The #headers flavor is interpreted as a normal library in Xcode but not in Buck

When depending on mylib#headers flavoured target, Buck only uses header files as expected and the library is not used at link time. On the other hand, Xcode see the mylib#headers flavoured target as a complete library and try to link against it, which is not what we want.

An easy example is to take the testcase under test/com/facebook/buck/cxx/testdata/explicit_header_only_caching

apple_binary(
    name = "binary",
    srcs = ["main.c"],
    deps = [
        ":dylib-exporting-lib1",
        ":lib3",
    ],
)

cxx_library(
    name = "dylib-exporting-lib1",
    srcs = ["stub.c"],
    linker_flags = ["-all_load"],
    preferred_linkage = "shared",
    deps = [":lib1"],
)

cxx_library(
    name = "lib1",
    srcs = ["lib1.c"],
    exported_headers = ["lib1.h"],
    header_namespace = "lib1",
    preferred_linkage = "static",
    deps = [":lib2"],
)

cxx_library(
    name = "lib2",
    srcs = ["lib2.c"],
    exported_headers = ["lib2.h"],
    header_namespace = "lib2",
    preferred_linkage = "static",
)

cxx_library(
    name = "lib3",
    srcs = ["lib3.c"],
    exported_headers = ["lib3.h"],
    header_namespace = "lib3",
    preferred_linkage = "static",
    deps = [":lib1#headers"],
)

You only need to remember that lib1 contain a function called lib1_function.

Building with buck

buck build //:binary

Looking at the linking stage of the binary, we can see:

-o
buck-out/gen/binary
-Xlinker
-map
-Xlinker
buck-out/gen/binary-LinkMap.txt
buck-out/gen/dylib-exporting-lib1#default,shared/libdylib-exporting-lib1.dylib
-Xlinker
-filelist
-Xlinker
buck-out/bin/binary#binary/filelist.txt

So it links against the libdylib-exporting-lib1.dylib and main.c.o and liblib3.a ( both contained inside filelist.txt).

The final binary does not have the implementation of lib1_function and use a reference to the one living inside libdylib-exporting-lib1.dylib.

This behavior is expected !

Building with Xcode

buck project --ide Xcode //:binary then building with Xcode

Looking at the linking stage of the binary, we can see:

/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang ... -filelist <path>/binary.LinkFileList -ldylib-exporting-lib1 -llib1 -llib3 ...

It links against the dylib-exporting-lib1.dylib, lib3.a, main.c.o (inside binary.LinkFileList) but also against lib1.a (it shouldn't).

The final binary does have the implementation of lib1_function and does not use the one living inside libdylib-exporting-lib1.dylib.

This behavior is wrong !

We can clearly see in Xcode that the target lib1 is not really compiled and that it compiles lib1#headers as a library which is not what we want here.