Add serde_codegen::expand to avoid public Syntex dependency

serde_codegen/src/lib.rs

@@ -22,6 +22,11 @@ extern crate syntax;
 #[cfg(not(feature = "with-syntex"))]
 extern crate rustc_plugin;
 
+#[cfg(feature = "with-syntex")]
+use std::io;
+#[cfg(feature = "with-syntex")]
+use std::path::Path;
+
 #[cfg(not(feature = "with-syntex"))]
 use syntax::feature_gate::AttributeType;
 
@@ -31,6 +36,13 @@ include!(concat!(env!("OUT_DIR"), "/lib.rs"));
 #[cfg(not(feature = "with-syntex"))]
 include!("lib.rs.in");
 
+#[cfg(feature = "with-syntex")]
+pub fn expand(src: &Path, dst: &Path) -> io::Result<()> {
+    let mut registry = syntex::Registry::new();
+    register(&mut registry);
+    registry.expand("", src, dst)
+}
+
 #[cfg(feature = "with-syntex")]
 pub fn register(reg: &mut syntex::Registry) {
     use syntax::{ast, fold};