Don't suggest cyclic associated type constraint

Author: compiler-errors

compiler/rustc_infer/src/infer/error_reporting/note_and_explain.rs

@@ -374,12 +374,18 @@ impl<T> Trait<T> for X {
     ) {
         let tcx = self.tcx;
 
+        // Don't suggest constraining a projection to something containing itself
+        if self.tcx.erase_regions(values.found).contains(self.tcx.erase_regions(values.expected)) {
+            return;
+        }
+
         let msg = || {
             format!(
                 "consider constraining the associated type `{}` to `{}`",
                 values.expected, values.found
             )
         };
+
         let body_owner = tcx.hir().get_if_local(body_owner_def_id);
         let current_method_ident = body_owner.and_then(|n| n.ident()).map(|i| i.name);
 

tests/ui/associated-types/dont-suggest-cyclic-constraint.rs

@@ -0,0 +1,11 @@
+use std::fmt::Debug;
+
+fn foo<I: Iterator>(mut iter: I, value: &I::Item)
+where
+    I::Item: Eq + Debug,
+{
+    debug_assert_eq!(iter.next(), Some(value));
+    //~^ ERROR mismatched types
+}
+
+fn main() {}

tests/ui/associated-types/dont-suggest-cyclic-constraint.stderr

@@ -0,0 +1,12 @@
+error[E0308]: mismatched types
+  --> $DIR/dont-suggest-cyclic-constraint.rs:7:35
+   |
+LL |     debug_assert_eq!(iter.next(), Some(value));
+   |                                   ^^^^^^^^^^^ expected `Option<<I as Iterator>::Item>`, found `Option<&<I as Iterator>::Item>`
+   |
+   = note: expected enum `Option<<I as Iterator>::Item>`
+              found enum `Option<&<I as Iterator>::Item>`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.