feat: Array indexing

[mark-koch]

This is the feature branch for array indexing. Tracked by #253.

The goal is to allow array indexing in @inout positions that ignores linearity constraints in the indices:

qs: array[qubit, 42]
cx(qs[0], qs[1])  # Ok
cx(qs[0], qs[0])  # Compiles, but will panic at runtime
q = qs[0]  # Error: Indexing only allowed in inout position

This is achieved by lowering array[qubit] to array[qubit | None] and making array.__getitem__ and array.__setitem__ swap in None whenever a qubit is taken out or put back in. The functions array.__getitem__ and array.__setitem__ take the array as @inout, so it suffices to apply the following desugaring:

cx(qs[expr1], qs[expr2])

# Desugars to
idx1 = expr1
idx2 = expr2
tmp1 = qs.__getitem__(idx1)
tmp2 = qs.__getitem__(idx2)
cx(tmp1, tmp2)
qs.__setitem__(tmp1, idx1)
qs.__setitem__(tmp2, idx2)

This also means that arrays containing structs might behave slightly unexpectedly:

@guppy.struct
class QubitPair:
    q1: qubit
    q2: qubit

ps: array[QubitPair, 42]
cx(ps[0].q1, ps[0].q2)  # Panics at runtime :(

# Since it desugars to
idx1 = 0
idx2 = 0
tmp1 = ps.__getitem__(idx1)
tmp2 = ps.__getitem__(idx2)  # Panic: Struct at index 0 has already been replaced with None
cx(tmp1.q1, tmp2.q2)
...

To solve this, we need to change the Hugr lowering of arrays to structually replace any occurrence of qubit with qubit | None, i.e. instead of doing array[QubitPair | None], we would need to do array[tuple[qubit | None, qubit | None]]. I'll leave this to a future PR in case we are interested in that.

The following feature PRs target this branch:

• feat: Type check array subscripts #420
• feat: Update linearity checker to handle subscripts #421
• feat: Hugr lowering for array indexing and add integration tests #422
• feat: Lower linear array elements to optionals #447

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.70330% with 6 lines in your changes missing coverage. Please review.

Project coverage is 92.70%. Comparing base (553ec51) to head (85dcceb).

Files with missing lines	Patch %	Lines
guppylang/checker/core.py	88.00%	3 Missing ⚠️
guppylang/prelude/_internal/compiler.py	95.31%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #415      +/-   ##
==========================================
+ Coverage   92.59%   92.70%   +0.10%     
==========================================
  Files          49       49              
  Lines        5363     5525     +162     
==========================================
+ Hits         4966     5122     +156     
- Misses        397      403       +6     

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[ss2165]

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