Improve pretty-printing for compound qualified paths.

Author: Patryk27

src/librustc/hir/print.rs

@@ -1523,7 +1523,17 @@ impl<'a> State<'a> {
                                         colons_before_params)
             }
             hir::QPath::TypeRelative(ref qself, ref item_segment) => {
-                self.print_type(qself);
+                // If we've got a compound-qualified-path, let's push an additional pair of angle
+                // brackets, so that we pretty-print `<<A::B>::C>` as `<A::B>::C`, instead of just
+                // `A::B::C` (since the latter could be ambiguous to the user)
+                if let hir::TyKind::Path(hir::QPath::Resolved(None, _)) = &qself.kind {
+                    self.print_type(qself);
+                } else {
+                    self.s.word("<");
+                    self.print_type(qself);
+                    self.s.word(">");
+                }
+
                 self.s.word("::");
                 self.print_ident(item_segment.ident);
                 self.print_generic_args(item_segment.generic_args(),

src/test/ui/qualified/qualified-path-params.rs

@@ -18,7 +18,7 @@ impl S {
 fn main() {
     match 10 {
         <S as Tr>::A::f::<u8> => {}
-    //~^ ERROR expected unit struct, unit variant or constant, found method `<S as Tr>::A::f<u8>`
+    //~^ ERROR expected unit struct, unit variant or constant, found method `<<S as Tr>::A>::f<u8>`
         0 ..= <S as Tr>::A::f::<u8> => {} //~ ERROR only char and numeric types are allowed in range
     }
 }

src/test/ui/qualified/qualified-path-params.stderr

@@ -1,4 +1,4 @@
-error[E0533]: expected unit struct, unit variant or constant, found method `<S as Tr>::A::f<u8>`
+error[E0533]: expected unit struct, unit variant or constant, found method `<<S as Tr>::A>::f<u8>`
   --> $DIR/qualified-path-params.rs:20:9
    |
 LL |         <S as Tr>::A::f::<u8> => {}