document how to use the heap and a dynamic allocator

examples/allocator.rs

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+//! How to use the heap and a dynamic memory allocator
+//!
+//! To compile this example you'll need to build the collections crate as part
+//! of the Xargo sysroot. To do that change the Xargo.toml file to look like
+//! this:
+//!
+//! ``` text
+//! [dependencies.core]
+//! [dependencies.collections] # new
+//!
+//! [dependencies.compiler_builtins]
+//! features = ["mem"]
+//! git = "https://github.com/rust-lang-nursery/compiler-builtins"
+//! stage = 1
+//! ```
+//!
+//! This example depends on the alloc-cortex-m crate so you'll have to add it
+//! to your Cargo.toml:
+//!
+//! ``` text
+//! # or edit the Cargo.toml file manually
+//! $ cargo add alloc-cortex-m
+//! ```
+
+#![feature(collections)]
+#![feature(used)]
+#![no_std]
+
+// This is the allocator crate; you can use a different one
+extern crate alloc_cortex_m;
+#[macro_use]
+extern crate collections;
+#[macro_use]
+extern crate cortex_m;
+extern crate cortex_m_rt;
+
+use cortex_m::asm;
+
+fn main() {
+    // Initialize the allocator
+    unsafe {
+        extern "C" {
+            // Start of the heap
+            static mut _sheap: usize;
+        }
+
+        // Size of the heap in words (1 word = 4 bytes)
+        // WARNING: The bigger the heap the greater the chance to run into a
+        // stack overflow (collision between the stack and the heap)
+        const SIZE: isize = 256;
+
+        // End of the heap
+        let _eheap = (&mut _sheap as *mut _).offset(SIZE);
+
+        alloc_cortex_m::init(&mut _sheap as *mut _, _eheap);
+    }
+
+    // Growable array allocated on the heap
+    let xs = vec![0, 1, 2];
+    hprintln!("{:?}", xs);
+}
+
+// As we are not using interrupts, we just register a dummy catch all handler
+#[allow(dead_code)]
+#[used]
+#[link_section = ".rodata.interrupts"]
+static INTERRUPTS: [extern "C" fn(); 240] = [default_handler; 240];
+
+extern "C" fn default_handler() {
+    asm::bkpt();
+}