[build2] Execute a test of a library with a macro enabled

[Boris Kolpackov]

artur.brzozowski via users <users at build2.org> writes:


> I am trying out build2 and I have to say the first impression is that
> of a very elegant tool.

Thank you, I am glad to hear that.


> Libsomething has some conditional compilation done with a preprocessor macro
> (say, `#ifdef TRACE`) in "something.hxx". I want to have TRACE defined for
> libsomething when I run `b test`.
> 
> ``` shell
> $ cat libsomething/tests/basics/buildfile
> import libs = libsomething%lib{something}
> 
> cxx.poptions += -DTRACE ## ADDED
> 
> exe{driver}: {hxx ixx txx cxx}{**} $libs
> ```
> 
> Simply adding the `cxx.poptions`, like above, does not seem to enable
> TRACE for libsomething for testing.

Right, this will define TRACE when compiling the test driver, but not
the imported library.

And this is by design since, at least in the general case, consumers
of the library imposing arbitrary (and potentially incompatible)
configuration on the library is not something that we would want.

As an example, imagine libsomething is imported by another project (say,
an application that uses it) and both the tests and this project are
built simultaneously (which is common in build2). Now, because the tests
enabled tracing, the application will also end up using libsomething
with tracing, which is probably not something that we want, at least
not implicitly like this.

Instead, the recommended way of dealing with something like this is
to make it explicit via a configuration variable. There is a section
in the packaging guide with some background:

https://build2.org/build2-toolchain/doc/build2-toolchain-packaging.xhtml#howto-debug-macro

With this approach, if you want to stick with the build2 build system
only, then you will need to configure your project with tracing enabled
manually. But if you want to wade into packaging, then we have support
for packages negotiating the desired configuration of their dependencies.
With this mechanism, you could create a separate tests package that
automatically configures libsomething with tracing enabled. See this
HOWTO article on separate tests packages:

https://github.com/build2/HOWTO/blob/master/entries/handle-tests-with-extra-dependencies.md