BE-512: HashQL: Allow cross-backend transitions for SwitchInt terminators

[indietyp]

🌟 What is the purpose of this PR?

SwitchInt terminators were previously blocked from generating cross-backend transitions during terminator placement, based on an assumption that coordinating branches across backends would be too complex. This restriction has been lifted, allowing SwitchInt to behave the same as Goto and permit cross-backend transitions. This enables more flexible execution placement for conditional branching logic.

🔍 What does this change?

• SwitchInt now calls add_goto_transitions instead of being a no-op, enabling cross-backend edge costs to be populated for blocks ending with a SwitchInt terminator.
• The test switchint_blocks_cross_backend has been renamed to switchint_allows_cross_backend and updated to assert that the Interpreter -> Embedding transition is now reachable with the expected cost.
• The switchint_edge_targets_are_branch_specific test has been updated to assert that cross-backend transitions are present rather than absent.
• The terminator placement snapshot has been updated to reflect the newly populated I->E and P->E transitions on edge[1].

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:

• are internal and do not require a docs change

🕸️ Does this require a change to the Turbo Graph?

The changes in this PR:

• do not affect the execution graph

🛡 What tests cover this?

• Updated unit tests in terminator_placement/tests.rs covering SwitchInt cross-backend transition behaviour.
• Updated snapshot test reflecting the new edge cost matrix output.

❓ How to test this?

1. Run cargo test in libs/@local/hashql/mir.
2. Confirm all terminator placement tests pass, including the renamed switchint_allows_cross_backend test.

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Project	Deployment	Actions	Updated (UTC)
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[cursor]

PR Summary

Medium Risk
Changes execution-planning cost computation for conditional branches, which could alter backend selection and runtime behavior across many queries. Logic change is small and covered by updated unit and snapshot tests, but affects a core planning pass.

Overview
SwitchInt terminators now permit cross-backend transitions during terminator placement by treating them like Goto when populating per-edge transition matrices.

Tests were updated to reflect the new reachable I->E/P->E transitions (including renamed switchint_allows_cross_backend) and the UI snapshot was regenerated to include the newly populated edge cost entries.

^{Reviewed by Cursor Bugbot for commit 026ce7b. Bugbot is set up for automated code reviews on this repo. 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-513: HashQL: Rework dynamic aggregate size estimation #8697
• BE-512: HashQL: Allow cross-backend transitions for SwitchInt terminators #8696 👈 (View in Graphite)
• BE-524: HashQL: Remove island schedule module #8694
• BE-525: HashQL: Remove implicit type-widening from Integer operations, and make comparison size aware #8693
• BE-515: HashQL: Remove OFFSET 0 materialization fence from generated query #8650
• BE-514: HashQL: implement iterative adjustment passes in placement solver #8635
• BE-500, BE-501: HashQL: Unify mixed parameter resolution in data dependency analysis #8607
• BE-494: HashQL: Loop-breaker selection for recursive inlining #8600
• BE-482: HashQL: Remove logical not from MIR and fix postgres boolean lowering #8595
• BE-474: HashQL: Take into account terminator target eligibility in execution backend placement #8587
• BE-455: HashQL: Introduce evaluation orchestrator #8586
• BE-523: HashQL: Suspendable interpreter with structured values and input system #8675
• BE-522: HashQL: Expand stdlib type system with temporal types #8674
• BE-306: HashQL: PostgreSQL translation #8526
• BE-457: HashQL: MIR execution pipeline extensions for postgres compilation #8525
• BE-456: HashQL: Entity type model enrichment and graph store query extensions #8524
• BE-429: HashQL: Add island dependency graph with data requirement resolution #8501
• BE-428: HashQL: Simplify traversal tracking to path recording #8498
• BE-434: HashQL: Refactor entity storage path resolution with centralized schema mapping #8496
• BE-427: Add hashql-macros crate with procedural macros for ID types and symbols #8495
• main

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

[augmentcode]

🤖 Augment PR Summary

Summary: Enables cross-backend transitions for blocks ending in SwitchInt terminators, aligning their behavior with Goto during terminator placement.
Changes: Updates the edge-cost matrix population logic and adjusts unit/snapshot tests to assert newly reachable cross-backend paths with expected costs.

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[augmentcode]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.43%. Comparing base (bb97a73) to head (026ce7b).

Additional details and impacted files

@@                          Coverage Diff                           @@
##           bm/be-524-hashql-remove-island-dag    #8696      +/-   ##
======================================================================
- Coverage                               63.43%   63.43%   -0.01%     
======================================================================
  Files                                    1382     1382              
  Lines                                  144756   144753       -3     
  Branches                                 5902     5902              
======================================================================
- Hits                                    91823    91820       -3     
  Misses                                  51974    51974              
  Partials                                  959      959              

Flag	Coverage Δ
apps.hash-ai-worker-ts	1.41% <ø> (ø)
apps.hash-api	0.00% <ø> (ø)
local.hash-backend-utils	2.81% <ø> (ø)
local.hash-graph-sdk	9.63% <ø> (ø)
local.hash-isomorphic-utils	0.00% <ø> (ø)
rust.hash-graph-api	2.52% <ø> (ø)
rust.hashql-compiletest	28.26% <ø> (ø)
rust.hashql-eval	79.70% <ø> (ø)
rust.hashql-mir	91.83% <100.00%> (-0.01%)	⬇️

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