Issues with passing same list of defines to both objc_library and swift_library

To follow what is detailed below check the BUILD file in this repo and note two objc_library targets (foo_objc_1, foo_objc_2) each depending on an swift_library target (foo_swift_1, foo_swift_2). With this setup and passing the same list of preprocessor macros to both libraries in the defines attribute undesired behavior is triggered.

ps: (1) In the build invocations below one can pass --action_env=$(date +%S) to force invalidation and avoid hitting cached artifacts in order to build the same target multiple times (2) The --subcommands flag is used to expose the compiler invocations.

Duplicated values and undesired split by space due to Bourne shell tokenization

The list of defines below is passed to both foo_objc_1 and foo_swift_1

DEFINES_1 = ["FOO=A B C"]

from here one can build foo_objc_1

bazel build --subcommands :foo_objc_1

and even though it compiles note the underlying clang invocation

external/local_config_cc/wrapped_clang -arch x86_64 '-D_FORTIFY_SOURCE=1' -fstack-protector -fcolor-diagnostics -Wall -Wthread-safety -Wself-assign -fno-omit-frame-pointer -O0 -DDEBUG 'DEBUG_PREFIX_MAP_PWD=.' -Wshorten-64-to-32 -Wbool-conversion -Wconstant-conversion -Wduplicate-method-match -Wempty-body -Wenum-conversion -Wint-conversion -Wunreachable-code -Wmismatched-return-types -Wundeclared-selector -Wuninitialized -Wunused-function -Wunused-variable -iquote . -iquote bazel-out/applebin_macos-darwin_x86_64-fastbuild-ST-cd2b3b8d4835/bin -Ibazel-out/applebin_macos-darwin_x86_64-fastbuild-ST-cd2b3b8d4835/bin -MD -MF bazel-out/applebin_macos-darwin_x86_64-fastbuild-ST-cd2b3b8d4835/bin/_objs/foo_objc_1/arc/foo.d '-DFOO=A B C' '-DFOO=A' -DB -DC -DOS_MACOSX -fno-autolink -isysroot __BAZEL_XCODE_SDKROOT__ -F__BAZEL_XCODE_SDKROOT__/System/Library/Frameworks -F__BAZEL_XCODE_DEVELOPER_DIR__/Platforms/MacOSX.platform/Developer/Library/Frameworks -fobjc-arc '-mmacosx-version-min=11.3' -O0 '-DDEBUG=1' -c foo.m -o bazel-out/applebin_macos-darwin_x86_64-fastbuild-ST-cd2b3b8d4835/bin/_objs/foo_objc_1/arc/foo.o

in particular

'-DFOO=A B C' '-DFOO=A' -DB -DC

Note how FOO is duplicated (which is expected since we're passing the same list to both). The '-DFOO=A B C' occurence comes from the fact that swift_library propagates defines to all targets that depend on it, important to note that KEY=VALUE pairs shouldn't be passed to swift_library in the first place but it's being done here to simulate a scenario where one would pass the same list of defines to both libraries.

The '-DFOO=A' -DB -DC occurence comes from the objc_library and the split is due to Bourne shell tokenization which is undesired. One could decide to then wrap the macro in quotes, e.g. \'FOO=X Y Z\' but this would cause a different issue (see section below).

Note the warnings triggered by this:

INFO: From Compiling foo.m:
In file included from <built-in>:379:
<command line>:4:9: warning: 'FOO' macro redefined [-Wmacro-redefined]
#define FOO A
        ^
<command line>:3:9: note: previous definition is here
#define FOO A B C
        ^
1 warning generated.
Target //:foo_objc_1 up-to-date:

Compilation error if consumer decides to wrap value in single quote

The list of defines below is passed to both foo_objc_2 and foo_swift_2

DEFINES_2 = ["\'FOO=X Y Z\'"]

one could do that to prevent the issue described in the previous section, this build will fail though

bazel build --subcommands :foo_objc_2

Note the clang invocation in this case

external/local_config_cc/wrapped_clang -arch x86_64 '-D_FORTIFY_SOURCE=1' -fstack-protector -fcolor-diagnostics -Wall -Wthread-safety -Wself-assign -fno-omit-frame-pointer -O0 -DDEBUG 'DEBUG_PREFIX_MAP_PWD=.' -Wshorten-64-to-32 -Wbool-conversion -Wconstant-conversion -Wduplicate-method-match -Wempty-body -Wenum-conversion -Wint-conversion -Wunreachable-code -Wmismatched-return-types -Wundeclared-selector -Wuninitialized -Wunused-function -Wunused-variable -iquote . -iquote bazel-out/applebin_macos-darwin_x86_64-fastbuild-ST-cd2b3b8d4835/bin -Ibazel-out/applebin_macos-darwin_x86_64-fastbuild-ST-cd2b3b8d4835/bin -MD -MF bazel-out/applebin_macos-darwin_x86_64-fastbuild-ST-cd2b3b8d4835/bin/_objs/foo_objc_2/arc/foo.d '-D'\''FOO=X Y Z'\''' '-DFOO=X Y Z' -DOS_MACOSX -fno-autolink -isysroot __BAZEL_XCODE_SDKROOT__ -F__BAZEL_XCODE_SDKROOT__/System/Library/Frameworks -F__BAZEL_XCODE_DEVELOPER_DIR__/Platforms/MacOSX.platform/Developer/Library/Frameworks -fobjc-arc '-mmacosx-version-min=11.3' -O0 '-DDEBUG=1' -c foo.m -o bazel-out/applebin_macos-darwin_x86_64-fastbuild-ST-cd2b3b8d4835/bin/_objs/foo_objc_2/arc/foo.o

in particular

'-D'\''FOO=X Y Z'\''' '-DFOO=X Y Z'

The '-D'\''FOO=X Y Z'\''' occurence comes from swift_library trying to wrap in quotes an already wrapped value and the second occurence '-DFOO=X Y Z' comes from objc_library receiving the correct value for that macro.

The build will fail due to how swift_library propagated the macro to its depender

<command line>:3:9: error: macro name must be an identifier
#define 'FOO X Y Z'
        ^
1 error generated.
Error in child process '/usr/bin/xcrun'. 1
Target //:foo_objc_2 failed to build

Takeaways

• Consumers should not pass the same list of defines to both objc_library and swift_library, instead explicitly pass expected values for each.
• objc_library preprocessor macros should be always wrapped in single quotes to prevent similar issues as described here due to Bourne shell tokenization.