This establishes that linking is used to propagate usage-requirements
between targets in CMake code. The use of the target_link_libraries
command as the API for this is chosen because introducing a new command
would introduce confusion due to multiple commands which differ only in
a subtle way.
This reverts commit 9cfe4f1b76.
It turns out that correctly adding the content to
the IMPORTED_LINK_INTERFACE_LIBARIES_<CONFIG> of an upstream target
from the buildsystem of a downstream project is not simple.
If upstream had added the INTERFACE content, the config-specific
properties would be determined by the DEBUG_CONFIGURATIONS of
upstream.
As downstream, we don't have any information about what
the DEBUG_CONFIGURATIONS of upstream were, so we can't determine
which configuration-specific properties to populate. The best we can do
is add it to all of them or add it to the ones downstream considers to
be DEBUG_CONFIGURATIONS, neither of which is a good solution.
So, removing the porcelain API for that is the best approach. A human
can still determine which properties to populate and use
the set_property API to populate the desired properies.
Another solution to this would be for upstream targets to publish
what they consider DEBUG_CONFIGURATIONS, but that can be added in
a future release.
The Config and IMPORTED_ variants may also contain generator
expressions.
If 'the implementation is the interface', then the result of
evaluating the expressions at generate time is used to populate
the IMPORTED_LINK_INTERFACE_LIBRARIES property.
1) In the case of non-static libraries, this is fine because the
user still has the option to populate the LINK_INTERFACE_LIBRARIES
with generator expressions if that is what is wanted.
2) In the case of static libraries, this prevents a footgun,
enforcing that the interface and the implementation are really
the same.
Otherwise, the LINK_LIBRARIES could contain a generator
expression which is evaluated with a different context at build
time, and when used as an imported target. That would mean that the
result of evaluating the INTERFACE_LINK_LIBRARIES property for
a static library would not necessarily be the 'link implementation'.
For example:
add_library(libone STATIC libone.cpp)
add_library(libtwo STATIC libtwo.cpp)
add_library(libthree STATIC libthree.cpp)
target_link_libraries(libtwo
$<$<STREQUAL:$<TARGET_PROPERTY:TYPE>,STATIC_LIBRARY>:libone>)
target_link_libraries(libthree libtwo)
If the LINK_LIBRARIES content was simply copied to the
IMPORTED_LINK_INTERFACE_LIBRARIES, then libthree links to libone, but
executables linking to libthree will not link to libone.
3) As the 'implementation is the interface' concept is to be
deprecated in the future anyway, this should be fine.
This makes
set(CMAKE_BUILD_INTERFACE_INCLUDES ON)
add the equivalent of
set_property(TARGET tgt APPEND PROPERTY
INTERFACE_INCLUDE_DIRECTORIES
$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR};${CMAKE_CURRENT_BINARY_DIR}>
)
to every target.
If the headers are in CMAKE_CURRENT_SOURCE_DIR, and the generated headers
are in CMAKE_CURRENT_BINARY_DIR, this is a convenient way to build a target
bar, which depends on foo, just by using target_link_libraries() and adding
the INTERFACE_INCLUDE_DIRECTORIES to the INCLUDE_DIRECTORIES of the target
being linked. There will be more-convenient porcelain API to consume the
property in the future.
This makes it possible to use:
target_link_libraries(foo LINK_INTERFACE_LIBRARIES bar)
where foo is an IMPORTED target. Other tll() signatures are not
allowed.
Previously we kept direct link dependencies in OriginalLinkLibraries.
The property exposes the information in the CMake language through the
get/set_property commands. We preserve the OriginalLinkLibraries value
internally to support old APIs like that for CMP0003's OLD behavior, but
the property is now authoritative. This follows up from commit d5cf644a
(Split link information processing into two steps, 2012-11-01).
This will be used later to populate the link interface properties when
exporting targets, and will later allow use of generator expressions
when linking to libraries with target_link_libraries.
Also make targets depend on the (config-specific) union of dependencies.
CMake now allows linking to dependencies or not depending on the config.
However, generated build systems are not all capable of processing
config-specific dependencies, so the targets depend on the union of
dependencies for all configs.
Stephen Kelly
Brad King