Do less panicking in general

src/libsyntax/parse/mod.rs

@@ -120,6 +120,10 @@ pub fn parse_expr_from_source_str<'a>(name: String,
     p.parse_expr()
 }
 
+/// Parses an item.
+///
+/// Returns `Ok(Some(item))` when successful, `Ok(None)` when no item was found, and`Err`
+/// when a syntax error occurred.
 pub fn parse_item_from_source_str<'a>(name: String,
                                       source: String,
                                       cfg: ast::CrateConfig,