fix the new unsoundness

Author: aliemjay

compiler/rustc_infer/src/infer/region_constraints/mod.rs

@@ -642,7 +642,14 @@ impl<'tcx> RegionConstraintCollector<'_, 'tcx> {
     ) -> ty::Region<'tcx> {
         let mut ut = self.unification_table_mut(); // FIXME(rust-lang/ena#42): unnecessary mut
         let root_vid = ut.find(vid).vid;
-        ut.probe_value(root_vid).0.unwrap_or_else(|| tcx.mk_re_var(root_vid))
+        let resolved = ut.probe_value(root_vid).0.unwrap_or_else(|| tcx.mk_re_var(root_vid));
+
+        // Don't resolve a variable to a region that it cannot name.
+        if self.var_universe(vid).can_name(self.universe(resolved)) {
+            resolved
+        } else {
+            tcx.mk_re_var(vid)
+        }
     }
 
     fn combine_map(&mut self, t: CombineMapType) -> &mut CombineMap<'tcx> {

tests/ui/higher-rank-trait-bounds/hrtb-exists-forall-trait-invariant.rs

@@ -2,9 +2,6 @@
 //
 // In particular, we test this pattern in trait solving, where it is not connected
 // to any part of the source code.
-//
-// check-pass
-// Oops!
 
 use std::cell::Cell;
 
@@ -28,5 +25,5 @@ fn main() {
     //     yielding `fn(&!b u32)`, in a fresh universe U1
     //   - So we get `?a = !b` but the universe U0 assigned to `?a` cannot name `!b`.
 
-    foo::<()>();
+    foo::<()>(); //~ ERROR implementation of `Trait` is not general enough
 }

tests/ui/higher-rank-trait-bounds/hrtb-exists-forall-trait-invariant.stderr

@@ -0,0 +1,11 @@
+error: implementation of `Trait` is not general enough
+  --> $DIR/hrtb-exists-forall-trait-invariant.rs:28:5
+   |
+LL |     foo::<()>();
+   |     ^^^^^^^^^^^ implementation of `Trait` is not general enough
+   |
+   = note: `()` must implement `Trait<for<'b> fn(Cell<&'b u32>)>`
+   = note: ...but it actually implements `Trait<fn(Cell<&'0 u32>)>`, for some specific lifetime `'0`
+
+error: aborting due to previous error
+

tests/ui/higher-rank-trait-bounds/hrtb-just-for-static.stderr

@@ -14,6 +14,12 @@ LL | fn give_some<'a>() {
    |              -- lifetime `'a` defined here
 LL |     want_hrtb::<&'a u32>()
    |     ^^^^^^^^^^^^^^^^^^^^ requires that `'a` must outlive `'static`
+   |
+note: due to current limitations in the borrow checker, this implies a `'static` lifetime
+  --> $DIR/hrtb-just-for-static.rs:9:15
+   |
+LL |     where T : for<'a> Foo<&'a isize>
+   |               ^^^^^^^^^^^^^^^^^^^^^^
 
 error: implementation of `Foo` is not general enough
   --> $DIR/hrtb-just-for-static.rs:30:5