Suggest path separator for single-colon typos

This commit adds guidance for when a user means to type a path, but ends
up typing a single colon, such as `<<Impl as T>:Ty>`.

This change seemed pertinent as the current error message is
particularly misleading, emitting `error: unmatched angle bracket`,
despite the angle bracket being matched later on, leaving the user to
track down the typo'd colon.

src/librustc_parse/parser/path.rs

@@ -71,7 +71,23 @@ impl<'a> Parser<'a> {
             debug!("parse_qpath: (decrement) count={:?}", self.unmatched_angle_bracket_count);
         }
 
+        let lo_colon = self.token.span;
+        if self.eat(&token::Colon) {
+            // <Bar as Baz<T>>:Qux
+            //                ^
+            let span = lo_colon.to(self.prev_span);
+            self.diagnostic()
+                .struct_span_err(span, "found single colon where type path was expected")
+                .span_suggestion(
+                    span,
+                    "use double colon",
+                    "::".to_string(),
+                    Applicability::MachineApplicable,
+                )
+                .emit();
+        } else {
             self.expect(&token::ModSep)?;
+        }
 
         let qself = QSelf { ty, path_span, position: path.segments.len() };
         self.parse_path_segments(&mut path.segments, style)?;

src/test/ui/parser/qualified-path-in-turbofish.fixed

@@ -0,0 +1,19 @@
+// run-rustfix
+trait T {
+    type Ty;
+}
+
+struct Impl;
+
+impl T for Impl {
+    type Ty = u32;
+}
+
+fn template<T>() -> i64 {
+    3
+}
+
+fn main() {
+    template::<<Impl as T>::Ty>();
+    //~^ ERROR found single colon where type path was expected
+}

src/test/ui/parser/qualified-path-in-turbofish.rs

@@ -0,0 +1,19 @@
+// run-rustfix
+trait T {
+    type Ty;
+}
+
+struct Impl;
+
+impl T for Impl {
+    type Ty = u32;
+}
+
+fn template<T>() -> i64 {
+    3
+}
+
+fn main() {
+    template::<<Impl as T>:Ty>();
+    //~^ ERROR found single colon where type path was expected
+}

src/test/ui/parser/qualified-path-in-turbofish.stderr

@@ -0,0 +1,8 @@
+error: found single colon where type path was expected
+  --> $DIR/qualified-path-in-turbofish.rs:17:27
+   |
+LL |     template::<<Impl as T>:Ty>();
+   |                           ^ help: use double colon: `::`
+
+error: aborting due to previous error
+