Add host.rs in hello-world example

examples/hello-world/platform/host/src/host.rs

@@ -0,0 +1,23 @@
+#![crate_type = "staticlib"]
+
+use std::ffi::CStr;
+use std::os::raw::c_char;
+
+extern "C" {
+    #[link_name = "main_1"]
+    fn str_from_roc() -> *const c_char;
+}
+
+// This magical #[export_name] annotation seems to be the only way to convince
+// LLVM to emit this as the symbol "start" rather than "_start" on macOS.
+// The linker will not recognize "_start" as the program entrypoint.
+//
+// See https://github.com/rust-lang/rust/issues/35052#issuecomment-235420755
+#[export_name = "\x01start"]
+pub extern "C" fn main(_argc: isize, _argv: *const *const u8) -> isize {
+    let c_str = unsafe { CStr::from_ptr(str_from_roc()) };
+
+    println!("Roc says: \"{}\"", c_str.to_str().unwrap());
+
+    0
+}