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)
```

1 files changed, 23 insertions, 0 deletions

diff --git a/src/libsyntax/ext/tt/macro_rules.rs b/src/libsyntax/ext/tt/macro_rules.rs
index 7401f256412..a9d0b739d6a 100644
--- a/src/libsyntax/ext/tt/macro_rules.rs
+++ b/src/libsyntax/ext/tt/macro_rules.rs
@@ -70,6 +70,29 @@ impl<'a> ParserAnyMacro<'a> {
             } else if !parser.sess.source_map().span_to_filename(parser.token.span).is_real() {
                 e.span_label(site_span, "in this macro invocation");
             }
+            match kind {
+                AstFragmentKind::Ty => {
+                    e.span_label(
+                        site_span,
+                        "this macro call doesn't expand to a type",
+                    );
+                }
+                AstFragmentKind::Pat if macro_ident.name == sym::vec => {
+                    e.span_label(
+                        site_span,
+                        "use a slice pattern here instead",
+                    );
+                    e.help("for more information, see https://doc.rust-lang.org/edition-guide/\
+                              rust-2018/slice-patterns.html");
+                }
+                AstFragmentKind::Pat => {
+                    e.span_label(
+                        site_span,
+                        "this macro call doesn't expand to a pattern",
+                    );
+                }
+                _ => {}
+            };
             e
         }));