Add regression test for #144033

tests/ui/higher-ranked/relate-bound-region-ice-144033.rs

@@ -0,0 +1,23 @@
+// Regression test for <https://github.com/rust-lang/rust/issues/144033>.
+// This used to ICE with "cannot relate bound region" instead of emitting
+// normal errors.
+
+trait FooMut {
+    fn bar<I>(self, _: I)
+    where
+        for<'b> &'b I: Iterator<Item = &'b ()>;
+}
+
+impl FooMut for () {
+    fn bar<I>(self, _: I)
+    where
+        for<'b> &'b I: Iterator,
+    {
+        let collection = std::iter::empty::<()>().map(|_| &());
+        self.bar(collection)
+        //~^ ERROR expected `&I` to be an iterator that yields `&()`
+        //~| ERROR mismatched types
+    }
+}
+
+fn main() {}

tests/ui/higher-ranked/relate-bound-region-ice-144033.stderr

@@ -0,0 +1,53 @@
+error[E0271]: expected `&I` to be an iterator that yields `&()`, but it yields `<&I as Iterator>::Item`
+  --> $DIR/relate-bound-region-ice-144033.rs:17:18
+   |
+LL |         self.bar(collection)
+   |              --- ^^^^^^^^^^ expected `&()`, found associated type
+   |              |
+   |              required by a bound introduced by this call
+   |
+   = note:    expected reference `&()`
+           found associated type `<&I as Iterator>::Item`
+   = help: consider constraining the associated type `<&I as Iterator>::Item` to `&()`
+   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
+note: the method call chain might not have had the expected associated types
+  --> $DIR/relate-bound-region-ice-144033.rs:16:51
+   |
+LL |         let collection = std::iter::empty::<()>().map(|_| &());
+   |                          ------------------------ ^^^^^^^^^^^^ `Iterator::Item` is `&()` here
+   |                          |
+   |                          this expression has type `Empty<()>`
+note: required by a bound in `FooMut::bar`
+  --> $DIR/relate-bound-region-ice-144033.rs:8:33
+   |
+LL |     fn bar<I>(self, _: I)
+   |        --- required by a bound in this associated function
+LL |     where
+LL |         for<'b> &'b I: Iterator<Item = &'b ()>;
+   |                                 ^^^^^^^^^^^^^ required by this bound in `FooMut::bar`
+
+error[E0308]: mismatched types
+  --> $DIR/relate-bound-region-ice-144033.rs:17:18
+   |
+LL |     fn bar<I>(self, _: I)
+   |            - expected this type parameter
+...
+LL |         let collection = std::iter::empty::<()>().map(|_| &());
+   |                                                       --- the found closure
+LL |         self.bar(collection)
+   |              --- ^^^^^^^^^^ expected type parameter `I`, found `Map<Empty<()>, {closure@...}>`
+   |              |
+   |              arguments to this method are incorrect
+   |
+   = note: expected type parameter `I`
+                      found struct `Map<std::iter::Empty<()>, {closure@$DIR/relate-bound-region-ice-144033.rs:16:55: 16:58}>`
+note: method defined here
+  --> $DIR/relate-bound-region-ice-144033.rs:6:8
+   |
+LL |     fn bar<I>(self, _: I)
+   |        ^^^          -
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0271, E0308.
+For more information about an error, try `rustc --explain E0271`.