refactor(tvix/eval): use Clap for arg+env parsing

Refactor the environment variable and argument parsing for the tvix repl
to use Clap instead of doing things ad-hoc, and thread through options
obtained from environment variables via explicit arguments rather than
obtaining them from the environment as they're needed. This makes adding
more flags more sustainable, and also makes the binary fully
self-documenting, including supported env vars, via `--help`.

Change-Id: Ib1f6a0cd20056e8c9196760ff755fa5729667760
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6653
Autosubmit: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/eval/src/main.rs

@@ -1,30 +1,35 @@
 use std::{
-    env, fs,
+    fs,
     path::{Path, PathBuf},
-    process,
 };
 
+use clap::Parser;
 use rustyline::{error::ReadlineError, Editor};
 
-fn main() {
-    let mut args = env::args();
-    if args.len() > 2 {
-        println!("Usage: tvix-eval [script]");
-        process::exit(1);
-    }
+#[derive(Parser)]
+struct Args {
+    /// Path to a script to evaluate
+    script: Option<PathBuf>,
 
-    if let Some(file) = args.nth(1) {
-        run_file(&file);
+    #[clap(flatten)]
+    eval_options: tvix_eval::Options,
+}
+
+fn main() {
+    let args = Args::parse();
+
+    if let Some(file) = &args.script {
+        run_file(file, args.eval_options)
     } else {
-        run_prompt();
+        run_prompt(args.eval_options)
     }
 }
 
-fn run_file(file: &str) {
+fn run_file(file: &Path, eval_options: tvix_eval::Options) {
     let contents = fs::read_to_string(file).expect("failed to read the input file");
     let path = Path::new(file).to_owned();
 
-    match tvix_eval::interpret(&contents, Some(path)) {
+    match tvix_eval::interpret(&contents, Some(path), eval_options) {
         Ok(result) => println!("=> {} :: {}", result, result.type_of()),
         Err(err) => eprintln!("{}", err),
     }
@@ -38,7 +43,7 @@ fn state_dir() -> Option<PathBuf> {
     path
 }
 
-fn run_prompt() {
+fn run_prompt(eval_options: tvix_eval::Options) {
     let mut rl = Editor::<()>::new().expect("should be able to launch rustyline");
 
     let history_path = match state_dir() {
@@ -63,7 +68,7 @@ fn run_prompt() {
                 }
 
                 rl.add_history_entry(&line);
-                match tvix_eval::interpret(&line, None) {
+                match tvix_eval::interpret(&line, None, eval_options) {
                     Ok(result) => {
                         println!("=> {} :: {}", result, result.type_of());
                     }