Exit nonzero on rustc -Wall
Previously `rustc -Wall /dev/null` would print a paragraph explaining that `-Wall` is not a thing in Rust, but would then exit 0. I believe exiting 0 is not the right behavior. For something like `rustc --version` or `rustc --help` or `rustc -C help` the user is requesting rustc to print some information; rustc prints that information and exits 0 because what the user requested has been accomplished. In the case of `rustc -Wall path/to/main.rs`, I don't find it correct to conceptualize this as "the user requested rustc to print information about the fact that Wall doesn't exist". The user requested a particular thing, and despite rustc knowing what they probably meant and informing them about that, the thing they requested has *not* been accomplished. Thus a nonzero exit code is needed.
844a657bb8