[6.3.0] Make variable substitution in C++ rule environment variables broken

[eric-skydio]

Description of the bug:

C++ rules support make variable substitution in the env= attribute, as used in the example below. Unfortunately, starting with Bazel 6.3.0, this seems to be broken, and instead produces this error:

ERROR: /tmp/repro/BUILD.bazel:1:8: in cc_test rule //:test: 
Traceback (most recent call last):
        File "/virtual_builtins_bzl/common/cc/cc_test.bzl", line 70, column 29, in _impl
        File "/virtual_builtins_bzl/common/cc/cc_test.bzl", line 30, column 47, in _cc_test_impl
        File "/virtual_builtins_bzl/common/cc/cc_helper.bzl", line 900, column 34, in _get_expanded_env
        File "/virtual_builtins_bzl/common/cc/cc_helper.bzl", line 783, column 46, in _expand
        File "/virtual_builtins_bzl/common/cc/cc_helper.bzl", line 717, column 27, in _expand_nested_variable
Error in extend: trying to mutate a frozen list value
ERROR: /tmp/repro/BUILD.bazel:1:8: Analysis of target '//:test' failed
ERROR: Analysis of target '//:test' failed; build aborted: 

What's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

.bazelversion

6.3.0

BUILD.bazel

cc_test(
    name="test",
    srcs=["test.cc"],
    env={"MY_VAR": "$(rootpath //:test.cc)"},
)

test.cc

int main() { return 0; }

Which operating system are you running Bazel on?

Ubuntu 18.04

What is the output of bazel info release?

release 6.3.0

If bazel info release returns development version or (@non-git), tell us how you built Bazel.

No response

What's the output of git remote get-url origin; git rev-parse master; git rev-parse HEAD ?

No response

Is this a regression? If yes, please try to identify the Bazel commit where the bug was introduced.

A bisect points at 2f9e8c1 being the culprit, @brentleyjones

Have you found anything relevant by searching the web?

No response

Any other information, logs, or outputs that you want to share?

No response

[keertk]

@bazel-io fork 6.3.1

[keertk]

@bazel-io fork 6.4.0

[brentleyjones]

Already fixed in the 6.3.1 and 6.4.0 branches:

• [6.3.1] Fix a bug where frozen targets list was mutated while expanding env attribute for cc_test #19052
• [6.4.0] Fix a bug where frozen targets list was mutated while expanding env attribute for cc_test #19053

[iancha1992]

As per #19070 (comment), this should be fixed in and after 6.3.1

[keertk]

@eric-skydio Thank you for reporting this issue.
We're looking to improve our release process so that we can catch more issues early on in the cycle (before the final release) and would love your input. What can we do to make it easier for you to test release candidates in the future?