Merge pull request #221 from jehiggs/master

Clarify behavior of `expand`

src/lib.rs

@@ -662,8 +662,15 @@ impl Config {
     /// This is only relevant for C and C++ files.
     ///
     /// # Panics
+    /// Panics if more than one file is present in the config, or if compiler
+    /// path has an invalid file name.
     ///
-    /// Panics if compiler path has an invalid file name.
+    /// # Example
+    /// ```no_run
+    /// let out = 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();
@@ -671,6 +678,10 @@ impl Config {
             cmd.env(a, b);
         }
         cmd.arg(compiler.family.expand_flag());
+
+        assert!(self.files.len() <= 1,
+                "Expand may only be called for a single file");
+
         for file in self.files.iter() {
             cmd.arg(file);
         }