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Help: Drop outdated documentation of import std implementation details

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Since commit fa10dc6c22 (Experimental/CXXModules: Implement EcoStd
Module Metadata parser, 2025-10-21, v4.3.0-rc1~483^2) the `import std`
implementation has changed.

Closes: #27706
This commit is contained in:
Brad King committed 2026-03-23 16:59:09 -04:00
1 parent 7178780043
commit 6818426bdd
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Help/manual/cmake-cxxmodules.7.rst
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@@ -471,45 +471,6 @@ Additionally, toolchains should set the following variables:
If a toolchain does not provide the ``CMAKE_CXX_MODULE_BMI_ONLY_FLAG``, it
will not be able to consume modules provided by ``IMPORTED`` targets.
Toolchain (``import std``)
^^^^^^^^^^^^^^^^^^^^^^^^^^
If the toolchain supports ``import std``, it must also provide a toolchain
identification module named ``${CMAKE_CXX_COMPILER_ID}-CXX-CXXImportStd``.
.. note::
Currently only CMake may provide these files due to the way they are
included. Once ``import std`` is no longer experimental, external
toolchains may provide support independently as well.
This module must provide the ``_cmake_cxx_import_std`` command. It will be
passed two arguments: the version of the C++ standard (e.g., ``23``) and the
name of a variable in which to place the result of its ``import std`` support.
The variable should be filled in with CMake source code which declares the
``__CMAKE::CXX${std}`` target, where ``${std}`` is the version passed in. If
the target cannot be made, the source code should instead set the
``CMAKE_CXX${std}_COMPILER_IMPORT_STD_NOT_FOUND_MESSAGE`` variable to the
reason that ``import std`` is not supported in the current configuration.
Note that CMake will guard the returned code with conditional checks to ensure
that the target is only defined once.
Ideally, the ``__CMAKE::CXX${std}`` target will be an ``IMPORTED``
``INTERFACE`` target with the ``std`` module sources attached to it. However,
it may be necessary to compile objects for some implementations. Object files
are required when there are symbols expected to be provided by the consumer of
the module by compiling it. There is a concern that, if this happens, more
than once within a program, this will result in duplication of these symbols
which may violate the :term:`ODR` for them.
As an example, if consumers of a module are expected to provide symbols for
that module, the use of the module is then a global property of the program
and cannot be abstracted away. Imagine that a library exposes a C API but
uses a C++ module internally. If it is supposed to provide the module
symbols, anything using the C API needs to cooperate with its internal module
usage if it wants to use the same module for its own purposes. If both end up
providing symbols for the imported module, there may be conflicts.
Configure
^^^^^^^^^