Improve wording and docs for qualified path recovery

Author: bobrippling

src/librustc_parse/parser/path.rs

@@ -81,17 +81,24 @@ impl<'a> Parser<'a> {
         Ok((qself, Path { segments: path.segments, span: lo.to(self.prev_span) }))
     }
 
+    /// Recover from an invalid single colon, when the user likely meant a qualified path.
+    ///
+    /// ```ignore (diagnostics)
+    /// <Bar as Baz<T>>:Qux
+    ///                ^ help: use double colon
+    /// ```
     fn recover_colon_before_qpath_proj(&mut self) -> bool {
         if self.token.kind != token::Colon {
             return false;
         }
 
-        // <Bar as Baz<T>>:Qux
-        //                ^
-        self.bump();
+        self.bump(); // colon
 
         self.diagnostic()
-            .struct_span_err(self.prev_span, "found single colon where type path was expected")
+            .struct_span_err(
+                self.prev_span,
+                "found single colon before projection in qualified path",
+            )
             .span_suggestion(
                 self.prev_span,
                 "use double colon",

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

@@ -15,5 +15,5 @@ fn template<T>() -> i64 {
 
 fn main() {
     template::<<Impl as T>::Ty>();
-    //~^ ERROR found single colon where type path was expected
+    //~^ ERROR found single colon before projection in qualified path
 }

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

@@ -15,5 +15,5 @@ fn template<T>() -> i64 {
 
 fn main() {
     template::<<Impl as T>:Ty>();
-    //~^ ERROR found single colon where type path was expected
+    //~^ ERROR found single colon before projection in qualified path
 }

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

@@ -1,4 +1,4 @@
-error: found single colon where type path was expected
+error: found single colon before projection in qualified path
   --> $DIR/qualified-path-in-turbofish.rs:17:27
    |
 LL |     template::<<Impl as T>:Ty>();