Account for multiple impl/dyn Trait in return type when suggesting '_

src/test/ui/suggestions/lifetimes/trait-object-nested-in-impl-trait.rs

@@ -0,0 +1,68 @@
+trait Foo {}
+impl<'a, T: Foo> Foo for &'a T {}
+impl<T: Foo + ?Sized> Foo for Box<T> {}
+
+struct Iter<'a, T> {
+    current: Option<Box<dyn Foo + 'a>>,
+    remaining: T,
+}
+
+impl<'a, T> Iterator for Iter<'a, T>
+where
+    T: Iterator,
+    T::Item: Foo + 'a,
+{
+    type Item = Box<dyn Foo + 'a>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let result = self.current.take();
+        self.current = Box::new(self.remaining.next()).map(|f| Box::new(f) as _);
+        result
+    }
+}
+
+struct Bar(Vec<Box<dyn Foo>>);
+
+impl Bar {
+    fn iter(&self) -> impl Iterator<Item = Box<dyn Foo>> {
+        Iter {
+            current: None,
+            remaining: self.0.iter(), //~ ERROR cannot infer an appropriate lifetime
+        }
+    }
+}
+
+struct Baz(Vec<Box<dyn Foo>>);
+
+impl Baz {
+    fn iter(&self) -> impl Iterator<Item = Box<dyn Foo>> + '_ {
+        Iter {
+            current: None,
+            remaining: self.0.iter(), //~ ERROR cannot infer an appropriate lifetime
+        }
+    }
+}
+
+struct Bat(Vec<Box<dyn Foo>>);
+
+impl Bat {
+    fn iter<'a>(&'a self) -> impl Iterator<Item = Box<dyn Foo>> + 'a {
+        Iter {
+            current: None,
+            remaining: self.0.iter(), //~ ERROR cannot infer an appropriate lifetime
+        }
+    }
+}
+
+struct Ban(Vec<Box<dyn Foo>>);
+
+impl Ban {
+    fn iter<'a>(&'a self) -> impl Iterator<Item = Box<dyn Foo>> {
+        Iter {
+            current: None,
+            remaining: self.0.iter(), //~ ERROR cannot infer an appropriate lifetime
+        }
+    }
+}
+
+fn main() {}