Test reproducing the bug we had

Author: amandasystems

tests/ui/nll/relate_tys/placeholder-outlives-existential.rs

@@ -0,0 +1,32 @@
+// Test that the preprocessing step correctly handles some
+// cases of placeholder leaks.
+//
+//@ compile-flags:-Zno-leak-check
+
+
+struct Co<'a>(&'a ());
+struct Inv<'a>(*mut &'a ());
+struct Contra<'a>(fn(&'a ()));
+
+// `exists<'e> forall<'p> 'p: 'e` -> ERROR
+fn p_outlives_e(
+    x: for<'e> fn(for<'p> fn(fn(fn(Contra<'e>, Co<'p>)))),
+) -> fn(fn(fn(for<'unify> fn(Contra<'unify>, Co<'unify>)))) {
+    x //~ ERROR mismatched types [E0308]
+}
+
+// `exists<'e> forall<'p> 'e: 'p` -> Ok, 'e: 'static
+fn e_outlives_p_static(
+    x: for<'e> fn(Inv<'e>, for<'p> fn(fn(fn(Contra<'p>, Co<'e>)))),
+) -> fn(Inv<'static>, fn(fn(for<'unify> fn(Contra<'unify>, Co<'unify>)))) {
+    x
+}
+
+// `exists<'e> forall<'p> 'e: 'p` -> Ok, 'e: 'static -> ERROR
+fn e_outlives_p_static_err<'not_static>(
+    x: for<'e> fn(Inv<'e>, for<'p> fn(fn(fn(Contra<'p>, Co<'e>)))),
+) -> fn(Inv<'not_static>, fn(fn(for<'unify> fn(Contra<'unify>, Co<'unify>)))) {
+    x //~ ERROR lifetime may not live long enough
+}
+
+fn main() {}

tests/ui/nll/relate_tys/placeholder-outlives-existential.stderr

@@ -0,0 +1,25 @@
+error[E0308]: mismatched types
+  --> $DIR/placeholder-outlives-existential.rs:15:5
+   |
+LL |     x
+   |     ^ one type is more general than the other
+   |
+   = note: expected fn pointer `fn(fn(fn(for<'unify> fn(Contra<'unify>, Co<'unify>))))`
+              found fn pointer `for<'e> fn(for<'e, 'p> fn(for<'e, 'p> fn(for<'e, 'p> fn(Contra<'e>, Co<'p>))))`
+
+error: lifetime may not live long enough
+  --> $DIR/placeholder-outlives-existential.rs:29:5
+   |
+LL | fn e_outlives_p_static_err<'not_static>(
+   |                            ----------- lifetime `'not_static` defined here
+...
+LL |     x
+   |     ^ returning this value requires that `'not_static` must outlive `'static`
+   |
+   = note: requirement occurs because of the type `Inv<'_>`, which makes the generic argument `'_` invariant
+   = note: the struct `Inv<'a>` 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 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.