Remove cfg-magic from fail* examples

Now, the user has to simply add a line that has been commented
to see the error. A lot simpler.

Author: LukasKalbertodt

examples/error_messages.rs

@@ -0,0 +1,30 @@
+//! This file showcases a few error messages emitted by `auto_impl`. You have
+//! to add specific lines to see the error. Then simply compile with:
+//!
+//! ```
+//! cargo build --example error_messages
+//! ```
+//!
+//! If you want to see nicer error messages, add `--features=nightly`.
+#![allow(unused_imports, dead_code)]
+
+use auto_impl::auto_impl;
+
+
+// Shows the error message for the case that `#[auto_impl]` was used with
+// incorrect proxy types. Only proxy types like `&` and `Box` are allowed. Add
+// this next line to see the error!
+//#[auto_impl(Boxxi)]
+trait Foo {
+    fn foo(&self) -> u32;
+}
+
+// Shows the error message for the case the `#[auto_impl]` wasn't applied to a
+// valid trait (in this case a struct). Add this next line to see the error!
+//#[auto_impl(&, Box)]
+struct Bar {
+    x: u32,
+}
+
+
+fn main() {}