rust: Add allocator support

Create a config `CONFIG_RUST_ALLOC` that will hook Rust's allocation system
into the `malloc`/`free` allocator provided on Zephyr.  This will allow the
`alloc` crate in rust to be used.

Signed-off-by: David Brown <[email protected]>

Author: d3zd3z

Kconfig

@@ -19,4 +19,16 @@ config RUST
 	help
 	  This option enables the use of applications written in Rust.
 
+if RUST
+
+config RUST_ALLOC
+	bool "Support an allocator in Rust code"
+	help
+	  If enabled, the Rust zephyr support library will include support for
+	  an allocator.  This allocator will use the currently configured
+	  Zephyr allocator (malloc/free).  This this enabled, Rust
+	  applications can use the `alloc` crate.
+
+endif # RUST
+
 endmenu

zephyr/src/alloc.rs

@@ -0,0 +1,41 @@
+//! A Rust global allocator that uses the stdlib allocator in Zephyr
+
+// This entire module is only use if CONFIG_RUST_ALLOC is enabled.
+extern crate alloc;
+
+use core::alloc::{GlobalAlloc, Layout};
+
+use alloc::alloc::handle_alloc_error;
+
+/// Define size_t, as it isn't defined within the FFI.
+#[allow(non_camel_case_types)]
+type c_size_t = usize;
+
+extern "C" {
+    fn malloc(size: c_size_t) -> *mut u8;
+    fn free(ptr: *mut u8);
+}
+
+pub struct ZephyrAllocator;
+
+unsafe impl GlobalAlloc for ZephyrAllocator {
+    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
+        let size = layout.size();
+        let align = layout.align();
+
+        // The C allocation library assumes an alignment of 8.  For now, just panic if this cannot
+        // be satistifed.
+        if align > 8 {
+            handle_alloc_error(layout);
+        }
+
+        malloc(size)
+    }
+
+    unsafe fn dealloc(&self, ptr: *mut u8, _layout: Layout) {
+        free(ptr)
+    }
+}
+
+#[global_allocator]
+static ZEPHYR_ALLOCATOR: ZephyrAllocator = ZephyrAllocator;

zephyr/src/lib.rs

@@ -129,3 +129,7 @@ pub mod raw {
 pub mod _export {
     pub use core::format_args;
 }
+
+/// If allocation has been requested, provide the allocator.
+#[cfg(CONFIG_RUST_ALLOC)]
+mod alloc;