Configuration order of stdio joining and redirection matters #116

Issue

Looks like the order in which joining (Expression::stdout_to_stderr, Expression::stderr_to_stdout) and redirection (Expression::std{out,err}_path) configuration methods are called results in different behaviors (could also be considered incorrect behavior).

If redirection is used before joining, the joining configuration is ignored:

//# duct = "0.13.7"

use duct::cmd;

fn main() {
    let cmd = cmd!("sh", "-c", "echo out && echo err 1>&2")
        .stdout_path("/tmp/out")
        .stderr_to_stdout();

    println!("{:?}", cmd);

    cmd.run().expect("command error");
}

❱ cargo play -q duct.rs 
Io(StderrToStdout, Io(StdoutPath("/tmp/out"), Cmd(["sh", "-c", "echo out && echo err 1>&2"])))
err

❱ cat /tmp/out
out

If redirection is set up after joining, it works as expected:

let cmd = cmd!("sh", "-c", "echo out && echo err 1>&2")
    .stderr_to_stdout()
    .stdout_path("/tmp/out");
}

❱ cargo play -q duct.rs 
Io(StdoutPath("/tmp/out"), Io(StderrToStdout, Cmd(["sh", "-c", "echo out && echo err 1>&2"])))

❱ cat /tmp/out
out
err

Discussion

Since Expression is structured as a binary tree, I'm not sure how to solve this. Rearranging tree nodes in a custom precedence order seems overkill.

Generating an intermediary flat structure from the tree, then sorting it by precedence, seems more viable. In this case, IoExpressionInner could implement Ord with a custom precedence ordering.