feat: Add TailLoop::BREAK_TAG and CONTINUE_TAG

[acl-cqc]

It doesn't seem obvious to me which way round these should be, so add constants, and use for the "definitive" make_break and make_continue.

No change to python as this doesn't even have make_break or make_continue ATM.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.78%. Comparing base (9962f97) to head (10e821e).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1626   +/-   ##
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  Coverage   85.78%   85.78%           
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  Files         136      136           
  Lines       25135    25143    +8     
  Branches    22061    22069    +8     
=======================================
+ Hits        21561    21569    +8     
  Misses       2425     2425           
  Partials     1149     1149           

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python	92.45% <ø> (ø)
rust	84.85% <100.00%> (+<0.01%)	⬆️

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[mark-koch]

Looks good to me, AFAIK the Python builder doesn't even have make_continue and make_break.

Should we use these CONTINUE_TAG and BREAK_TAG constants also during validation?

[acl-cqc]

Should we use these CONTINUE_TAG and BREAK_TAG constants also during validation?

Yes, I guess so - but I'm looking for validation code that uses constant 0 / 1 for tags/tail-loops and not finding anything. If you can point me where you're looking then I'd be happy to fix?

[acl-cqc]

As per discussion on Slack - the other change we could make would be at

hugr/hugr-core/src/ops/controlflow.rs

Line 44 in c987d2f

let sum_type = Type::new_sum([self.just_inputs.clone(), self.just_outputs.clone()]);

where we could instead do something like

let mut sum_rows = [type_row![], type_row![]];
sum_rows[BREAK_TAG] = self.just_outputs.clone();
sum_rows[CONTINUE_TAG] = self.just_inputs.clone();

but we deem this "not worth it".