[libcxx-commits] [libcxx] [libcxx][clang-modules] Fix headers being marked as textual (PR #130723)

[Takuto Ikuta via libcxx-commits]

atetubou wrote:

I'm still not sure what is the problem using `textual` specifiers in the module. And which of the following explanation is relevant to this change?

> When building libcxx as a module, it fails to build because it's missing various definitions.
> 
> Consider the following code for module A:
> 
> ```
> #include <module B that includes config_site>
> #include <config>
> #include <module C that includes config>
> ```
> 
> With the module map file:
> 
> ```
> module std {
>    module A { header "a.h" }
>    module B { header "b.h" }
>    module C { header "c.h" }
> }
> ```
> 
> Macro visibility rules state that "[macros] are visible if they are from the current submodule or translation unit, or if they were exported from a submodule that has been imported."
> 
> * Module A has visibility of all macros exported by modules B and C (because they were exported from an imported submodule)
> * Module B and C have visibility over all macros exported by module A (because they are from the current submodule)
> 
> 1. When we #include the first line, module B successfully includes config_site.
> 
> Module B exports: config_site contents, config_site header guard
> 
> 2. Module A now includes  directly
> 

Include directly what?

> Because it can see module B's exports, it stops at the header guard for config_site and does not include config_site. This is not an issue, however, because it has visibility onto everything exported from config_site thanks to module B.
> 
> Module A exports: config contents, config header guard, config's includes' content and header guard (_except config_site_)
> 
> 3. Module C now includes config. Based on the above visibility rules, it can see everything exported from module A.
> 
> Because of that, we see the header guard for config and do not import it at all. This is mostly OK, as we can see the config that module A exported. However, if we ever try and access config_site, it will fail, as A did not export config_site.



https://github.com/llvm/llvm-project/pull/130723