BE-258: HashQL: Add Changed enum to track MIR transformation pass modifications

[indietyp]

Add Changed enum to MIR transform passes for tracking modifications

🌟 What is the purpose of this PR?

This PR adds a Changed enum to MIR transform passes to track whether a pass modified the code. This enables more efficient pass pipelines by allowing subsequent passes to be skipped when no changes were made.

🔍 What does this change?

• Adds a new Changed enum (Yes, No, Unknown) that transform passes return to indicate modifications
• Updates all transform pass implementations to return the appropriate Changed value
• Modifies test harnesses to capture and display the Changed value in snapshots
• Updates documentation to describe the new feature in the testing guide
• Improves benchmarks to track and propagate change status through pass pipelines

🛡 What tests cover this?

• All existing transform pass tests have been updated to include the Changed value in snapshots
• The test harness now verifies the correct Changed value is returned

Pre-Merge Checklist 🚀

🚢 Has this modified a publishable library?

This PR:

• does not modify any publishable blocks or libraries, or modifications do not need publishing

📜 Does this require a change to the docs?

The changes in this PR:

• require changes to docs which are made as part of this PR

🕸️ Does this require a change to the Turbo Graph?

The changes in this PR:

• do not affect the execution graph

[cursor]

PR Summary

Enables pass-level change tracking across MIR by returning Changed from transforms and surfacing it in tests and benchmarks.

• Add Changed enum with helpers (conservative, optimistic, From<bool>) and change TransformPass::run to return it
• Update passes (cfg_simplify, sroa, inst_simplify, dse, dbe, cp, ssa_repair) to detect and return Changed; early-exit with No where applicable
• Adjust compiletest suites and unit test harnesses to capture and snapshot Changed (new separator in .snap files)
• Update benchmarks to propagate/max Changed across pipeline stages
• Documentation: testing skill and MIR builder guide updated; reference paths switched to references/*

^{Written by Cursor Bugbot for commit 65c3381. This will update automatically on new commits. Configure here.}

[indietyp]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• BE-227: HashQL: Implement call graph analysis for MIR #8214
• BE-258: HashQL: Add Changed enum to track MIR transformation pass modifications #8210 👈 (View in Graphite)
• BE-255: HashQL: Implement Copy Propagation (CP) pass for MIR #8207
• BE-231: Add InstSimplify pass for MIR instruction simplification and constant folding #8206
• BE-254: Add CodSpeed benchmarking for HashQL MIR transformation passes #8204
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[codecov]

Codecov Report

❌ Patch coverage is 89.79592% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.15%. Comparing base (049e36c) to head (65c3381).

Files with missing lines	Patch %	Lines
libs/@local/hashql/mir/src/pass/mod.rs	23.07%	10 Missing ⚠️

Additional details and impacted files

@@                             Coverage Diff                             @@
##           bm/be-255-hashql-add-copy-propagation-pass    #8210   +/-   ##
===========================================================================
  Coverage                                       59.14%   59.15%           
===========================================================================
  Files                                            1197     1197           
  Lines                                          113673   113726   +53     
  Branches                                         5056     5060    +4     
===========================================================================
+ Hits                                            67234    67275   +41     
- Misses                                          45663    45675   +12     
  Partials                                          776      776           

Flag	Coverage Δ
rust.hashql-compiletest	46.65% <ø> (ø)
rust.hashql-mir	88.23% <89.79%> (-0.07%)	⬇️

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[codspeed-hq]

CodSpeed Performance Report

Merging #8210 will improve performance by ×2.7

_{Comparing bm/be-258-hashql-implement-transformationpass-changed-detection (65c3381) with bm/be-255-hashql-add-copy-propagation-pass (049e36c)}

Summary

⚡ 7 improvements
✅ 10 untouched

Benchmarks breakdown

	Benchmark	BASE	HEAD	Efficiency
⚡	complex	25.3 µs	11.8 µs	×2.1
⚡	diamond	20.7 µs	7.8 µs	×2.7
⚡	complex	18.9 µs	8 µs	×2.4
⚡	diamond	15.5 µs	7.3 µs	×2.1
⚡	diamond	49.8 µs	38.2 µs	+30.54%
⚡	linear	11.9 µs	7 µs	+70.67%
⚡	complex	67.9 µs	55.5 µs	+22.49%