How do I use compilation-transform-file-match-alist?

[Yuri Khan]

Hello,

I have a project that I compile using a Docker container. This part works.

I want to have multiple working copies of the project lying on my file
system, so that I could work on several branches at the same time
without losing context when switching.

Each working copy is naturally located under a different directory.

I want the working copy on the host to be mounted into a fixed
directory name in the container. This greatly simplifies efficient use
of ccache.

    /home/me/work/PROJECT/
        master/             -> /home/me/work/PROJECT/_branch
        foo-branch/
        bar-branch/

Compilation will sometimes output warnings and error messages. These
refer to files by their full absolute name in the container:

    /home/me/work/PROJECT/_branch/some/source.cpp:42: error:
some_identifier undefined

By default, compilation-mode will display those as is, and when I try
to visit the offending file, Emacs will not find it, because the
directory is different.

I have found a variable, compilation-transform-file-match-alist, that
looks like it will instruct Emacs to map file names according to a
list of regexp/replacement pairs. So I could, in theory, put
PROJECT/_branch on the left and PROJECT/master on the right, to get:

    /home/me/work/PROJECT/master/some/source.cpp:42: error:
some_identifier undefined

However, I seem to be unable to easily set it for compilation-mode
buffers. I cannot set it globally because I want different
replacements in PROJECT/master, PROJECT/foo-branch and
PROJECT/bar-branch. I naturally try to put it in
PROJECT/master/.dir-locals.el:

    ((nil . ((compilation-transform-file-match-alist .
              (("/PROJECT/_branch/" . "/PROJECT/master/"))))))

But it looks like compilation-mode buffers do not apply
directory-local variables.

How is the feature intended to be used?

I guess I can hook ‘compilation-mode-hook’ to invoke ‘hack-local-variables’?