add a "hello-rust" example

examples/hello-rust/platform/src/lib.rs

@@ -0,0 +1,58 @@
+#![allow(non_snake_case)]
+
+use core::ffi::c_void;
+use core::mem::MaybeUninit;
+use roc_std::{RocCallResult, RocStr};
+
+extern "C" {
+    #[link_name = "roc__mainForHost_1_exposed"]
+    fn roc_main(output: *mut RocCallResult<RocStr>) -> ();
+}
+
+#[no_mangle]
+pub unsafe fn roc_alloc(size: usize, _alignment: u32) -> *mut c_void {
+    return libc::malloc(size);
+}
+
+#[no_mangle]
+pub unsafe fn roc_realloc(
+    c_ptr: *mut c_void,
+    new_size: usize,
+    _old_size: usize,
+    _alignment: u32,
+) -> *mut c_void {
+    return libc::realloc(c_ptr, new_size);
+}
+
+#[no_mangle]
+pub unsafe fn roc_dealloc(c_ptr: *mut c_void, _alignment: u32) {
+    return libc::free(c_ptr);
+}
+
+#[no_mangle]
+pub fn rust_main() -> isize {
+    let mut call_result: MaybeUninit<RocCallResult<RocStr>> = MaybeUninit::uninit();
+
+    unsafe {
+        roc_main(call_result.as_mut_ptr());
+
+        let output = call_result.assume_init();
+
+        match output.into() {
+            Ok(roc_str) => {
+                let len = roc_str.len();
+                let str_bytes = roc_str.get_bytes() as *const libc::c_void;
+
+                if libc::write(1, str_bytes, len) < 0 {
+                    panic!("Writing to stdout failed!");
+                }
+            }
+            Err(msg) => {
+                panic!("Roc failed with message: {}", msg);
+            }
+        }
+    }
+
+    // Exit code
+    0
+}