Prepend project bin/ to PATH in bundle exec for load-relative Ruby

[masak1yu]

Summary

When Ruby is compiled with --enable-load-relative (as done by mise), RubyGems generates binstubs with a #!/bin/sh shell wrapper that resolves ruby relative to the binstub directory:

bindir="${0%/*}"
exec "$bindir/ruby" "-x" "$0" "$@"

When BUNDLE_PATH is set to vendor/bundle, set_path prepends vendor/bundle/ruby/X.Y.Z/bin to PATH. bundle exec then finds the vendor binstub first via Bundler.which, but since its shebang is #!/bin/sh (not #!/usr/bin/env ruby), ruby_shebang? returns false and Bundler falls back to kernel_exec. The OS executes the shell wrapper, which looks for $bindir/ruby in the vendor bin directory — where no ruby binary exists — and fails:

vendor/bundle/ruby/3.3.0/bin/rspec: line 6: vendor/bundle/ruby/3.3.0/bin/ruby: No such file or directory

Fix

This PR prepends the project bin/ directory (where Bundler generates binstubs with #!/usr/bin/env ruby via env_shebang: true) to PATH before the vendor bin path in SharedHelpers#set_path. This ensures Bundler.which finds the correctly-shimmed project binstub first, allowing ruby_shebang? to return true and kernel_load to execute the command in-process.

The project bin/ is only added when the directory actually exists, avoiding side effects for projects without binstubs.

Root Cause Analysis

Three factors combine to produce this bug:

1. --enable-load-relative Ruby (mise builds): LIBRUBY_RELATIVE=yes triggers bash_prolog_script in Gem::Installer#shebang, wrapping every vendor binstub in a #!/bin/sh prologue
2. set_path only prepends vendor bin: vendor/bundle/ruby/X.Y.Z/bin is added to PATH but bin/ is not, so the broken vendor binstub is found first
3. ruby_shebang? doesn't match #!/bin/sh: The shell wrapper falls through to kernel_exec, which executes it as a shell script that fails to find $bindir/ruby

Related Issues

• Bundler tries to use wrong version of ruby #8441

Test Plan

• Added spec: project bin/ directory is prepended to PATH before vendor bundle bin when it exists
• Added spec: project bin/ is not added to PATH when the directory doesn't exist
• Verified manually: bundle exec rspec works on a project with BUNDLE_PATH=vendor/bundle and mise Ruby 3.3.10