Add INTERFACE libraries to generated buildsystem if they have SOURCES

INTERFACE libraries were created with the intention of collecting usage
requirements for use by other targets via `target_link_libraries`.
Therefore they were not allowed to have SOURCES and were not included in
the generated buildsystem.  In practice, this has become limiting:

* Header-only libraries do have sources, they just do not compile.
  Developers should be able to edit those sources (the header files)
  in their IDE.

* Header-only libraries may need to generate some of their header
  files via custom commands.

Some projects work around these limitations by pairing each interface
library with an `add_custom_target` that makes the header files and
custom commands appear in the generated buildsystem and in IDEs.

Lift such limitations by allowing INTERFACE libraries to have SOURCES.
For those with sources, add a corresponding build target to the
generated buildsystem.

Fixes: #19145

4 files changed, 27 insertions, 0 deletions

diff --git a/Tests/RunCMake/VS10Project/InterfaceLibSources-check.cmake b/Tests/RunCMake/VS10Project/InterfaceLibSources-check.cmake
new file mode 100644
index 0000000..bcdc101
--- /dev/null
+++ b/Tests/RunCMake/VS10Project/InterfaceLibSources-check.cmake
@@ -0,0 +1,25 @@
+set(vcProjectFile "${RunCMake_TEST_BINARY_DIR}/iface.vcxproj")
+if(NOT EXISTS "${vcProjectFile}")
+  set(RunCMake_TEST_FAILED "Project file ${vcProjectFile} does not exist.")
+  return()
+endif()
+
+set(found_iface_h 0)
+file(STRINGS "${vcProjectFile}" lines)
+foreach(line IN LISTS lines)
+  if(line MATCHES "<([A-Za-z0-9_]+) +Include=.*iface\\.h")
+    set(rule "${CMAKE_MATCH_1}")
+    if(NOT rule STREQUAL "None")
+      set(RunCMake_TEST_FAILED "iface.h referenced as ${rule} instead of None in\n  ${vcProjectFile}")
+      return()
+    endif()
+    if(found_iface_h)
+      set(RunCMake_TEST_FAILED "iface.h referenced multiple times in\n  ${vcProjectFile}")
+      return()
+    endif()
+    set(found_iface_h 1)
+  endif()
+endforeach()
+if(NOT found_iface_h)
+  set(RunCMake_TEST_FAILED "iface.h not referenced in\n  ${vcProjectFile}")
+endif()
diff --git a/Tests/RunCMake/VS10Project/InterfaceLibSources.cmake b/Tests/RunCMake/VS10Project/InterfaceLibSources.cmake
new file mode 100644
index 0000000..3672be1
--- /dev/null
+++ b/Tests/RunCMake/VS10Project/InterfaceLibSources.cmake
@@ -0,0 +1 @@
+add_library(iface INTERFACE iface.h)
diff --git a/Tests/RunCMake/VS10Project/RunCMakeTest.cmake b/Tests/RunCMake/VS10Project/RunCMakeTest.cmake
index 93ef603..e9f251a 100644
--- a/Tests/RunCMake/VS10Project/RunCMakeTest.cmake
+++ b/Tests/RunCMake/VS10Project/RunCMakeTest.cmake
@@ -6,6 +6,7 @@ cmake_policy(SET CMP0054 NEW)
 run_cmake(VsCsharpSourceGroup)
 run_cmake(VsCSharpCompilerOpts)
 run_cmake(ExplicitCMakeLists)
+run_cmake(InterfaceLibSources)
 run_cmake(RuntimeLibrary)
 run_cmake(SourceGroupCMakeLists)
 run_cmake(SourceGroupTreeCMakeLists)
diff --git a/Tests/RunCMake/VS10Project/iface.h b/Tests/RunCMake/VS10Project/iface.h
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/Tests/RunCMake/VS10Project/iface.h