[acecilia]

Problem

rb_binary launchers fail with a LoadError when they run with a working directory other than the bazel run default:

ruby: No such file or directory -- lib/gem/add.rb (LoadError)

This affects:

• bazel run --run_under="cd ... &&" ... (e.g. wrappers that preserve the caller's cwd)
• running the generated launcher binary directly (e.g. from an IDE or a script)

Root cause

generate_rb_binary_script only locates the main script through runfiles when it lives outside the main repository (../-prefixed short path). Mains in the main repository are baked into the launcher as raw relative paths, so they only resolve when the cwd matches bazel run's default (<launcher>.runfiles/_main).

Fix

Locate the main script through runfiles for non-Windows platforms regardless of repository, with a runtime fallback to the previous relative-path behavior:

• external-repository mains: unchanged (strict rlocation, fails with a hint if missing)
• main-repository mains: rlocation, falling back to the relative path when the lookup misses (e.g. a main not included in runfiles), so plain bazel run behaves exactly as before
• Windows: unchanged (raw path, see the existing runfiles-library caveat)

Related

• Generate convenience target alongside binary.sh target that changes the current working directory #115 asks for a convenience target that runs tools with the caller's working directory. This PR is a prerequisite for that feature: without runfiles-based main resolution, such a target would fail with a LoadError whenever the rb_binary main lives in the main repository.

Testing

• examples/gem: bazel run lib/gem:add-numbers 2 — unchanged ✅
• direct launcher invocation from / — previously LoadError, now prints 3 ✅
• bazel run --run_under="cd / &&" lib/gem:add-numbers -- 2 — now prints 3 ✅
• added a CI step to examples-gem (non-Windows) exercising direct launcher invocation from a foreign cwd
• bazel run :buildifier.check ✅
• bazel test --ignore_dev_dependency //ruby/tests:all ✅ (7/7)

Created with AI ✨ but reviewed thoroughly by me 👨‍💻