book: add an example of conditional compilation of resources and tasks

book/src/by-example/tips.md

@@ -20,6 +20,20 @@ rewrite code. If you consistently use `lock`s to access the data behind shared
 resources then your code will continue to compile when you change the priority
 of tasks.
 
+## Conditional compilation
+
+You can use conditional compilation (`#[cfg]`) on resources (`static [mut]`
+items) and tasks (`fn` items). The effect of using `#[cfg]` attributes is that
+the resource / task will *not* be injected into the prelude of tasks that use
+them (see `resources`, `spawn` and `schedule`) if the condition doesn't hold.
+
+The example below logs a message whenever the `foo` task is spawned, but only if
+the program has been compiled using the `dev` profile.
+
+``` rust
+{{#include ../../../examples/cfg.rs}}
+```
+
 ## Running tasks from RAM
 
 The main goal of moving the specification of RTFM applications to attributes in

ci/script.sh

@@ -68,6 +68,8 @@ main() {
 
                 generics
                 ramfunc
+
+                cfg
             )
 
             for ex in ${exs[@]}; do

examples/cfg.rs

@@ -0,0 +1,54 @@
+//! examples/cfg.rs
+
+#![deny(unsafe_code)]
+#![deny(warnings)]
+#![no_main]
+#![no_std]
+
+extern crate panic_semihosting;
+
+#[cfg(debug_assertions)]
+use cortex_m_semihosting::hprintln;
+use rtfm::app;
+
+#[app(device = lm3s6965)]
+const APP: () = {
+    #[cfg(debug_assertions)] // <- `true` when using the `dev` profile
+    static mut COUNT: u32 = 0;
+
+    #[init]
+    fn init() {
+        // ..
+    }
+
+    #[task(priority = 3, resources = [COUNT], spawn = [log])]
+    fn foo() {
+        #[cfg(debug_assertions)]
+        {
+            *resources.COUNT += 1;
+
+            spawn.log(*resources.COUNT).ok();
+        }
+
+        // this wouldn't compile in `release` mode
+        // *resources.COUNT += 1;
+
+        // ..
+    }
+
+    #[cfg(debug_assertions)]
+    #[task]
+    fn log(n: u32) {
+        hprintln!(
+            "foo has been called {} time{}",
+            n,
+            if n == 1 { "" } else { "s" }
+        )
+        .ok();
+    }
+
+    extern "C" {
+        fn UART0();
+        fn UART1();
+    }
+};