fix cmake dependency hell

rerun_cpp/CMakeLists.txt

@@ -31,10 +31,12 @@ endif()
 
 # ------------------------------------------------------------------------------
 # Setup rerun_c dependency.
+add_library(rerun_c STATIC IMPORTED)
+
 # Inside the repo build ourselves, otherwise default to a local `lib` folder.
 if(DEFINED RERUN_REPOSITORY)
     include(build_rerun_c.cmake)
-    build_rerun_c(RERUN_C_LIB_DEFAULT)
+    build_rerun_c(rerun_c)
 else()
     set(RERUN_C_DEFAULT_LIB_DIR ${CMAKE_CURRENT_SOURCE_DIR}/lib/)
 
@@ -55,21 +57,19 @@ else()
     else()
         message(WARNING "Unsupported platform ${RERUN_C_LIB_DEFAULT}, can't find rerun_c library.")
     endif()
-endif()
 
-set(RERUN_C_LIB ${RERUN_C_LIB_DEFAULT} CACHE PATH "\
-Where to find the rerun_c library.\n\
-If not specified, a local rerun_c with the current system architecture will be used."
-)
+    set(RERUN_C_LIB ${RERUN_C_LIB_DEFAULT} CACHE PATH "\
+        Where to find the rerun_c library.\n\
+        If not specified, a local rerun_c with the current system architecture will be used."
+    )
 
-if("${RERUN_C_LIB}" STREQUAL "")
-    message(FATAL_ERROR "RERUN_C_LIB is not set.")
+    if("${RERUN_C_LIB}" STREQUAL "")
+        message(FATAL_ERROR "RERUN_C_LIB is not set.")
+    endif()
+
+    set_target_properties(rerun_c PROPERTIES IMPORTED_LOCATION ${RERUN_C_LIB})
 endif()
 
-# For cleanliness, create a rerun_c target with the correct dependencies.
-# This allows other targets to depend on rerun_c directly if they need to.
-add_library(rerun_c STATIC IMPORTED)
-set_target_properties(rerun_c PROPERTIES IMPORTED_LOCATION ${RERUN_C_LIB})
 
 if(APPLE)
     target_link_libraries(rerun_c INTERFACE "-framework CoreFoundation" "-framework IOKit")

rerun_cpp/build_rerun_c.cmake

@@ -2,7 +2,7 @@
 #
 # This works only within a checkout of the Rerun repository, since the Rerun C rust source
 # is not part of the Rerun C++ SDK distribution bundle (instead, pre-built libraries are provided).
-function (build_rerun_c OUT_C_LIB)
+function (build_rerun_c rerun_c)
     if(APPLE)
         set(RERUN_C_BUILD_ARTIFACT ${CMAKE_CURRENT_SOURCE_DIR}/../target/release/librerun_c.a)
     elseif(UNIX) # if(LINUX) # CMake 3.25
@@ -22,16 +22,15 @@ function (build_rerun_c OUT_C_LIB)
         COMMENT "Building rerun_c from source"
     )
 
-    # It is necessary to add a custom target that depends on the output of the custom command,
-    # otherwise, cmake has no rule to build the output.
+    # In CMake you can't depend on an output file directly. We have to wrap this in a target that rerun_c then depends on.
     add_custom_target(rerun_c_build DEPENDS "${RERUN_C_BUILD_ARTIFACT}")
+    add_dependencies(rerun_c rerun_c_build)
+    set_target_properties(rerun_c PROPERTIES IMPORTED_LOCATION ${RERUN_C_LIB})
 
     # Put `rerun.h` into the same place where it's on a user's machine and apply CMake variables like version number.
     configure_file(
         "${CMAKE_CURRENT_SOURCE_DIR}/../crates/rerun_c/src/rerun.h"
         "${CMAKE_CURRENT_SOURCE_DIR}/src/rerun/c/rerun.h"
         NEWLINE_STYLE LF # Specify line endings, otherwise CMake wants to change them on Windows.
     )
-
-    set(${OUT_C_LIB} ${RERUN_C_BUILD_ARTIFACT} PARENT_SCOPE)
 endfunction()