[C++20] [Modules] [std module] Implicitly import std if standard library header is included

[apolukhin]

What do you think about the following idea: add a preprocessor macro that replaces content of the header with import std; if the macro is defined by user.

For example libcxx/include/vector can be changed in the following way:

#ifndef _LIBCPP_VECTOR
#define _LIBCPP_VECTOR

#if defined(LIBCPP_IMPLICIT_MODULES_USE)
import std;
#  include <version>
#else

...  // all the remaining code from <vector>

#endif // LIBCPP_IMPLICIT_MODULES_USE

#endif // _LIBCPP_VECTOR

Motivation:

1. Implicit import std; in standard library header would improve compile times if module std is available.
2. Migration from includes to import std; for big projects is not trivial. The above approach eases that migration
3. It's a possible alternative solution to [C++23] [Modules] [std module] Skip including standard headers if the std module is imported #80663

[llvmbot]

@llvm/issue-subscribers-clang-modules

Author: Antony Polukhin (apolukhin)

What do you think about the following idea: add a preprocessor macro that replaces content of the header with `import std;` if the macro is defined by user.

For example libcxx/include/vector can be changed in the following way:

#ifndef _LIBCPP_VECTOR
#define _LIBCPP_VECTOR

#if defined(LIBCPP_IMPLICIT_MODULES_USE)
import std;
#  include <version>
#else

...  // all the remaining code from <vector>

#endif // LIBCPP_IMPLICIT_MODULES_USE

#endif // _LIBCPP_VECTOR

Motivation:

1. Implicit import std; in standard library header would improve compile times if module std is available.
2. Migration from includes to import std; for big projects is not trivial. The above approach eases that migration
3. It's a possible alternative solution to [C++23] [Modules] [std module] Skip including standard headers if the std module is imported #80663

[ChuanqiXu9]

If I read correctly, this is an issue to libc++ instead of clang? I'll try to add a tag to libc++.

For the issue itself, I feel good.

[philnik777]

I'm not particularly enthusiastic about this. AFAIK you can achieve the same thing by using Clang modules, which work today without any additional changes to libc++.