make find_libilo_a honour CARGO_TARGET_DIR and .cargo/config.toml

[danieljohnmorris]

Summary

The AOT linker helper find_libilo_a (in src/vm/compile_cranelift.rs) hard-coded $CARGO_MANIFEST_DIR/target/{release,debug}/libilo.a as its only search path. Fix worktrees that redirect cargo's target dir out of the tree, which is the convention we adopted for isolated parallel builds (.cargo/config.toml containing [build]\ntarget-dir = "/tmp/ilo-targets/<worktree-name>"), ended up with the staticlib at the redirected path while the lookup kept probing target/. Every aot_compile_* test failed with cannot find libilo.a, and each fix PR carried a ln -sf /tmp/ilo-targets/<name>/release/libilo.a target/release/libilo.a workaround.

That workaround was load-bearing in at least #499, #511, #506, and #512. It's fragile (litters the worktree, can race the build), and every future fix agent had to re-discover it. This PR makes the helper match where cargo actually puts the staticlib.

Repro before / after

In any worktree with .cargo/config.toml redirecting build.target-dir:

Before:

$ rm -f target/release/libilo.a
$ cargo test --release --features cranelift --lib compile_cranelift::tests::aot_compile_simple_multiply
...
thread '...' panicked at: cannot find libilo.a, build with `cargo build --release --features cranelift` first.

After (this worktree's target/release/libilo.a does not exist):

$ ls target/release/libilo.a
ls: target/release/libilo.a: No such file or directory
$ cargo test --release --features cranelift --lib compile_cranelift
...
test result: ok. 163 passed; 0 failed; 0 ignored

No symlink. The lookup reads .cargo/config.toml, finds target-dir = "/tmp/ilo-targets/ilo-fix-aot-lookup", and links from there.

What is in the diff

One commit (1c8fc33d):

• find_libilo_a probes in order: CARGO_TARGET_DIR env var, build.target-dir from $CARGO_MANIFEST_DIR/.cargo/config.toml, $CARGO_MANIFEST_DIR/target, workspace parent target/. Each step tries release then debug.
• Helpers extracted: cargo_config_target_dir (ad-hoc TOML parser, no new dep, looks only for target-dir = "..." inside [build]), libilo_a_in (probe one target dir).
• Unit tests for both helpers: double-quoted and single-quoted values, ignoring target-dir outside [build], missing-file behaviour, release-over-debug preference, debug fallback, no-match returns None.
• CHANGELOG entry under Unreleased > Changed.

The end-to-end integration test is the existing aot_compile_* suite plus tests/coverage_cranelift_compile.rs. This very worktree exercises the new lookup path: it redirects target-dir to /tmp/ilo-targets/ilo-fix-aot-lookup and links successfully without any symlink. I deliberately did not write a unit test that mutates CARGO_TARGET_DIR directly, because that would race other tests sharing the process; the helper-level tests plus the end-to-end suite cover it cleanly.

Test plan

• cargo build --release --features cranelift clean
• cargo test --release --features cranelift --lib compile_cranelift passes (163 tests, includes the new helper tests and all AOT tests)
• cargo test --release --features cranelift full suite passes (50 test binaries, 0 failed)
• cargo fmt --check clean
• cargo clippy --release --features cranelift --all-targets -- -D warnings clean
• Verified target/release/libilo.a does not exist in this worktree, AOT tests still link from /tmp/ilo-targets/ilo-fix-aot-lookup/release/libilo.a

Follow-ups

• None. Closes pending.md #5av.

[codecov]

Codecov Report

❌ Patch coverage is 84.61538% with 20 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/vm/compile_cranelift.rs	84.61%	20 Missing ⚠️

📢 Thoughts on this report? Let us know!