Assert that there's only one file in config when using expand

src/lib.rs

@@ -609,6 +609,17 @@ impl Config {
     /// Run the compiler, returning the macro-expanded version of the input files.
     ///
     /// This is only relevant for C and C++ files.
+    ///
+    /// # Panics
+    /// Panics if more than one file is present in the config, as macro
+    /// expansion only makes sense for a single file.
+    ///
+    /// # Example
+    /// ```no_run
+    /// gcc::Config::new()
+    ///             .file("src/foo.c")
+    ///             .expand()
+    /// ```
     pub fn expand(&self) -> Vec<u8> {
         let compiler = self.get_compiler();
         let mut cmd = compiler.to_command();
@@ -616,6 +627,10 @@ impl Config {
             cmd.env(a, b);
         }
         cmd.arg(compiler.family.expand_flag());
+
+        assert_eq!(self.files.len(), 1,
+                  "Expand may only be called for a single file");
+
         for file in self.files.iter() {
             cmd.arg(file);
         }