
+}
--- /dev/null
+error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds
+ --> $DIR/hidden-lifetimes.rs:28:54
+ |
+LL | fn hide_ref<'a, 'b, T: 'static>(x: &'a mut &'b T) -> impl Swap + 'a {
+ | ^^^^^^^^^^^^^^
+ |
+note: hidden type `&'a mut &'b T` captures the lifetime 'b as defined on the function body at 28:17
+ --> $DIR/hidden-lifetimes.rs:28:17
+ |
+LL | fn hide_ref<'a, 'b, T: 'static>(x: &'a mut &'b T) -> impl Swap + 'a {
+ | ^^
+
+error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds
+ --> $DIR/hidden-lifetimes.rs:45:70
+ |
+LL | fn hide_rc_refcell<'a, 'b: 'a, T: 'static>(x: Rc<RefCell<&'b T>>) -> impl Swap + 'a {
+ | ^^^^^^^^^^^^^^
+ |
+note: hidden type `std::rc::Rc<std::cell::RefCell<&'b T>>` captures the lifetime 'b as defined on the function body at 45:24
+ --> $DIR/hidden-lifetimes.rs:45:24
+ |
+LL | fn hide_rc_refcell<'a, 'b: 'a, T: 'static>(x: Rc<RefCell<&'b T>>) -> impl Swap + 'a {
+ | ^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0700`.