Bazel select() based on build config

I am trying to provide some preprocessor definitions at compile time based on whether the user runs bazel test or bazel build.

Specifically, I want to have a conditional dependency of a cc_library.deps and a conditional definition in cc_library.defines.

I found that select() is the way to go but I cannot figure out how to know what action the user runs.

Solution

I'm not aware of any way to detect the current command (build vs test) using select(), but I think you can achieve something similar with custom keys.

You could define a config_setting block like the following:

# BUILD
config_setting(
  name = "custom",
  values = {
    "define": "enable_my_flag=true"
  }
)

and use it in your library to control the defines:

# BUILD - continued
cc_library(
  name = "mylib",
  hdrs = ["mylib.h"],
  srcs = ["mylib.cc"],
  defines = select({
    ":custom": ["MY_FLAG"],
    "//conditions:default": [],
  })
)

Now building the library using bazel build :mylib will result in the default case - no defines to be present, but if you build using bazel build :mylib --define enable_my_flag=true then the other branch will be selected and MY_FLAG will be defined.

This can be easily extended to the test case, for example by adding the --define to your .bazelrc:

# .bazelrc
test --define enable_my_flag=true

Now every time you run bazel test :mylib_test the define flag will be appended and the library will be built with MY_FLAG defined.

Out of curiosity why do you want to run the test on a library built with a different set of defines/dependencies? That might defeat the purpose of the test since in the end you're testing something different from the library you're going to use.