Add FuncVec convenience type

[alexreinking]

Add a FuncVec type that decays to Func in the n=1 case. This:

1. Allows one to write Func output = pipeline.outputs() when you know that your pipeline is single-output.
2. Sets the stage for future scheduling directives to return FuncVecs with the same single-func affordances (hoist_invariants in the future)
3. Simplifies the creation of groups of related funcs: see the apps/ changes.

Breaking changes

None — the FuncVec type inherits from std::vector without adding state.

Checklist

• Tests added or updated (not required for docs, CI config, or typo fixes)
• Documentation updated (if public API changed)
• Python bindings updated (if public API changed)
• Benchmarks are included here if the change is intended to affect performance.
• Commits include AI attribution where applicable (see Code of Conduct)

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[abadams]

What do you think about: name + "[" + std::to_string(i) + "]"

[alexreinking]

I'm not sure what [ would do in a Func name.

[alexreinking]

I think I prefer to keep f0, f1, ..., fN rather than burn two characters (on []) of std::string's short-string optimizations.

[shoaibkamil]

I think [ eventually gets legalized into ___ (three underscoeres)
except if the Func is used as an input/output generator, in which case it gets rejected. Seems safer to avoid [.

[abadams]

I really like e.g. gPyramid[3] because it reads way better in the profiler output and corresponds more closely to source code. That's what I've been using locally.

[abadams]

If we hard-reject it for output funcs though, that seems decisive

[abadams]

How about this: Have the constructor also take an optional suffix string. Right now it's a prefix string and a count. If it had a suffix too I could just say:

FuncVec v("foo[", 17, "]");

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.06%. Comparing base (ca8cb9a) to head (8ed11d4).

Files with missing lines	Patch %	Lines
src/Func.h	33.33%	2 Missing ⚠️

Additional details and impacted files

@@                      Coverage Diff                       @@
##           alexreinking/rfactor-hoisting    #9387   +/-   ##
==============================================================
  Coverage                          70.06%   70.06%           
==============================================================
  Files                                261      261           
  Lines                              79471    79483   +12     
  Branches                           19379    19382    +3     
==============================================================
+ Hits                               55683    55693   +10     
+ Misses                             17913    17911    -2     
- Partials                            5875     5879    +4     

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