[bug #47715] Implicit rule defined for multiple targets at once only runs one time

[Tristan Wibberley]

Follow-up Comment #6, bug #47715 (project make):

Please find here detailed thoughts on the documentation to cover this
situation.

I will add to https://savannah.gnu.org/bugs/?42125 with a suggested mechanism
to choose the behaviour of multiple targets that I think is probably
straightforward to implement and generally useful. I will be heppy to try to
implement it myself given a little pointer to the right places to change.


I definitely saw these nodes to find an explanation after encountering the
issue:

-
https://www.gnu.org/software/make/manual/html_node/Multiple-Targets.html#Multiple-Targets
-
https://www.gnu.org/software/make/manual/html_node/Static-versus-Implicit.html#Static-versus-Implicit
- various content lists in a few nodes.

I would have seen this node previously when learning how to use make in detail
but that was a long time ago, but I was searching contents lists for
"multiple" because I thought there would be a detailed section as for general
rules and static pattern rules.

-
https://www.gnu.org/software/make/manual/html_node/Pattern-Intro.html#Pattern-Intro


I didn't see the relevant information in the pattern rules node, I think a few
reasons combined to make me disregard it - this is even despite me thinking in
the back of my mind that there was something about multiple targets in
implicit rules:

- I'd learned the term "Implicit Rule" and searched through the contents for
that term to find more information

- The relevant paragraph in the Pattern Rules section is dense and doesn't
have an example of this behaviour in a form like the other examples in the
documentation which I've become accustomed to.

- The section that compares static pattern rules to implicit rules didn't
mention this as a difference yet it is a big and confusing difference and the
behaviour of static rules is demonstrated with clear examples.

- I felt like I have been learning a lot of make and pushing the limit of what
is normal and my experience with software is that there are odd behaviours in
such an endeavour that would normally be bugs.


Suggested changes to documentation:

- Add a section, if not already present, named "Multiple Targets in a Rule" to
each rule type that supports them with a worked example for each.
          I think "each" means "general", "static pattern", "implicit pattern"
but not "suffix".

- Add an overall section on Multiple Targets in a Rule, if not already
present. Make it briefly compare, in text only, the behaviour of each type of
rule and link to the multiple rules section of each type.

- Add a table-structured comparison of features against rule types as a brief
mnemonic.

- If the behaviour of multiple rules for one target has similar differences
then maybe do the same for that so the same confusion doesn't happen in the
future there either since this would be the cheap moment to make such a
change.

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