2 files changed, 74 insertions, 1 deletions

diff --git a/book/en/src/by-example/tips.md b/book/en/src/by-example/tips.md
index 79b9d71e..4936bd27 100644
--- a/book/en/src/by-example/tips.md
+++ b/book/en/src/by-example/tips.md
@@ -113,3 +113,67 @@ Here's an example where `heapless::Pool` is used to "box" buffers of 128 bytes.
 ``` console
 $ cargo run --example binds
 {{#include ../../../../ci/expected/pool.run}}```
+
+## Inspecting the expanded code
+
+`#[rtfm::app]` is a procedural macro that produces support code. If for some
+reason you need to inspect the code generated by this macro you have two
+options:
+
+You can inspect the file `rtfm-expansion.rs` inside the `target` directory. This
+file contains the expansion of the `#[rtfm::app]` item (not your whole program!)
+of the *last built* (via `cargo build` or `cargo check`) RTFM application. The
+expanded code is not pretty printed by default so you'll want to run `rustfmt`
+over it before you read it.
+
+``` console
+$ cargo build --example foo
+
+$ rustfmt target/rtfm-expansion.rs
+
+$ tail -n30 target/rtfm-expansion.rs
+```
+
+``` rust
+#[doc = r" Implementation details"]
+const APP: () = {
+    use lm3s6965 as _;
+    #[no_mangle]
+    unsafe fn main() -> ! {
+        rtfm::export::interrupt::disable();
+        let mut core = rtfm::export::Peripherals::steal();
+        let late = init(
+            init::Locals::new(),
+            init::Context::new(rtfm::Peripherals {
+                CBP: core.CBP,
+                CPUID: core.CPUID,
+                DCB: core.DCB,
+                DWT: core.DWT,
+                FPB: core.FPB,
+                FPU: core.FPU,
+                ITM: core.ITM,
+                MPU: core.MPU,
+                SCB: &mut core.SCB,
+                SYST: core.SYST,
+                TPIU: core.TPIU,
+            }),
+        );
+        core.SCB.scr.modify(|r| r | 1 << 1);
+        rtfm::export::interrupt::enable();
+        loop {
+            rtfm::export::wfi()
+        }
+    }
+};
+```
+
+Or, you can use the [`cargo-expand`] subcommand. This subcommand will expand
+*all* the macros, including the `#[rtfm::app]` attribute, and modules in your
+crate and print the output to the console.
+
+[`cargo-expand`]: https://crates.io/crates/cargo-expand
+
+``` console
+$ # produces the same output as before
+$ cargo expand --example smallest | tail -n30
+```
diff --git a/macros/src/lib.rs b/macros/src/lib.rs
index 441d6b5e..736289cb 100644
--- a/macros/src/lib.rs
+++ b/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()
 }