C++ modules have two variants which are of importance to CMake:
- `CXX_MODULES`: interface modules (those using `export module M;`,
`export module M:part;`, or `module M:internal_part;`)
- `CXX_MODULE_HEADER_UNITS`: importable header units
Creating C++ modules or partitions are *not* supported in any other
source listing. This is because the source files must be installed (so
their scope matters), but not part of usage requirements (what it means
for a module source to be injected into a consumer is not clear at this
moment). Due to the way `FILE_SET` works with scopes, they are a perfect
fit as long as `INTERFACE` is not allowed (which it is not).
These fields are specified by our `P1689r3` paper, but are not actually
needed. The dependencies of the scanning results themselves can be
captured via normal depfile logic. Avoid saving this possibly-large
information in the scanning results. It is not needed by later steps.
Optionally enable this infrastructure through an undocumented
`CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP` variable. Currently this is
experimental and intended for use by compiler writers to implement their
scanning tools. Warn as such when the feature is activated. Later when
compilers provide the needed scanning tools we can enable this variable
from our corresponding compiler information modules. It is never meant
to be set by project code.
When enabled, generate a build graph similar to what we use for Fortran
module dependencies. There are some differences needed because we can
scan dependencies without explicit preprocessing, and can directly
compile the original source afterward.
Co-Author: Ben Boeckel <ben.boeckel@kitware.com>
Initialize it with placeholder content. This document will serve to
contain documentation for experimental features that are under
development and not yet included in official documentation.
