Enable no_legacy_features on the Linux and QNX toolchains

[nradakovic]

Summary

Moves both the Linux (GCC) and QNX (QCC) cc_toolchain configs to an explicit, no_legacy_features-based configuration. With legacy feature injection disabled, every compile, link, and archive flag is now supplied by a feature defined in the templates. Adds test coverage for the newly explicit features and refreshes the documentation.

Toolchain changes

• Enable no_legacy_features on both templates and explicitly implement every compile, link, and archive feature previously provided by Bazel's legacy machinery.
• Add guarded legacy features that are safe to enable by default (no-ops until the relevant build mode is active): per_object_debug_info, fission_support, linkstamps, includes, force_pic_flags, strip_debug_symbols, static_libgcc. fully_static_link stays opt-in since it emits -static unconditionally.
• Fix include_paths to use the correct build variables (quote_include_paths / include_paths / system_include_paths); the previous *_include_directories names were never populated, so no -iquote/-I/-isystem flags were emitted.

Tests

• New feature-verification tests that genuinely exercise the explicit features: preprocessor_defines_test (local_defines), include_dir_test (the includes attribute), user_link_flags_test (linkopts/--defsym), random_seed_test, and fully_static_link_test (verifies no shared objects are mapped; marked incompatible with toolchains lacking static archives, e.g. AutoSD).
• New opt_in_features/ package with manual tests for the disabled-by-default / build-mode-gated features, plus a genuine force-PIC check run in a dedicated --force_pic CI step.
• Wire the new tests into //:feature_verification_tests and update the x86_64-linux workflow.

Documentation

• Rewrite docs/features.md to reflect the explicit feature set, with per-feature platform tags and default-enabled/opt-in status.
• Remove the now-complete planning docs (linux_toolchain_feature_set.md, qnx_toolchain_feature_set.md), migrating their sysroot and tool-wiring notes into a new "Toolchain Feature Wiring" section in docs/maintenance.md; update the README accordingly.
• Refresh docs/test_suite.md and docs/tests_and_validation.md for the new tests and layout.

Resolves #76 #77 #78 #79 #80

[Copilot]

Pull request overview

This PR migrates the Linux (GCC) and QNX (QCC) Bazel cc_toolchain templates to a fully explicit feature configuration by enabling no_legacy_features, ensuring that compile/link/archive behavior is driven by features declared in the templates rather than Bazel’s legacy feature injection.

Changes:

• Enable no_legacy_features in both Linux and QNX toolchain templates and explicitly add the core compile/link/archive features that were previously implicitly provided.
• Add new feature-verification tests (plus a new opt_in_features/ package for manual/build-mode-gated checks) and wire them into the feature verification suite.
• Refresh and restructure documentation to reflect the explicit feature set and updated validation/test layout, and update the Linux CI workflow to exercise --force_pic.

Reviewed changes

Copilot reviewed 21 out of 21 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/feature_verification/user_link_flags_test.cpp	Adds runtime/link-failure-based verification for forwarding linkopts via user link flags feature.
tests/feature_verification/random_seed_test.cpp	Adds a build-and-run smoke test exercising code patterns affected by -frandom-seed.
tests/feature_verification/preprocessor_defines_test.cpp	Adds a compile-time check verifying local_defines are routed via preprocessor_defines.
tests/feature_verification/opt_in_features/opt_in_smoke.cpp	Shared smoke-test main for manual opt-in feature targets.
tests/feature_verification/opt_in_features/force_pic_test.cpp	Adds a --force_pic verification by asserting PIE (ET_DYN) via ELF header inspection.
tests/feature_verification/opt_in_features/BUILD	Introduces manual tests for opt-in/build-mode-gated features.
tests/feature_verification/include_dir/custom_math.h	Adds a header used to verify include search paths emitted for includes-exposed directories.
tests/feature_verification/include_dir_test.cpp	Adds test verifying includes attribute -> include_paths feature behavior.
tests/feature_verification/fully_static_link_test.cpp	Adds runtime verification of fully static linking by scanning /proc/self/maps for .so mappings.
tests/feature_verification/BUILD	Wires new feature tests into the feature verification package.
tests/BUILD	Adds new feature verification tests into the top-level tests suite.
templates/qnx/cc_toolchain_config.bzl.template	Enables no_legacy_features and adds explicit core compile/link/archive features for QNX.
templates/linux/cc_toolchain_config.bzl.template	Enables no_legacy_features and adds explicit core + guarded legacy-equivalent features for Linux.
README.md	Updates repository documentation links to reflect the new consolidated features documentation.
docs/tests_and_validation.md	Updates the description of feature verification coverage and the new opt-in tests package.
docs/test_suite.md	Documents the newly added feature verification tests and the opt-in/manual test package.
docs/qnx_toolchain_feature_set.md	Removes planning doc now superseded by explicit feature wiring + consolidated features doc.
docs/maintenance.md	Adds “Toolchain Feature Wiring” section explaining how non-feature wiring complements no_legacy_features.
docs/linux_toolchain_feature_set.md	Removes planning doc now superseded by explicit feature wiring + consolidated features doc.
docs/features.md	Adds consolidated documentation for toolchain features across Linux and QNX.
.github/workflows/x86_64-linux.yml	Adds a dedicated CI step to exercise the force-PIE behavior under --force_pic.

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