feat: allow authoring module units

[JohelEGP]

Partially addresses #269.
Resolves #1190.

• Emit the module directive, global module fragment, and module declaration at the top.
• Allow exporting declarations.

The above means that module units aren't affected by #470.

I started including an expensive header.
So I isolated the problem to a module.
And timeliness was restored.

Test it on a recent Clang version.

hello.cpp2:

export module hello;
export hello: () -> std::string = "Hello";

main.cpp2:

#include <iostream>
import hello;
t: @struct type = {}
main: () = std::cout << hello() << ", modules!\n";

Commands:

cppfront hello.cpp2
cppfront main.cpp2
clang++ -std=c++20 -stdlib=libc++ -fmodules -fbuiltin-module-map -fmodule-output=hello.pcm -I /path/to/cpp2util/ -c -x c++-module hello.cpp
clang++ -std=c++20 -stdlib=libc++ -fmodules -fbuiltin-module-map -fprebuilt-module-path=. -I /path/to/cpp2util/ main.cpp hello.o
./a.out

Hello, modules!

hello.cpp

module;

#include "cpp2util.h"

export module hello;

//=== Cpp2 type declarations ====================================================



//=== Cpp2 type definitions and function declarations ===========================


#line 2 "hello.cpp2"
export [[nodiscard]] auto hello() -> std::string;


//=== Cpp2 function definitions =================================================


#line 2 "hello.cpp2"
[[nodiscard]] auto hello() -> std::string { return "Hello";  }

main.cpp

#include "cpp2util.h"


//=== Cpp2 type declarations ====================================================


#line 3 "main.cpp2"
class t;


//=== Cpp2 type definitions and function declarations ===========================

#include <iostream>
import hello;
#line 3 "main.cpp2"
class t {};
auto main() -> int;


//=== Cpp2 function definitions =================================================


#line 4 "main.cpp2"
auto main() -> int { std::cout << hello() << ", modules!\n";  }

Testing summary:

100% tests passed, 0 tests failed out of 692

Total Test time (real) =  34.15 sec

Acknowledgements:

• Tested thanks to https://github.com/modern-cmake/cppfront.
• Commit messages follow Conventional Commits 1.0.0.
• Comments and commit messages follow Semantic Line Breaks.

[JohelEGP]

Check the generated program on Compiler Explorer: https://cpp2.godbolt.org/z/csGKWqxfh.

[JohelEGP]

On GCC: https://cpp2.godbolt.org/z/n9TPeh58Y.
I had to use std::string_view for it to work.
Opened https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110893
and https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110894
for now.

[hsutter]

I appreciate this and I very much want to support authoring and consuming modules (*), but I don't think modules are ready for us yet. The support on the three major compilers is still very brittle, and I keep expecting that to change imminently but I want to keep holding off until that changes. If cppfront starts supporting modules authoring now, I fear it'll lead to lots of bug reports that will mostly be compiler bugs beyond our control, and so will be a distraction for cppfront.

So, with appreciation for the work that went into this PR, I'll still close this one for now. Sorry! I do appreciate it and I do look forward to when we can do this on all the major compilers.

(*) beyond the implicit as-if-import-std that already works in -pure-cpp2 mode, but even that has a big workaround to "fake it till [they] make it" in cpp2util.h

[JohelEGP]

That's fair.

Let's keep watching the growth of C++ modules.

[abuehl]

Could this be reconsidered now? We have converted a ~100 KLOC C++20 project (product: https://www.cadifra.com/) last year to using modules on MSVC. The conversion was quite some work, but we are very happy now with the MS compiler and the result of the conversion. Cppfront mistreating the module keyword is blocking us from using cppfront. I know that cppfront is prerelease, but I would still love to try using it on our project. I don't even mind if the cpp2 syntax changes. We could easily track such changes, but mistreating the module keyword is a showstopper for us. See also bug #1358.

[zaucy]

I've been having some good experience with modules on both MSVC and clang in my cpp2b project. I would love to see cpp2 support modules soon as well!