Improving diagnostic messages when invoking tasks involving a disabler object

[mitchgrout]

My project includes a pretty sizable number of external tools, many of which are optional (e.g. linters, formatters, bonus generation steps, ...). I make use of CI runners to run particular tasks, and I can't guarantee that all of them have the same configurations as each other. This means I have a lot of statements like, let's say...

doxygen = find_program('doxygen', required: false, disabler: true)
custom_target('doxygen', command: doxygen, output: 'index.html')

Running meson compile -C build doxygen, I can have one of two outputs:

1. If doxygen was found, then I get my output as expected
2. If not found, then I get ERROR: Can't invoke target doxygen: target not found

I understand why this output is present, but it is in my opinion uninformative, as it is indistinguishable from making a simple typo, like meson compile -C build Doxygen. To someone who is not familiar with the build configuration, it also only adds confusion in my experience.

A thought could be to attach to a disabler object a reason for being disabled, e.g. making the disabler() method signature into

disabler(
  str [reason] # The reason for the disabler being present
)

and adding disabler.reason() to extract the specified reason, e.g.

message(disabler().reason()) # 'no reason provided'
message(disabler('program not found').reason()) # 'program not found'

A disabler generated by a failed find_program call could then set its reason to something informative such as...

doxygen = find_program('doxygen', required: false, disabler: true)
if is_disabler(doxygen)
  message(doxygen.reason()) # 'program "doxygen" not found'
endif

and rather than not emitting a target due to a disabler being present, providing an always-failing task which emits the reason from the disabler.

---

As it stands, in order to get useful output, I need to copy-paste code blobs like...

doxygen = find_program('doxygen', required: false)
if not doxygen.found()
  command = [find_program('fail.sh'), 'program "doxygen" not found']
else
  command = doxygen
endif

custom_target('doxygen', command: command, output: 'index.html')

such that if I run meson compile -C build doxygen without the tool available, I get understandable diagnostics.