Rewrite flags.rs::parse_flags

[bartlomieju]

With recent numerous changes to CLI and more to come (#2213, #2215) I found myself in situation when it's harder and harder to properly test CLI parsing logic and not break anything.

That's why I'd like to propose yet another rewrite to:

deno/cli/flags.rs

Line 134 in 40d8ef1

pub fn parse_flags(matches: ArgMatches) -> DenoFlags {

Currently it only returns DenoFlags and choosing the actual subcommand and construction of argument array that is passed to script in done in main.rs. That's not elegant, and logic is split into multiple files and makes it impossible to properly test all entry points.

As previously suggested in #2157 it should be flags_from_vec that is only public function in flags.rs that takes care of parsing of CLI flags and choosing appropriate action.

So my proposition is to change what's returned from this function to:

enum DenoSubcommand {
   Fmt = "fmt",
   Info = "info",
   Types = "types",
   Fetch = "fetch",
   Repl = "repl",
   Run = "run",
}

// last parameter is `rest_argv` which is passed down 
// to script and available as Deno.args
fn flags_from_vec(args: Vec<String>) -> (DenoFlags, DenoSubcommand, Vec<String>)

Then we'll be able to nicely test everything:

  #[test]
  fn test_set_flags_x() {
    let cli_args = svec!["deno", "-r", "-D", "run", "script.ts", "--foo", "bar", "bazz"];
    let (flags, cmd, args) = flags_from_vec(cli_args);
    assert_eq!(cmd, DenoSubcommand::Run);
    assert_eq!(args, svec!["deno", "script.ts", "--foo", "bar", "bazz");
    assert_eq!(
      flags,
      DenoFlags {
        log_debug: true,
        reload: true,
        ..DenoFlags::default()
      }
    );
  }

With DenoSubcommand there will be simple match on it in main.rs that basically selects function to call, as DenoFlags and script args will be already parsed.

CC @ry

[ry]

SGTM