rustc: Switch dsymutil status => output

[alexcrichton]

Sometimes dsymutil writes to stdout/stderr which rust isn't reading, which may
cause a deadlock.

Closes #16060

[lilyball]

Wouldn't it be cleaner to use something like

match Command::new("dsymutil").arg(out_filename).stdout(io::process::Ignored)
                              .stderr(io::process::Ignored).status() {
    Ok(..) => {}
    ...

This way you're not unnecessarily buffering output just to throw it away.

Heck, it looks like reading the output actually spawns two tasks (one for stdout and one for stderr). That's even more wasted work than merely buffering the output.

[lilyball]

As mentioned in #16060 this could even decide to inherit the stderr fd instead of ignoring it, so any dsymutil errors will be visible to the user.

[alexcrichton]

An allocation or a task spawning or two at that stage of compilation is just a drop in the bucket at that point, there's no need to actually worry about it. This has been the source of many annoying warnings being printed in the past, which is why I believe it's silenced.

[lilyball]

Well, ignoring preserves silence. I guess I won't object if you go ahead with .output(), but without a comment someone else may come along later and wonder why you're requesting output and promptly ignoring it. Explicitly closing stdout/stderr makes it clear what you're doing.