bootstrap submodule sync overwrites local changes

[Aaron1011]

When bootstrap updates a submodule, it performs a hard reset, discarding all local changes:

rust/src/bootstrap/lib.rs

Lines 631 to 632 in 1481fd9

	self.run(Command::new("git").args(&["reset", "-q", "--hard"]).current_dir(&absolute_path));
	self.run(Command::new("git").args(&["clean", "-qdfx"]).current_dir(absolute_path));

This was a nasty surprise when I was editing my local src/llvm-project, and then did a git pull and ./x.py build.
Fortunately, I only had some extra debugging prints added, but there was no indication that my local changes would be overwritten.

We should require explicit confirmation before potentially deleting local changes like this.

[chenyukang]

There is an option in config.toml:
Automatic submodule handling can still be disabled with build.submodules = false.

We'd better get confirmation if there is no explicit config parameter get from config.toml?

[nagisa]

This has been a frequent source of surprises for me as well going back all the way to the addition of x.py. More generally I feel it is important that we figure out what the out-of-the-box experience is tailored for – the developers or people building rustc for distribution. Today this is a mixed bag.

[jyn514]

I think a simple fix would be to use git stash + git apply to save changes before doing a hard reset; if that causes issues for some reason, we can remove all the --force flags so that we don't overwrite local changes and give a hard error instead.

[dotdot0]

Can I work on this issue? Though I need Some guidance I am new to the project

[jyn514]

@pratushrai0309 first, reproduce the original issue so you can be sure your changes are having an effect. Then modify the git commands linked in the original post (#103485 (comment)) to add git stash save before and git stash apply after. If that saves your changes then your work is done and you can open a PR; otherwise you'll have to do some experimenting with git to see what works.

[dotdot0]

So what exactly I should do to reproduce the original Issue. This is my first time contributing to the compiler.

[dotdot0]

@rustbot claim

[dotdot0]

That's what I have done

        self.run(Command::new("git").args(&["stash", "save"]).current_dir(&absolute_path));
        self.run(Command::new("git").args(&["stash", "apply"]).current_dir(&absolute_path));
        self.run(Command::new("git").args(&["reset", "-q", "--hard"]).current_dir(&absolute_path));
        self.run(Command::new("git").args(&["clean", "-qdfx"]).current_dir(absolute_path));