More explicit diagnostic when using a vec![] in a pattern

```
error: unexpected `(` after qualified path
  --> $DIR/vec-macro-in-pattern.rs:3:14
   |
LL |         Some(vec![x]) => (),
   |              ^^^^^^^
   |              |
   |              unexpected `(` after qualified path
   |              in this macro invocation
   |              use a slice pattern here instead
   |
   = help: for more information, see https://doc.rust-lang.org/edition-guide/rust-2018/slice-patterns.html
   = note: this warning originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
```

src/libsyntax/ext/expand.rs

@@ -686,12 +686,13 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
         );
     }
 
-    fn parse_ast_fragment(&mut self,
-                          toks: TokenStream,
-                          kind: AstFragmentKind,
-                          path: &Path,
-                          span: Span)
-                          -> AstFragment {
+    fn parse_ast_fragment(
+        &mut self,
+        toks: TokenStream,
+        kind: AstFragmentKind,
+        path: &Path,
+        span: Span,
+    ) -> AstFragment {
         let mut parser = self.cx.new_parser_from_tts(&toks.into_trees().collect::<Vec<_>>());
         match parser.parse_ast_fragment(kind, false) {
             Ok(fragment) => {
@@ -700,6 +701,21 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
             }
             Err(mut err) => {
                 err.set_span(span);
+                match kind {
+                    AstFragmentKind::Ty => {
+                        err.span_label(
+                            span,
+                            "this macro call doesn't expand to a type",
+                        );
+                    }
+                    AstFragmentKind::Pat => {
+                        err.span_label(
+                            span,
+                            "this macro call doesn't expand to a pattern",
+                        );
+                    }
+                    _ => {}
+                };
                 err.emit();
                 self.cx.trace_macros_diag();
                 kind.dummy(span)