working code

Cargo.lock

@@ -0,0 +1,6 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+[[package]]
+name = "wasi-tutorial"
+version = "0.1.0"
+

input.txt

@@ -0,0 +1 @@
+Hello

output.txt

@@ -0,0 +1 @@
+Hello

src/main.rs

@@ -1,3 +1,32 @@
-fn main() {
-    println!("Hello, world!");
+use std::env;
+use std::fs;
+use std::io::{Read, Write};
+
+fn process(input_fname: &str, output_fname: &str) -> Result<(), String> {
+    let mut input_file =
+        fs::File::open(input_fname).map_err(|err| format!("error opening input: {}", err))?;
+    let mut contents = Vec::new();
+    input_file
+        .read_to_end(&mut contents)
+        .map_err(|err| format!("read error: {}", err))?;
+
+    let mut output_file = fs::File::create(output_fname)
+        .map_err(|err| format!("error opening output '{}': {}", output_fname, err))?;
+    output_file
+        .write_all(&contents)
+        .map_err(|err| format!("write error: {}", err))
 }
+
+fn main() {
+    let args: Vec<String> = env::args().collect();
+    let program = args[0].clone();
+
+    if args.len() < 3 {
+        eprintln!("{} <input_file> <output_file>", program);
+        return;
+    }
+
+    if let Err(err) = process(&args[1], &args[2]) {
+        eprintln!("{}", err)
+    }
+}