feat: support env from a bzl file in bundle_fetch (env_bzl)

ruby/extensions.bzl

@@ -30,6 +30,12 @@ ruby_bundle_fetch = tag_class(attrs = {
     "name": attr.string(doc = "Resulting repository name for the bundle"),
     "srcs": attr.label_list(),
     "env": attr.string_dict(),
+    "env_bzl": attr.label(doc = """\
+Optional .bzl file exporting a dict named `bundle_env`, merged into the generated \
+rb_bundle_install `env`. Unlike `env` (static strings), values are resolved at \
+BUILD-evaluation time, so they can come from generated repositories (e.g. a \
+machine-resolved toolchain path).\
+""",),
     "extra_args": attr.string_list(doc = "Extra arguments appended to `bundle install`. Supports `$(location ...)` against `data`."),
     "data": attr.label_list(doc = "Files referenced from `extra_args` via `$(location ...)`."),
     "binstubs": attr.bool(default = True, doc = "Run `bundle binstubs --all` after install. Set False for cross-platform bundles."),
@@ -81,6 +87,7 @@ def _ruby_module_extension(module_ctx):
                 name = bundle_fetch.name,
                 srcs = bundle_fetch.srcs,
                 env = bundle_fetch.env,
+                env_bzl = bundle_fetch.env_bzl,
                 extra_args = bundle_fetch.extra_args,
                 data = [str(label) for label in bundle_fetch.data],
                 binstubs = bundle_fetch.binstubs,

ruby/private/bundle_fetch.bzl

@@ -233,6 +233,15 @@ def _rb_bundle_fetch_impl(repository_ctx):
         },
     )
 
+    if repository_ctx.attr.env_bzl:
+        env_load = 'load("{}", "bundle_env")'.format(str(repository_ctx.attr.env_bzl))
+        env_value = "bundle_env"
+        if repository_ctx.attr.env:
+            env_value = "bundle_env | {}".format(repr(repository_ctx.attr.env))
+    else:
+        env_load = ""
+        env_value = repr(repository_ctx.attr.env)
+
     repository_ctx.template(
         "BUILD",
         repository_ctx.attr._build_tpl,
@@ -246,7 +255,8 @@ def _rb_bundle_fetch_impl(repository_ctx):
             "{gems}": _join_and_indent(gem_full_names),
             "{gem_fragments}": "".join(gem_fragments),
             "{gem_install_fragments}": "".join(gem_install_fragments),
-            "{env}": repr(repository_ctx.attr.env),
+            "{env_load}": env_load,
+            "{env}": env_value,
             "{extra_args}": repr(repository_ctx.attr.extra_args),
             "{data}": _join_and_indent(repository_ctx.attr.data),
             "{binstubs}": repr(repository_ctx.attr.binstubs),
@@ -309,6 +319,11 @@ rb_bundle_fetch = repository_rule(
                   "`$(location ...)` expansion against `data` (forwarded to the " +
                   "generated rb_bundle_install `env`).",
         ),
+        "env_bzl": attr.label(
+            doc = "Optional .bzl file exporting `bundle_env` (dict), merged into the " +
+                  "generated rb_bundle_install `env`. Values are resolved at " +
+                  "BUILD-evaluation time, so they can come from generated repositories.",
+        ),
         "extra_args": attr.string_list(
             doc = "Extra arguments appended to the `bundle install` command line " +
                   "run by the generated rb_bundle_install target. Supports " +

ruby/private/bundle_fetch/BUILD.tpl

@@ -1,6 +1,7 @@
 """@generated by @rules_ruby//:ruby/private/bundle_fetch.bzl"""
 
 load("@rules_ruby//ruby:defs.bzl", "rb_bundle_install", "rb_gem", "rb_gem_install")
+{env_load}
 
 package(default_visibility = ["//visibility:public"])