Hi,

here a list of things we could still do in order to improve the Rust
bootstrapping process.

(1) For all Mozilla Rusts except the latest one, pass
"--disable-docs --disable-compilerdocs" in order to disable building (and
especially TESTING) the docs.  Testing the docs takes a very long time
and no one will ever read the docs of the compilers used for bootstrapping.

For that, we have to find out where these options can be passed.
We use a "./x.py" bootstrapping method so I don't think that the GNU-style
"./configure" ever gets called (or does it?).

(2) Bootstrap rust 1.29.0 directly via mrustc 0.9.
See https://github.com/thepowersgang/mrustc/issues/140 for why that is
currently not working.

(3) Parallelize things again.  In the past I've disabled parts of the
parallel build because of bugs and/or memory constraints, but after (2) we
could check whether they are working now.

(4) Factor out the mrustc bootstrapper into a mrustc-bootstrapped-package
procedure which changes a package in order to use mrustc, including the
phases.  That way, we can easily choose which of the Mozilla Rusts we want
to bootstrap.
Currently, we also have to change the package definition of unrelated packages
(to remove the bootstrapping phases there and replace them by regular phases)
which is not so nice.  That would mean that there are multiple different
mrustc packages at runtime, like mrustc-1.19.0@0.9 and mrustc-1.29.0@0.9--not
sure about the naming.