My attempt at a Rust replacement for 0install's _runenv.py

runenv.rs

/* Rust version of this Python code:

import os, sys, json
envname = os.path.basename(sys.argv[0])
args = json.loads(os.environ["0install-runenv-" + envname])
os.execv(args[0], args + sys.argv[1:])
*/


extern mod std;

use std::json;
use std::json::{List, String};
use core::libc::funcs::posix88::unistd;

macro_rules! expect(
	($e:expr, $v:ident, $err:expr) => (
		match $e {
			$v(ref x) => x,
			_ => fail!($err)
		}
	);
)

fn json_list_to_str_vector(jp: &json::Json) -> ~[~str] {
	let jl = expect!(*jp, List, ~"Not a JSON list");
	let mut results = ~[];

	for vec::each(*jl) |x: &json::Json| {
		let s: ~str = copy *expect!(*x, String, ~"Not a JSON string");
		results.push(s);
	}

	results
}

// This should really be in the standard library
fn execv(argv: &mut [~str]) -> ! {
	let mut argv_raw: ~[*i8] = ~[];

	for vec::each_mut(argv) |x: &mut ~str| {
		let l = x.len();
		unsafe {
			str::raw::set_len(x, l);		// Ensure null-terminated
			do str::as_c_str(*x) |x: *i8| {
				argv_raw.push(x);
			}
		}		
	}
	argv_raw.push(ptr::null());
	do vec::as_imm_buf(argv_raw) |buf, _len| {
		unsafe { unistd::execv(*buf, cast::transmute(buf)) };
	};
	fail!(fmt!("execv failed: %?", argv));
}

// argv[0] is the name of the symlink used to invoke us (e.g. "make"). We look for a corresponding
// environment variable to tell us how to invoke make. This variable is a list of arguments for execv.
fn main() {
	let our_args = os::args();
	let prog :path::PosixPath = path::Path(our_args[0]);
	let basename :&str = prog.filename().expect(fmt!("Not a file '%?'", prog));
	let var :~str = ~"0install-runenv-" + basename;
	let json_str = os::getenv(var).expect(fmt!("Environment varibale '%s' not set", var));
	let j: json::Json = json::from_str(json_str).unwrap();

	let mut prog_args: ~[~str] = json_list_to_str_vector(&j);
	prog_args += our_args.tail();

	execv(prog_args);
}