Fix handling of parse errors when using include!().

Makes the compilation abort when a parse error is encountered while
trying to parse an item in an included file. The previous behaviour was
to stop processing the file when a token that can't start an item was
encountered, without producing any error. Fixes #21146.

src/libsyntax/ext/source_util.rs

@@ -111,10 +111,14 @@ pub fn expand_include<'cx>(cx: &'cx mut ExtCtxt, sp: Span, tts: &[ast::TokenTree
         fn make_items(mut self: Box<ExpandResult<'a>>)
                       -> Option<SmallVector<P<ast::Item>>> {
             let mut ret = SmallVector::zero();
-            loop {
+            while self.p.token != token::Eof {
                 match self.p.parse_item_with_outer_attributes() {
                     Some(item) => ret.push(item),
-                    None => break
+                    None => self.p.span_fatal(
+                        self.p.span,
+                        &format!("expected item, found `{}`",
+                                 self.p.this_token_to_string())[]
+                    )
                 }
             }
             Some(ret)