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

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

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, 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.

This fix prepends the project `bin/` directory (where Bundler generates
binstubs with `#!/usr/bin/env ruby`) to PATH before the vendor bin
path, so `Bundler.which` finds the correctly-shimmed binstub first.
The project bin/ is only added when the directory actually exists.

Fixes an interaction between load-relative Ruby builds (mise, some
asdf configurations) and `BUNDLE_PATH=vendor/bundle`.

Related: #8441

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: masak1yu

bundler/lib/bundler/shared_helpers.rb

@@ -342,6 +342,9 @@ def set_path
       validate_bundle_path
       paths = (ENV["PATH"] || "").split(File::PATH_SEPARATOR)
       paths.unshift "#{Bundler.bundle_path}/bin"
+      bin_dir = Bundler.settings[:bin] || "bin"
+      bin_path = Pathname.new(bin_dir).expand_path(Bundler.root)
+      paths.unshift bin_path.to_s if bin_path.directory?
       Bundler::SharedHelpers.set_env "PATH", paths.uniq.join(File::PATH_SEPARATOR)
     end
 

spec/bundler/shared_helpers_spec.rb

@@ -343,6 +343,37 @@
       it_behaves_like "ENV['PATH'] gets set correctly"
     end
 
+    context "when project bin/ directory exists" do
+      before do
+        Dir.mkdir bundled_app(".bundle")
+        Dir.mkdir(bundled_app("bin")) unless File.directory?(bundled_app("bin"))
+        ENV["PATH"] = "/usr/bin"
+      end
+
+      it "prepends project bin/ to ENV['PATH'] before bundle path bin" do
+        subject.set_bundle_environment
+        paths = ENV["PATH"].split(File::PATH_SEPARATOR)
+        bundle_bin_index = paths.index("#{Bundler.bundle_path}/bin")
+        project_bin_index = paths.index(bundled_app("bin").to_s)
+        expect(project_bin_index).not_to be_nil
+        expect(project_bin_index).to be < bundle_bin_index
+      end
+    end
+
+    context "when project bin/ directory does not exist" do
+      before do
+        Dir.mkdir bundled_app(".bundle")
+        FileUtils.rm_rf(bundled_app("bin"))
+        ENV["PATH"] = "/usr/bin"
+      end
+
+      it "does not prepend project bin/ to ENV['PATH']" do
+        subject.set_bundle_environment
+        paths = ENV["PATH"].split(File::PATH_SEPARATOR)
+        expect(paths).not_to include(bundled_app("bin").to_s)
+      end
+    end
+
     context "ENV['PATH'] already contains the bundle bin path" do
       let(:bundle_path) { "#{Bundler.bundle_path}/bin" }