Use autocxx

[Chris00]

After a lot of work, I have the example 1 working with a mix of automatic (using autocxx) and manual (using cxx as you do). I believe this approach scales better as most of the "bare" Rust bindings for C++ methods is done automatically. You can see the code here.

I also took different design decisions. In particular I (ab)used the Deref trait to express sub-class relationship. It seems to work well (Deref is tried transitively by Rust when seeking a method) but that is something worth discussing. Let me know what you think.

[mkovaxx]

@Chris00 I really like this idea! I'm not familiar with autocxx yet, but I'd be happy to try and see how far we get! :)
I suggest we work together on a branch here, so I've invited you as a contributor!

[Chris00]

Thanks. I will create an autocxx branch. First, only mfem-sys will change and, when we agree on it, then we can discuss the mfem interface.

BTW, is there any reason that the mfem directory is not a git submodule?

[mkovaxx]

Thanks. I will create an autocxx branch. First, only mfem-sys will change and, when we agree on it, then we can discuss the mfem interface.

Sounds good to me!

BTW, is there any reason that the mfem directory is not a git submodule?

If you mean crates/mfem-cpp/mfem, that's already a submodule, see:
https://github.com/mkovaxx/mfem-rs/blob/6f541e6cf2956176590d980edc07c41f33de4cd8/.gitmodules

Maybe you mean something else?

[Chris00]

that's already a submodule,

My bad, I looked to fast (too late in the night 😁).

[Chris00]

I've created an autocxx branch (for mfem-sys only). Let me know what you think. You may want to compile it with

cargo run -p mfem-sys --example ex1 -- --mesh your.mesh

and see the generated functions with

cargo doc -p mfem-sys --document-private-items

[mkovaxx]

I'm hitting a strange snag like this. Still trying to figure it out...

$ cargo run -p mfem-sys --example ex1 -- --mesh ~/prog/mfem/data/escher.mesh
   Compiling serde v1.0.210
   Compiling thiserror v1.0.59
   Compiling cxx v1.0.121
   Compiling autocxx v0.27.1
   Compiling clap v4.5.4
error: the `cargo::` syntax for build script output instructions was added in Rust 1.77.0, but the minimum supported Rust version of `autocxx v0.27.1` is 1.77.
See https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script for more information about build script outputs.
warning: build failed, waiting for other jobs to finish...

It's weird, because there's no cargo:: used anywhere in our build.rs files AFAICT...

[mkovaxx]

What version of the Rust toolchain are you on?

I get that strange error above with both 1.77.0 and 1.78.0.
With the latest, 1.84.1, I get a different error:

error[E0308]: mismatched types
   --> crates/mfem-sys/build.rs:35:33
    |
35  |         build.flag_if_supported(f);
    |               ----------------- ^ expected `&str`, found `String`
    |               |
    |               arguments to this method are incorrect
    |
note: method defined here
   --> /Users/k/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cc-1.0.95/src/lib.rs:697:12
    |
697 |     pub fn flag_if_supported(&mut self, flag: &str) -> &mut Build {
    |            ^^^^^^^^^^^^^^^^^
help: consider borrowing here
    |
35  |         build.flag_if_supported(&f);
    |                                 +

For more information about this error, try `rustc --explain E0308`.

[Chris00]

Well, the signature is

pub fn flag_if_supported(&mut self, flag: impl AsRef<OsStr>) -> &mut Build

What version of cc is in your Cargo.lock? Mine is

[[package]]
name = "cc"
version = "1.2.11"

[Chris00]

error: the cargo:: syntax for build script output instructions was added in Rust 1.77.0, but the minimum supported Rust version of autocxx v0.27.1 is 1.77.
See https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script for more information about build script outputs.
warning: build failed, waiting for other jobs to finish...

It's weird, because there's no `cargo::` used anywhere in our `build.rs` files AFAICT...

I guess autocxx-build is also outputting some directives with that format. I've added a minimal Rust version.

[Chris00]

What version of the Rust toolchain are you on?

I'm using the nightly version (but an "old" one, I don't update it that often).

[mkovaxx]

I've opened a PR #11 against the autocxx branch, trying to make it work on my setup.

[Chris00]

The linking problem with Windows in the CI comes from the fact that they left a function signature in mesh.hpp without an implementation. It's easy to fix either with an additional commit removing the signature (if we want to stay as close as possible to v4.7) or using a later commit of the official repository — they later removed the function signature. What's your take on this? If the former, we could use again https://github.com/mkovaxx/mfem