write generated code to disk for easier inspection

now that the generated code is actually readable let's make it easier to access

this commit also documents how to inspect the generated code via
`rtfm-expansion.rs` and `cargo-expand`

macros/src/lib.rs

@@ -4,6 +4,8 @@
 extern crate proc_macro;
 
 use proc_macro::TokenStream;
+use std::{fs, path::Path};
+
 use syn::parse_macro_input;
 
 mod analyze;
@@ -304,5 +306,12 @@ pub fn app(args: TokenStream, input: TokenStream) -> TokenStream {
     let analysis = analyze::app(&app);
 
     // Code generation
-    codegen::app(&input.ident, &app, &analysis).into()
+    let ts = codegen::app(&input.ident, &app, &analysis);
+
+    // Try to write the expanded code to disk
+    if Path::new("target").exists() {
+        fs::write("target/rtfm-expansion.rs", ts.to_string()).ok();
+    }
+
+    ts.into()
 }