add closure example that is passed to the host

examples/closure/Closure.roc

@@ -0,0 +1,8 @@
+app Closure provides [ closure ] imports []
+
+closure : {} -> Int
+closure = 
+    x = 42
+
+    \{} -> x
+

examples/closure/platform/Cargo.lock

@@ -0,0 +1,23 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+[[package]]
+name = "host"
+version = "0.1.0"
+dependencies = [
+ "roc_std 0.1.0",
+]
+
+[[package]]
+name = "libc"
+version = "0.2.79"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
+name = "roc_std"
+version = "0.1.0"
+dependencies = [
+ "libc 0.2.79 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[metadata]
+"checksum libc 0.2.79 (registry+https://github.com/rust-lang/crates.io-index)" = "2448f6066e80e3bfc792e9c98bf705b4b0fc6e8ef5b43e5889aff0eaa9c58743"

examples/closure/platform/Cargo.toml

@@ -0,0 +1,13 @@
+[package]
+name = "host"
+version = "0.1.0"
+authors = ["Richard Feldman <oss@rtfeldman.com>"]
+edition = "2018"
+
+[lib]
+crate-type = ["staticlib"]
+
+[dependencies]
+roc_std = { path = "../../../roc_std" }
+
+[workspace]

examples/closure/platform/host.c

@@ -0,0 +1,7 @@
+#include <stdio.h>
+
+extern int rust_main();
+
+int main() {
+  return rust_main();
+}

examples/closure/platform/src/lib.rs

@@ -0,0 +1,38 @@
+use std::mem::MaybeUninit;
+use std::time::SystemTime;
+
+extern "C" {
+    #[link_name = "closure_1_exposed"]
+    fn closure(output: *mut u8) -> ();
+
+    #[link_name = "closure_1_size"]
+    fn closure_size() -> i64;
+}
+
+#[no_mangle]
+pub fn rust_main() -> isize {
+    println!("Running Roc closure");
+    let start_time = SystemTime::now();
+    let (function_pointer, closure_data) = unsafe {
+        let mut output: MaybeUninit<(fn(i64) -> i64, i64)> = MaybeUninit::uninit();
+
+        closure(output.as_mut_ptr() as _);
+
+        output.assume_init()
+    };
+    let answer = function_pointer(closure_data);
+    let end_time = SystemTime::now();
+    let duration = end_time.duration_since(start_time).unwrap();
+
+    println!(
+        "Roc closure took {:.4} ms to compute this answer: {:?}",
+        duration.as_secs_f64() * 1000.0,
+        // truncate the answer, so stdout is not swamped
+        answer
+    );
+
+    println!("closure size {:?}", unsafe { closure_size() });
+
+    // Exit code
+    0
+}