Add parsing of command-line arguments

Currently only supports `--help` argument and settings input file
implicitly from the positional arguments.

src/main.rs

@@ -1,3 +1,68 @@
+// Try to keep this string updated with the argument parsing, otherwise it will
+// get confusing for users.
+static USAGE: &'static str = "usage: pine [options] input
+
+options:
+  --help    print all options";
+
 fn main() {
-	println!("Hello, world!");
+	// Throw away the first argument, which usually is the executable name.
+	let args = std::env::args().skip(1).collect::<Vec<_>>();
+
+	// If there is no arguments, we short circuit to avoid having to perform the
+	// expensive command-line argument generation and parsing step. We can allow
+	// ourselves to do this since, unlike i.e. rustc, we don't print full usage
+	// information on invocation of only the binary, but instead we behave more
+	// like clang or go.
+	if args.len() == 0 {
+		eprintln!("pine: \x1b[1;31merror\x1b[0m: no input files");
+		std::process::exit(1);
+	}
+
+	let mut path: Option<&str> = None;
+	// Handle command-line arguments.
+	let mut i = 0;
+	loop {
+		if i == args.len() {
+			break;
+		}
+
+		let arg = args[i].as_str();
+
+		if arg.starts_with("--") {
+			match &arg[2..] {
+				"help" => {
+					println!("{}\n", USAGE);
+					return;
+				},
+				_ => {
+					eprintln!("pine: \x1b[1;31merror\x1b[0m: unknown argument '{}'", arg);
+					std::process::exit(1);
+				},
+			};
+		} else if arg.starts_with("-") {
+			// We don't handle arguments that start with a single dash, this might be
+			// added later. For now we just exit with an error.
+			eprintln!("pine: \x1b[1;31merror\x1b[0m: unknown argument '{}'", arg);
+			std::process::exit(1);
+		} else {
+			if path.is_some() {
+				eprintln!("pine: \x1b[1;31merror\x1b[0m: multiple file names provided (first two are `{}` and `{}`)", path.unwrap(), arg);
+				std::process::exit(1);
+			}
+
+			// Use this argument as an input file.
+			path = Some(arg);
+		}
+
+		i += 1;
+	}
+
+	if path.is_none() {
+		eprintln!("pine: \x1b[1;31merror\x1b[0m: no input files");
+		std::process::exit(1);
+	}
+
+	let path = path.unwrap();
+	eprintln!("compiling `{}`", path);
 }