Add some unsoundness tests for opaques capturing hidden regions not in substs

tests/ui/impl-trait/alias-liveness/rpit-hidden-erased-unsoundness.rs

@@ -0,0 +1,25 @@
+// This test should never pass!
+
+#![feature(type_alias_impl_trait)]
+
+trait Captures<'a> {}
+impl<T> Captures<'_> for T {}
+
+struct MyTy<'a, 'b>(Option<*mut &'a &'b ()>);
+unsafe impl Send for MyTy<'_, 'static> {}
+
+fn step1<'a, 'b: 'a>() -> impl Sized + Captures<'b> + 'a {
+    MyTy::<'a, 'b>(None)
+}
+
+fn step2<'a, 'b: 'a>() -> impl Sized + 'a {
+    step1::<'a, 'b>()
+    //~^ ERROR hidden type for `impl Sized + 'a` captures lifetime that does not appear in bounds
+}
+
+fn step3<'a, 'b: 'a>() -> impl Send + 'a {
+    step2::<'a, 'b>()
+    // This should not be Send unless `'b: 'static`
+}
+
+fn main() {}

tests/ui/impl-trait/alias-liveness/rpit-hidden-erased-unsoundness.stderr

@@ -0,0 +1,18 @@
+error[E0700]: hidden type for `impl Sized + 'a` captures lifetime that does not appear in bounds
+  --> $DIR/rpit-hidden-erased-unsoundness.rs:16:5
+   |
+LL | fn step2<'a, 'b: 'a>() -> impl Sized + 'a {
+   |              --           --------------- opaque type defined here
+   |              |
+   |              hidden type `impl Captures<'b> + 'a` captures the lifetime `'b` as defined here
+LL |     step1::<'a, 'b>()
+   |     ^^^^^^^^^^^^^^^^^
+   |
+help: to declare that `impl Sized + 'a` captures `'b`, you can add an explicit `'b` lifetime bound
+   |
+LL | fn step2<'a, 'b: 'a>() -> impl Sized + 'a + 'b {
+   |                                           ++++
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0700`.

tests/ui/impl-trait/alias-liveness/rpit-hide-lifetime-for-swap.rs

@@ -0,0 +1,32 @@
+// This test should never pass!
+
+use std::cell::RefCell;
+use std::rc::Rc;
+
+trait Swap: Sized {
+    fn swap(self, other: Self);
+}
+
+impl<T> Swap for Rc<RefCell<T>> {
+    fn swap(self, other: Self) {
+        <RefCell<T>>::swap(&self, &other);
+    }
+}
+
+fn hide<'a, 'b: 'a, T: 'static>(x: Rc<RefCell<&'b T>>) -> impl Swap + 'a {
+    x
+    //~^ ERROR hidden type for `impl Swap + 'a` captures lifetime that does not appear in bounds
+}
+
+fn dangle() -> &'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(long.clone()).swap(hide(short));
+    let res: &'static [i32; 3] = *long.borrow();
+    res
+}
+
+fn main() {
+    println!("{:?}", dangle());
+}

tests/ui/impl-trait/alias-liveness/rpit-hide-lifetime-for-swap.stderr

@@ -0,0 +1,18 @@
+error[E0700]: hidden type for `impl Swap + 'a` captures lifetime that does not appear in bounds
+  --> $DIR/rpit-hide-lifetime-for-swap.rs:17:5
+   |
+LL | fn hide<'a, 'b: 'a, T: 'static>(x: Rc<RefCell<&'b T>>) -> impl Swap + 'a {
+   |             --                                            -------------- opaque type defined here
+   |             |
+   |             hidden type `Rc<RefCell<&'b T>>` captures the lifetime `'b` as defined here
+LL |     x
+   |     ^
+   |
+help: to declare that `impl Swap + 'a` captures `'b`, you can add an explicit `'b` lifetime bound
+   |
+LL | fn hide<'a, 'b: 'a, T: 'static>(x: Rc<RefCell<&'b T>>) -> impl Swap + 'a + 'b {
+   |                                                                          ++++
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0700`.

tests/ui/impl-trait/alias-liveness/tait-hidden-erased-unsoundness.rs

@@ -0,0 +1,28 @@
+// This test should never pass!
+
+#![feature(type_alias_impl_trait)]
+
+trait Captures<'a> {}
+impl<T> Captures<'_> for T {}
+
+struct MyTy<'a, 'b>(Option<*mut &'a &'b ()>);
+unsafe impl Send for MyTy<'_, 'static> {}
+
+fn step1<'a, 'b: 'a>() -> impl Sized + Captures<'b> + 'a {
+    MyTy::<'a, 'b>(None)
+}
+
+mod tait {
+    type Tait<'a> = impl Sized + 'a;
+    pub(super) fn step2<'a, 'b: 'a>() -> Tait<'a> {
+        super::step1::<'a, 'b>()
+        //~^ ERROR hidden type for `Tait<'a>` captures lifetime that does not appear in bounds
+    }
+}
+
+fn step3<'a, 'b: 'a>() -> impl Send + 'a {
+    tait::step2::<'a, 'b>()
+    // This should not be Send unless `'b: 'static`
+}
+
+fn main() {}

tests/ui/impl-trait/alias-liveness/tait-hidden-erased-unsoundness.stderr

@@ -0,0 +1,13 @@
+error[E0700]: hidden type for `Tait<'a>` captures lifetime that does not appear in bounds
+  --> $DIR/tait-hidden-erased-unsoundness.rs:18:9
+   |
+LL |     type Tait<'a> = impl Sized + 'a;
+   |                     --------------- opaque type defined here
+LL |     pub(super) fn step2<'a, 'b: 'a>() -> Tait<'a> {
+   |                             -- hidden type `impl Captures<'b> + 'a` captures the lifetime `'b` as defined here
+LL |         super::step1::<'a, 'b>()
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0700`.