Debian Bullseye has just been released, and it carries rustc 1.48.0.
This actually implies a regression that we can't really do anything about in
rhg. See https://github.com/rust-lang/rust/issues/88825. In short, closed (or
bad) standard file descriptors are reopened silently with no way of telling by
the Rust runtime before main is executed. This means that closed fds are not
forwarded to the subprocess we run in case of fallback. This is a bit sad, but
probably not something worth worrying too much about.