Make ext::base::expr_to_string work correctly with include! macro invocations

Author: jseyfried

src/libsyntax/ext/base.rs

@@ -820,6 +820,12 @@ impl<'a> ExtCtxt<'a> {
 /// compilation on error, merely emits a non-fatal error and returns None.
 pub fn expr_to_string(cx: &mut ExtCtxt, expr: P<ast::Expr>, err_msg: &str)
                       -> Option<(InternedString, ast::StrStyle)> {
+    // Update `expr.span`'s expn_id now in case expr is an `include!` macro invocation.
+    let expr = expr.map(|mut expr| {
+        expr.span.expn_id = cx.backtrace;
+        expr
+    });
+
     // we want to be able to handle e.g. concat("foo", "bar")
     let expr = cx.expander().fold_expr(expr);
     match expr.node {