Install gems into separate BUNDLE_PATH and use bundle exec

examples/gem/lib/gem/BUILD

@@ -23,6 +23,7 @@ rb_binary(
     main = "add.rb",
     deps = [
         ":add",
+        "//:gem",
         "@bundle",
     ],
 )

ruby/private/binary.bzl

@@ -37,7 +37,7 @@ Use a built-in `args` attribute to pass extra arguments to the script.
     ),
 }
 
-def generate_rb_binary_script(ctx, binary, args = []):
+def generate_rb_binary_script(ctx, binary, bundler = False, args = []):
     windows_constraint = ctx.attr._windows_constraint[platform_common.ConstraintValueInfo]
     is_windows = ctx.target_platform_has_constraint(windows_constraint)
     toolchain = ctx.toolchains["@rules_ruby//ruby:toolchain_type"]
@@ -57,6 +57,11 @@ def generate_rb_binary_script(ctx, binary, args = []):
         script = ctx.actions.declare_file("{}.rb.sh".format(ctx.label.name))
         template = ctx.file._binary_sh_tpl
 
+    if bundler:
+        bundler_command = "bundle exec"
+    else:
+        bundler_command = ""
+
     args = " ".join(args)
     args = ctx.expand_location(args)
 
@@ -68,15 +73,16 @@ def generate_rb_binary_script(ctx, binary, args = []):
             "{args}": args,
             "{binary}": binary_path,
             "{toolchain_bindir}": toolchain_bindir,
+            "{bundler_command}": bundler_command,
             "{ruby_binary_name}": toolchain.ruby.basename,
         },
     )
 
     return script
 
 def rb_binary_impl(ctx):
+    bundler = False
     env = {}
-    script = generate_rb_binary_script(ctx, ctx.executable.main)
     transitive_data = get_transitive_data(ctx.files.data, ctx.attr.deps).to_list()
     transitive_srcs = get_transitive_srcs(ctx.files.srcs, ctx.attr.deps).to_list()
     tools = []
@@ -94,9 +100,16 @@ def rb_binary_impl(ctx):
         if file.basename == "Gemfile":
             env["BUNDLE_GEMFILE"] = file.path
 
+    for dep in ctx.attr.deps:
+        if dep.label.workspace_name == "bundle":
+            bundler = True
+            env["BUNDLE_PATH"] = dep.label.workspace_root
+
     runfiles = ctx.runfiles(transitive_data + transitive_srcs + tools)
     env.update(ctx.attr.env)
 
+    script = generate_rb_binary_script(ctx, ctx.executable.main, bundler)
+
     return [
         DefaultInfo(
             executable = script,

ruby/private/binary/binary.cmd.tpl

@@ -1,2 +1,2 @@
 @set PATH={toolchain_bindir};%PATH%
-@{ruby_binary_name} {binary} {args} %*
+@{bundler_command} {ruby_binary_name} {binary} {args} %*

ruby/private/binary/binary.sh.tpl

@@ -1,4 +1,4 @@
 #!/usr/bin/env bash
 
 export PATH={toolchain_bindir}:$PATH
-{ruby_binary_name} {binary} {args} $@
+{bundler_command} {ruby_binary_name} {binary} {args} $@

ruby/private/bundle.bzl

@@ -6,6 +6,7 @@ _BINSTUB_CMD = """@ruby -x "%~f0" %*
 
 def _rb_bundle_impl(repository_ctx):
     binstubs_path = repository_ctx.path("bin")
+    bundle_path = repository_ctx.path(".")
     gemfile = repository_ctx.path(repository_ctx.attr.gemfile)
     toolchain_path = repository_ctx.path(Label("@rules_ruby_dist//:BUILD")).dirname
 
@@ -37,6 +38,7 @@ def _rb_bundle_impl(repository_ctx):
             "BUNDLE_BIN": repr(binstubs_path),
             "BUNDLE_GEMFILE": gemfile.basename,
             "BUNDLE_IGNORE_CONFIG": "1",
+            "BUNDLE_PATH": repr(bundle_path),
             "BUNDLE_SHEBANG": repr(ruby),
             "PATH": path_separator.join([repr(ruby.dirname), repository_ctx.os.environ["PATH"]]),
         },

ruby/private/gem_push.bzl

@@ -14,8 +14,9 @@ def _rb_gem_push_impl(ctx):
 
     script = generate_rb_binary_script(
         ctx,
-        ruby_toolchain.gem,
-        ["push", ctx.file.gem.short_path],
+        binary = ruby_toolchain.gem,
+        bundler = False,
+        args = ["push", ctx.file.gem.short_path],
     )
 
     runfiles = ctx.runfiles(srcs + tools)