Document how x.py works

[mark-i-m]

https://internals.rust-lang.org/t/compiling-rustc-with-non-standard-flags/8950/3

[jyn514]

@mark-i-m do you think https://rustc-dev-guide.rust-lang.org/building/bootstrapping.html answers this well enough? Or was there something more you wanted to document?

[mark-i-m]

I think my intent was more related to how x.py is implemented and how to hack on it...

[jyn514]

Hmm, maybe I could link to https://github.com/rust-lang/rust/blob/master/src/bootstrap/README.md#extending-rustbuild in https://rustc-dev-guide.rust-lang.org/building/bootstrapping.html#contributing-to-bootstrap.

[mark-i-m]

Perhaps? Or else move that readme to a short chapter in the guide. Not sure...

[jyn514]

@Mark-Simulacrum do you have opinions on whether this should be documented in the dev-guide or in-tree?

[Mark-Simulacrum]

Open to either; rustc-dev-guide is somewhat less likely to receive updates. x.py is not that complicated itself, just is incredibly stateful (filesystem), so that pretty significantly increases complexity of changing it. Not sure what kind of docs are intended here.

[jyn514]

This looks roughly the same as rust-lang/rust#90686 to me; I'm going to close it since the other issue was also closed (the documentation now lives in-tree in src/bootstrap/README.md).

Happy to add more docs but I think we should use a more specific issue for that.