Order-only dependencies on a generator

[lhearachel]

As part of a project that I help maintain, we generate some binary data files using an assembler and assembler macros, e.g.,

#include "macros/scrcmd.inc"
#include "res/text/gmm/message_bank_acuity_cavern.h"

    .data

    ScriptEntry _000E
    ScriptEntry _0014
    ScriptEntry _002F
    .short 0xFD13

    // truncated

We run each of these *.s files through the following generator:

field_script_gen = generator(make_script_bin_sh,
    arguments: make_script_bin_args,
    output: '@BASENAME@',
    depfile: '@[email protected]',
    depends: [
        message_banks_index, # TODO: Can this be an order-only dependency...?
        trdata_naix,
    ],
)

(Aside: the shell-script wrapper is so that we can execute an objcopy to extract the binary component of the assembler output, rather than effectively run one input file through two generators.)

As the TODO comment notes, we have a dependency on message_banks_index declared, which is a target declared earlier in the build-chain:

message_banks = custom_target('pl_msg.narc',
    output: [
        'pl_msg.narc',
        'pl_msg.naix',
    ],
    input: gmm_to_bin_gen.process(message_bank_files),
    command: [
        narc_exe, 'create',
        '--naix',
        '--order', message_banks_narc_order,
        '--output', '@OUTPUT0@',
        '@PRIVATE_DIR@',
    ]
)
message_banks_index = message_banks[1]

This target is listed in depends only to ensure that some generated content is built ahead of running the generator (in the example above, res/text/gmm/message_bank_acuity_cavern.h). Per the generator docs, this seems to be the right approach for declaring such a dependency.

However, this results in a change to any of the static files in message_bank_files to flag all of our assembly-generators as requiring rebuild; ideally, we would like to be able to declare message_banks_index as an order-only dependency and rely on the depfile to denote which specific headers are implicit dependencies. We could list all of the specific headers that are tracked, but there are at least a hundred such headers, and that solution feels brittle for any user who forks our project.

Is there a way to declare such a dependency as order-only for back-ends that support it? If not, where does that code live, and would it be a reasonable and/or welcome contribution?