feat(kotlin): Karbine ABI compile-avoidance (opt-in) — closes #11

[Sam Gammon (sgammon)]

Implements #11: consume Elide's Karbine --abi-only capability so dependents key on a body-stable ABI header jar, pruning rebuilds on method-body edits.

What

New opt-in build setting //config/kotlinc:abi_compile_avoidance (default off). When enabled, a kt-only elide_kotlin_* target's JavaInfo.compile_jar comes from a dedicated, codegen-free elide kotlinc --abi-only header action instead of java_common.run_ijar:

• Body edit → byte-identical header → dependents pruned. Replaces the body-sensitive Kotlin run_ijar jar (the "Bug 1" cascade).
• Critical-path collapse: dependents wait on the fast header action, not the full compile (which runs in parallel).
• Sound: const/inline/signature changes still change the header, so dependents that need to rebuild still do.

Scope

elide kotlinc --abi-only emits Kotlin ABI only, so mixed kt+java targets keep the run_ijar jar (correct, just not pruned). Mixed-source support filed upstream as WHIPLASH#1149.

Tests (per the issue's "verify empirically" ask)

• Analysis tests (//tests, CI-gating): flag-on emits the ElideKotlincAbi --abi-only action and wires compile_jar to the _abi.jar; mixed kt+java does not take the abi path and keeps the ijar.
• e2e/abi_avoidance/run.sh (real-elide soundness harness): asserts body & default-arg edits keep the header identical, while const/inline/signature edits change it — plus the two --abi-only behaviors the rule relies on (accepts -d <jar>; Kotlin-only on mixed sources). All 7 assertions green against 1.3.4+20260623.

Default-off, so zero behavior change until opted in. //tests/... 35/35.

[codspeed-hq]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

✅ 3 untouched benchmarks

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_{Comparing fix/issue-11 (963cee2) with main (1bff695)}

[Copilot]

Pull request overview

Adds an opt-in Kotlin build optimization that leverages Elide Karbine’s kotlinc --abi-only mode to produce a body-stable ABI/header jar for Kotlin-only targets, allowing Bazel dependents to key off the header jar and avoid rebuild cascades on method-body-only edits.

Changes:

• Introduces //config/kotlinc:abi_compile_avoidance (default off) to enable ABI compile-avoidance for Kotlin-only targets.
• Implements a dedicated ElideKotlincAbi header action in run_kotlinc and wires JavaInfo.compile_jar to the produced _abi.jar when enabled (fallback to run_ijar for mixed kt+java).
• Adds analysis tests validating action emission/wiring behavior and an e2e/abi_avoidance harness for empirical soundness checks.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/kotlin_rule_test.bzl	Adds analysis tests covering the ABI header action and mixed-source fallback behavior.
elide/rules/kotlin.bzl	Routes Kotlin rule JavaInfo.compile_jar through a new compile_jar_override returned from run_kotlinc.
elide/private/compile_common.bzl	Implements --abi-only header action generation and supports overriding compile_jar in make_java_info.
e2e/abi_avoidance/run.sh	Adds a local soundness script validating header stability vs ABI-affecting edits and key --abi-only behaviors.
e2e/abi_avoidance/README.md	Documents the ABI avoidance soundness harness and how/why to run it.
config/kotlinc/BUILD.bazel	Adds the new abi_compile_avoidance build setting.

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