include_bytes! now registers the file included

This is a little bit tricky, since with include_str!, we know that we
are including utf-8 content, so it's safe to store the source as a
String in a FileMap. We don't know that for include_bytes!, but I don't
think we actually need to track the contents anyways, so I'm passing "".

new_filemap does check for the zero length content, and it should be
reasonable, howeven I'm not sure if it would be better to pass None
instead of Some(Rc::new("")) as the src component of a FileMap.

Fixes bug #24348

src/libsyntax/ext/source_util.rs

@@ -184,6 +184,11 @@ pub fn expand_include_bytes(cx: &mut ExtCtxt, sp: Span, tts: &[ast::TokenTree])
             return DummyResult::expr(sp);
         }
         Ok(..) => {
+            // Add this input file to the code map to make it available as
+            // dependency information, but don't enter it's contents
+            let filename = format!("{}", file.display());
+            cx.codemap().new_filemap(filename, "".to_string());
+
             base::MacEager::expr(cx.expr_lit(sp, ast::LitBinary(Rc::new(bytes))))
         }
     }