add subset relations test using polonius

It's a relatively simple smoke-test for subset errors, executed outside
of the polonius compare-mode.

src/test/ui/nll/polonius/subset-relations.rs

@@ -0,0 +1,30 @@
+// Checks that Polonius can compute cases of universal regions errors:
+// "illegal subset relation errors", cases where analysis finds that
+// two free regions outlive each other, without any evidence that this
+// relation holds.
+
+// ignore-compare-mode-nll
+// compile-flags: -Z borrowck=mir -Zpolonius
+
+// returning `y` requires that `'b: 'a`, but it's not known to be true
+fn missing_subset<'a, 'b>(x: &'a u32, y: &'b u32) -> &'a u32 {
+    y //~ ERROR
+}
+
+// `'b: 'a` is explicitly declared
+fn valid_subset<'a, 'b: 'a>(x: &'a u32, y: &'b u32) -> &'a u32 {
+    y
+}
+
+// because of `x`, it is implied that `'b: 'a` holds
+fn implied_bounds_subset<'a, 'b>(x: &'a &'b mut u32) -> &'a u32 {
+    x
+}
+
+// `'b: 'a` is declared, and `'a: 'c` is known via implied bounds:
+// `'b: 'c` is therefore known to hold transitively
+fn transitively_valid_subset<'a, 'b: 'a, 'c>(x: &'c &'a u32, y: &'b u32) -> &'c u32  {
+    y
+}
+
+fn main() {}

src/test/ui/nll/polonius/subset-relations.stderr

@@ -0,0 +1,14 @@
+error: lifetime may not live long enough
+  --> $DIR/subset-relations.rs:11:5
+   |
+LL | fn missing_subset<'a, 'b>(x: &'a u32, y: &'b u32) -> &'a u32 {
+   |                   --  -- lifetime `'b` defined here
+   |                   |
+   |                   lifetime `'a` defined here
+LL |     y
+   |     ^ function was supposed to return data with lifetime `'a` but it is returning data with lifetime `'b`
+   |
+   = help: consider adding the following bound: `'b: 'a`
+
+error: aborting due to previous error
+