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.