Rollup merge of #22285 - kmcallister:pub-macro, r=nick29581

It's not clear what this means, because a macro in item position can expand to zero or more items.  For now we disallow it, which is technically a

    [breaking-change]

but is landing without an RFC.  The `pub` keyword previously had no effect, which seems quite unintended.

Fixes #18317.
Fixes #14660.

src/libsyntax/parse/parser.rs

@@ -4650,6 +4650,17 @@ impl<'a> Parser<'a> {
         self.parse_method(attrs, visa)
     }
 
+    fn complain_if_pub_macro(&mut self, visa: Visibility, span: Span) {
+        match visa {
+            Public => {
+                self.span_err(span, "can't qualify macro invocation with `pub`");
+                self.span_help(span, "try adjusting the macro to put `pub` inside \
+                                      the invocation");
+            }
+            Inherited => (),
+        }
+    }
+
     /// Parse a method in a trait impl, starting with `attrs` attributes.
     pub fn parse_method(&mut self,
                         attrs: Vec<Attribute>,
@@ -4664,6 +4675,10 @@ impl<'a> Parser<'a> {
                 && (self.look_ahead(2, |t| *t == token::OpenDelim(token::Paren))
                     || self.look_ahead(2, |t| *t == token::OpenDelim(token::Brace))) {
                 // method macro.
+
+                let last_span = self.last_span;
+                self.complain_if_pub_macro(visa, last_span);
+
                 let pth = self.parse_path(NoTypesAllowed);
                 self.expect(&token::Not);
 
@@ -5867,6 +5882,9 @@ impl<'a> Parser<'a> {
                     || self.look_ahead(2, |t| *t == token::OpenDelim(token::Brace))) {
             // MACRO INVOCATION ITEM
 
+            let last_span = self.last_span;
+            self.complain_if_pub_macro(visibility, last_span);
+
             // item macro.
             let pth = self.parse_path(NoTypesAllowed);
             self.expect(&token::Not);