repro: referencing Communication's mw_com_rst breaks //:docs (demo, do not merge)

[antonkri]

Draft / demonstration only — not for merge. //:docs is deliberately broken.

Problem

Communication generates verification reports for its dependable elements
(mw_com, message_passing) via the dependable_element rule from
eclipse-score/tooling. We would like to
mount those pages into the reference integration documentation instead of only
linking them (#378). This PR shows why that does not work today.

The change

One line in the root BUILD:

docs(
    bundles = DOCS_BUNDLES,
    data = ["@score_communication//score/mw/com/dependability:mw_com_rst"],
    known_good = "known_good.json",
    source_dir = "docs",
)

<name>_rst is the sphinx_docs_library that dependable_element generates in
"Step 4" precisely so that an external Sphinx build can include the element's pages.
docs(data = ...) is the documented way to add files to this project's root bundle, so
this is the natural attempt.

Result

bazel build --nobuild \
  --extra_toolchains=//bazel/toolchains:score_ref_int_libclang_toolchain \
  //:docs

ERROR: BUILD:35:5: in _docs_bundle rule //:docs_bundle: non-test target
'//:docs_bundle' depends on testonly target
'@@score_communication+//score/mw/com/dependability:mw_com_rst'
and doesn't have testonly attribute set
ERROR: Analysis of target '//:docs' failed; build aborted: Analysis failed

CI hits the same error through //:docs_shim. Communication is pinned at 313f1178
per known_good.json. The --extra_toolchains flag is unrelated — without it analysis
stops earlier on a missing libclang toolchain.
dependable_element_message_passing_rst behaves identically.

Why testonly = False is not the answer

dependable_element has a testonly parameter that only defaults to True, so the
obvious idea is to flip it. It does not help: the graph underneath <name>_rst is built
out of Bazel test rules, which are unconditionally testonly.

<name>_rst has srcs = [":<name>_index"], and <name>_index reaches test rules
through three independent attributes:

$ bazel query 'kind(".*_test rule", deps(@score_communication//score/mw/com/dependability:mw_com_rst))'
@score_communication//score/mw/com/impl:frontend_component
@score_communication//score/mw/com/impl/bindings:bindings
@score_communication//score/mw/com/impl/bindings/lola:lola_component
@score_communication//score/mw/com/impl/bindings/mock_binding:mock_component
@score_communication//third_party/score_baselibs:bitmanipulation
@score_communication//third_party/score_baselibs:bitmanipulation_component
@score_communication//third_party/score_baselibs:containers
@score_communication//third_party/score_baselibs:containers_component
@score_communication//third_party/score_baselibs:futurecpp
@score_communication//third_party/score_baselibs:futurecpp_component
@score_communication//third_party/score_baselibs:os
@score_communication//third_party/score_baselibs:os_component

Attribute of _dependable_element_index	Points at	Rule class	Read in the rule impl?
components (mandatory)	<x>_component / <x> from component()	_component_test (component.bzl:359)	yes, via aspect
deps	<dep> of other dependable elements	_dependable_element_test (dependable_element.bzl:2036, test = True)	only dep.label.name (line 693)
tests	real test targets	cc_test etc.	never — no ctx.attr.tests anywhere

The components row is the important one: component() creates its main target as a
test rule, and components is a mandatory attribute of dependable_element. The
contamination is therefore structural, not an artifact of the two optional attributes.

Possible directions (for discussion)

1. deps and tests are cheap wins. ctx.attr.tests is never read, and
   ctx.attr.deps is only used for dep.label.name — no providers, no files (the real
   provider edge is processed_deps → <dep>_index). Making deps a string_list and
   dropping tests removes two of three paths with no loss of functionality.
2. components needs a split into a docs-producing target and a traceability test
   target — the same split that already exists for <name> vs <name>_doc.
3. Or keep linking instead of mounting, as done in docs: link Communication's dependable_element verification reports #378.

Any fix belongs in eclipse-score/tooling, not here. This PR only makes the current
state reproducible.

Contents

• BUILD — the one data = [...] line that breaks //:docs
• repro/dependable_element_testonly/README.md — full analysis, commands, expected output

[github-actions]

The created documentation from the pull request is available at: docu-html