MPR#4958: negative flags

[damiendoligez]

PR transferred from https://caml.inria.fr/mantis/view.php?id=4958
[original reporter: @ygrek]

Provide the ability to specify negative flags, e.g.

flag_and_dep ["tag1","tag2"] ~not:["tag3"] cmd;

cmd is activated if tag1 and tag2 are set, and tag3 is not set

It will make the fix for #225 (MPR#4943) more simple and robust.

[gasche]

I am worried of the unplanned consequences of losing the monotonicity guarantee on flag declaration semantics. I would rather not do this unless there is evidence we cannot do without.

[ygrek]

can you elaborate more on monotonicity guarantee please?

[gasche]

Currently, we have the guarantee that adding more tags to a file will only enable more flags to be applied when compiling the file.

That said, thinking about it more, it is not clear-cut that we rely on it today. One situation I had in mind was the ability to process tags in several batches instead of just one, in fact but this does not affect flag declarations that are computed at build time, not a tag-declaration time. Reopening.

[ygrek]

I also think this is related to the tags (not) being delarative - e.g. #153