Note that it may not make sense to include a source copy of rustc, as that will itself require an (earlier) stage of rustc to build. mrustc can offer a bootstrap for 1.54, but depending on the versions required, you may need upwards of 10 additional rustc sources. On Thu, 25 Jan 2024 at 10:04, Arthur Cohen wrote: > Hi Richard, > > On 1/23/24 08:23, Richard Biener wrote: > > On Mon, Jan 22, 2024 at 7:51 PM Arthur Cohen > wrote: > >> > >> Hi everyone, > >> > >> In order to increase the development speed of Rust features, we are > >> seeking feedback on reusing some Rust components directly within our > >> front-end. As mentioned in other conferences, the most important > >> component we would like to integrate into the front-end is the polonius > >> borrow-checker. Another component we've become interested in is the > >> `rustc_format_parser` library, responsible for handling parsing and > >> handling of format arguments as described in the documentation for > >> `std::fmt` [1]. > >> > >> However, since these libraries are written in Rust, GCC in itself is not > >> yet able to compile them. They all depend on the Rust standard library, > >> which we cannot yet compile and link properly. This obviously raises a > >> question - how to actually compile, integrate and distribute these > >> components? > >> > >> We do have the option to rewrite them from scratch, but we feel that > >> spending time on these existing, correct, battle-tested and easily > >> integrable components instead of focusing on other aspects of the > >> language would be a mistake. Spending this time instead on Rust features > >> that we are missing for compiling these components would, in our > >> opinion, yield more results, as it would also help in compiling other > >> Rust programs. > >> > >> We could either distribute these components as compiled libraries, or > >> look at integrating the official Rust compiler to our build system as a > >> temporary measure. I am aware that this would mean restricting the Rust > >> GCC front-end to platforms where the official Rust compiler is also > >> available, which is less than ideal. > > > > But that's only for the host part - you can still cross compile to > another > > target and possibly, once the GCC frontend can compile these libraries > > itself, also use them to bootstrap a hosted version on that target - > > speaking of .. > > > >> However, this would only be > >> temporary - as soon as the Rust front-end is able to compile these > >> components, we would simply reuse them and compile them with gccrs as > >> part of our bootstrapping process. > > > > .. gccrs would then need to be able to build itself without those > modules, > > at least during stage1 so that the stage1 compiler can then be used to > > build them. Or you go like m2 and build a "mini-rust" that's just > capable > > of building the modules. > > Right, that makes a lot of sense. We should definitely be able to build > the format string parser without a format string parser, as it does not > use format strings for error handling or anything. And even if it did, > it would be pretty easy to remove that and do the formatting by hand. > > Similarly, the borrow checker is not "needed" for compilation and we do > plan on building stage1 without it, while making it mandatory for > stage2/3 builds. > > > I think re-using parts already available is very sensible at this > point. Note > > that while we might temporarily require a host rust compiler to boostrap > > gccrs I'd rather not have the build system download something from the > > internet - so at least the sources of those dependences need to be in the > > GCC repository, possibly in a new toplevel directory. > > Okay, that makes a lot of sense. I was thinking of adding a basic check > for the Rust compiler to be present when compiling these components - > and error out if that isn't the case. Are you suggesting we embed a full > copy of rustc in GCC and build it from source when compiling the Rust > frontend? Or am I misunderstanding? > > >> The documentation for `std::fmt` [1] describes all of the features > >> available in Rust format strings. It also contains a grammar for the > >> format-string parser, which we would need to re-implement on top of > >> supporting all the formatting features. As a prototype, I wrote an FFI > >> interface to the `rustc_format_parser` library and integrated it to our > >> macro expansion system, which took me less than a couple hours. In less > >> than an afternoon, we had bindings for all of the exported types and > >> functions in the library and had access to a compliant and performant > >> Rust format string parser. But re-implementing a correct > >> `format_args!()` parser - with the same performance as the Rust one, and > >> the same amount of features - would probably take days, if not weeks. > >> And we are talking about one of the simplest components we aim to reuse. > >> Something like a fully-compliant trait solver for the Rust programming > >> language would take months if not years to implement properly from > scratch. > >> > >> I would like to stress once again that relying on distributing compiled > >> libraries or using `rustc` in our build system would be temporary, and > >> that these measures would be removed as soon as gccrs is able to compile > >> these components from source. > >> > >> I am looking for comments on this decision as well as the best practices > >> we could adopt. Have there been any similar decisions for other > >> self-hosted front-ends? Any previous email threads/commits that you > >> could point me to would be greatly appreciated. > > > > Not in this very same way but the D frontend is a shim around the > official > > DMD frontend and the Go frontend imports the golang standard library > > (but the frontend itself is written in C++ and doesn't use any part of > it). > > > > The C++ frontend uses part of the C++ standard library (but it can of > > course build that itself - but it requires a host C++ compiler with > library). > > Thanks for the pointers. I was wondering if this is something that the > Ada frontend had faced at the beginning, but I've been told it does not > have a lot of dependencies anyway so this might not be helpful. > > > > > Richard. > > > >> Thanks, > >> > >> Arthur > >> > >> [1]: https://doc.rust-lang.org/std/fmt/ > > Thanks a lot for taking the time, I really appreciate it. > > Best, > > Arthur >