refactor(hugr-llvm)!: Optimise the llvm types used to represent hugr sums.

[doug-q]

We rework LLVMSumType and LLVMValueType to have llvm representations that

• elide the tag field when the sum type has only one variant
• elide any "uninformational" fields, i.e. Type::UNIT.
• elide the struct wrapper when there is only a single field
• reuse fields from other variants where possible.

In particular, the hugr bool_t() is now represented by i1.

BREAKING CHANGE: LLVMSumValue::get_tag_type renamed to tag_type. LLVMSumType::try_new2 renamed to LLVMSumType::try_new. LLVMSumType::get_variant removed.

[hugrbot]

This PR contains breaking changes to the public Rust API.

cargo-semver-checks summary

--- failure inherent_method_missing: pub method removed or renamed ---

Description:
A publicly-visible method or associated fn is no longer available under its prior name. It may have been renamed or removed entirely.
      ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
     impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.38.0/src/lints/inherent_method_missing.ron

Failed in:
LLVMSumValue::get_tag_type, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-llvm/src/sum.rs:283
LLVMSumType::try_new2, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-llvm/src/sum.rs:35
LLVMSumType::get_tag_type, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-llvm/src/sum.rs:123
LLVMSumType::get_variant, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-llvm/src/sum.rs:139
LLVMSumType::try_new2, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-llvm/src/sum.rs:35
LLVMSumType::get_tag_type, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-llvm/src/sum.rs:123
LLVMSumType::get_variant, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-llvm/src/sum.rs:139

[doug-q]

@peter-campora I made this for you. Would you mind reviewing?

[zrho]

LGTM but I currently lack context to find anything subtle here.

[qartik]

The LLVM output looks a lot more readable now!

[mark-koch]

Very nice, the LLVM is a lot more readbale now!

My only real concerns are empty sums and my comment in build_untag

Also, it took me a bit to wrap my head around what's going on in layout_variants_impl. Left a few refactoring suggestions, but that's up to you

[doug-q]

Great review, thank you @mark-koch .

[codecov]

Codecov Report

Attention: Patch coverage is 85.93407% with 64 lines in your changes missing coverage. Please review.

Project coverage is 86.55%. Comparing base (b38a2e8) to head (55932b7).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
hugr-llvm/src/sum.rs	86.04%	33 Missing and 9 partials ⚠️
hugr-llvm/src/extension/collections/array.rs	20.00%	1 Missing and 11 partials ⚠️
hugr-llvm/src/sum/layout.rs	95.41%	6 Missing ⚠️
hugr-llvm/src/emit/ops.rs	0.00%	0 Missing and 3 partials ⚠️
hugr-llvm/src/extension/collections/list.rs	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1855      +/-   ##
==========================================
+ Coverage   86.52%   86.55%   +0.02%     
==========================================
  Files         194      195       +1     
  Lines       35250    35569     +319     
  Branches    32063    32382     +319     
==========================================
+ Hits        30501    30786     +285     
- Misses       2973     3011      +38     
+ Partials     1776     1772       -4     

Flag	Coverage Δ
python	92.34% <ø> (ø)
rust	85.98% <85.93%> (+0.03%)	⬆️

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[doug-q]

@mark-koch I believe I've address all of your comments. I split layout_variants_impl as you suggested and I think it turned out quite a bit better.

I have added test cases for emission build_get_tag and build_untag for each of the flavours of sum type.

I have restored handling for void sums, but I think we might need a bit more.

[mark-koch]

Thanks Doug, looks good!

However, something seems off in the layout since tests are failing the asserts?