Suppress space after idents with "ModName" style in serialization of exported macros.

Fixes issue #20701

Author: vadimcn

src/libsyntax/print/pprust.rs

@@ -1164,11 +1164,20 @@ impl<'a> State<'a> {
 
     pub fn print_tts(&mut self, tts: &[ast::TokenTree]) -> IoResult<()> {
         try!(self.ibox(0));
+        let mut suppress_space = false;
         for (i, tt) in tts.iter().enumerate() {
-            if i != 0 {
+            if i != 0 && !suppress_space {
                 try!(space(&mut self.s));
             }
             try!(self.print_tt(tt));
+            // There should be no space between the module name and the following `::` in paths,
+            // otherwise imported macros get re-parsed from crate metadata incorrectly (issue #20701)
+            suppress_space = match tt {
+                &ast::TtToken(_, token::Ident(_, token::ModName)) |
+                &ast::TtToken(_, token::MatchNt(_, _, _, token::ModName)) |
+                &ast::TtToken(_, token::SubstNt(_, token::ModName)) => true,
+                _ => false
+            }
         }
         self.end()
     }