fix pretty print for qpath

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -2328,11 +2328,12 @@ impl<'a> State<'a> {
             self.print_path(path, false, depth);
         }
         self.s.word(">");
-        self.s.word("::");
-        let item_segment = path.segments.last().unwrap();
-        self.print_ident(item_segment.ident);
-        if let Some(ref args) = item_segment.args {
-            self.print_generic_args(args, colons_before_params)
+        for item_segment in &path.segments[qself.position..] {
+            self.s.word("::");
+            self.print_ident(item_segment.ident);
+            if let Some(ref args) = item_segment.args {
+                self.print_generic_args(args, colons_before_params)
+            }
         }
     }
 

src/test/pretty/qpath-associated-type-bound.rs

@@ -0,0 +1,16 @@
+// pp-exact
+
+
+mod m {
+    pub trait Tr {
+        type Ts: super::Tu;
+    }
+}
+
+trait Tu {
+    fn dummy() { }
+}
+
+fn foo<T: m::Tr>() { <T as m::Tr>::Ts::dummy(); }
+
+fn main() { }