Support Cabal conditions and flags

[avdv]

First off, sorry to whoever is looking at this PR it is such a big change. I have been working on this on and off for a few weeks...

This adds support for conditional blocks in .cabal files, generating corresponding select statements in BUILD files.

For example, the cabal project in example/package-d (see https://github.com/avdv/gazelle_cabal/blob/02193153cceeeb5dc900596caf6e43848ff3b5df/example/package-d/package-d.cabal) defines flags and uses them in conditional blocks. Running gazelle generates the following BUILD file:

load("@bazel_skylib//lib:selects.bzl", "selects")
load("@bazel_skylib//rules:common_settings.bzl", "bool_flag")
load("@rules_haskell//haskell:defs.bzl", "haskell_binary", "haskell_library")

bool_flag(
    name = "database",
    build_setting_default = False,
    visibility = ["//visibility:public"],
)

config_setting(
    name = "flag_database",
    flag_values = {
        ":database": "True",
    },
    visibility = ["//visibility:public"],
)

bool_flag(
    name = "experimental",
    build_setting_default = False,
    visibility = ["//visibility:public"],
)

config_setting(
    name = "flag_experimental",
    flag_values = {
        ":experimental": "True",
    },
    visibility = ["//visibility:public"],
)

bool_flag(
    name = "network-support",
    build_setting_default = True,
    visibility = ["//visibility:public"],
)

config_setting(
    name = "flag_network-support",
    flag_values = {
        ":network-support": "True",
    },
    visibility = ["//visibility:public"],
)

# AND-chained config settings
selects.config_setting_group(
    name = "database_and_network-support",
    match_all = [
        ":flag_database",
        ":flag_network-support",
    ],
    visibility = ["//visibility:public"],
)

# rule generated from package-d/package-d.cabal by gazelle_cabal
haskell_library(
    name = "package-d",
    srcs = [
        "src/PackageD/Core.hs",
        "src/PackageD/Internal/Base.hs",
    ] + select({
        ":flag_network-support": [
            "src/PackageD/Internal/NetworkImpl.hs",
            "src/PackageD/Network.hs",
        ],
        "//conditions:default": [],
    }) + select({
        ":flag_experimental": [
            "src/PackageD/Experimental.hs",
        ],
        "//conditions:default": [],
    }) + select({
        ":flag_database": [
            "src/PackageD/Database.hs",
            "src/PackageD/Internal/DatabaseImpl.hs",
        ],
        "//conditions:default": [],
    }) + select({
        ":database_and_network-support": [
            "src/PackageD/NetworkDatabase.hs",
        ],
        "//conditions:default": [],
    }),
    ghcopts = [
        "-DVERSION_package_d=\"0.1.0.0\"",
        "-Wall",
    ],
    hidden_modules = ["PackageD.Internal.Base"] + select({
        ":flag_network-support": [
            "PackageD.Internal.NetworkImpl",
        ],
        "//conditions:default": [],
    }) + select({
        ":flag_database": [
            "PackageD.Internal.DatabaseImpl",
        ],
        "//conditions:default": [],
    }),
    version = "0.1.0.0",
    visibility = ["//visibility:public"],
    deps = ["@stackage//:base"] + select({
        ":flag_network-support": [
            "@stackage//:network",
        ],
        "//conditions:default": [],
    }) + select({
        ":flag_experimental": [
            "@stackage//:text",
        ],
        "//conditions:default": [],
    }) + select({
        ":flag_database": [
            "@stackage//:direct-sqlite",
        ],
        "//conditions:default": [],
    }),
)

# rule generated from package-d/package-d.cabal by gazelle_cabal
haskell_binary(
    name = "package-d-cli",
    srcs = ["app/Main.hs"],
    ghcopts = ["-DVERSION_package_d=\"0.1.0.0\""] + select({
        ":flag_network-support": [
            "-DNETWORK_SUPPORT",
        ],
        "//conditions:default": [],
    }) + select({
        ":flag_database": [
            "-DDATABASE_SUPPORT",
        ],
        "//conditions:default": [],
    }) + select({
        ":database_and_network-support": [
            "-DDATABASE_AND_NETWORK_SUPPORT",
        ],
        "//conditions:default": [],
    }),
    main_file = "app/Main.hs",
    version = "0.1.0.0",
    visibility = ["//visibility:public"],
    deps = [
        ":package-d",
        "@stackage//:base",
    ] + select({
        ":flag_network-support": [
            "@stackage//:optparse-applicative",
        ],
        "//conditions:default": [],
    }),
)

I have used this successfully on the xmobar project which makes extensive use of flags and conditions.

• Add DNF implementation
• Format .go
• Support conditional cabal files
• Add example project using flags and conditions

[Copilot]

Pull request overview

Adds first-class support for Cabal conditional blocks and flags by extending cabalscan to emit configurable attributes and gazelle_cabal to generate corresponding Bazel bool_flag, config_setting, select(...), and selects.config_setting_group(...) constructs, plus an example project to validate the behavior end-to-end.

Changes:

• Extend Cabal scanning output to include flags and conditional/select-shaped attributes (DNF-based condition handling).
• Teach Gazelle rule generation + dependency resolution to carry configurable lists through to generated BUILD attributes.
• Add a new example (example/package-d) and CI coverage to validate flag-driven selects.

Reviewed changes

Copilot reviewed 32 out of 32 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
gazelle_cabal/lang_test.go	Formatting/import order adjustments in tests.
gazelle_cabal/lang.go	Generates bool_flag/config_setting/config_setting_group and adds configurable list handling.
gazelle_cabal/dependency_resolution_test.go	Updates tests to use configurable list structures.
gazelle_cabal/dependency_resolution.go	Updates dependency resolution to support configurable deps/opts/tools and expression walking.
gazelle_cabal/BUILD.bazel	Adds buildtools dependency for manipulating bzl AST.
example/package-d/test_flags.sh	Adds a script to validate different flag combinations via bazel run.
example/package-d/src/PackageD/NetworkDatabase.hs	Example module gated by combined flags.
example/package-d/src/PackageD/Network.hs	Example module gated by network flag.
example/package-d/src/PackageD/Internal/NetworkImpl.hs	Example internal module for network support.
example/package-d/src/PackageD/Internal/DatabaseImpl.hs	Example internal module for database support.
example/package-d/src/PackageD/Internal/Base.hs	Core example module always included.
example/package-d/src/PackageD/Experimental.hs	Example module gated by experimental flag.
example/package-d/src/PackageD/Database.hs	Example module gated by database flag.
example/package-d/src/PackageD/Core.hs	Core library module.
example/package-d/package-d.cabal	New example Cabal file defining flags + conditional stanzas.
example/package-d/app/Main.hs	Example executable using CPP macros driven by flags.
example/package-d/CHANGELOG.md	Example package changelog.
example/MODULE.bazel	Adds bazel_skylib and extra packages for the new example.
cabalscan/tests/CabalScan/RuleGeneratorSpec.hs	Minor formatting cleanup in existing test.
cabalscan/tests/CabalScan/DNFSpec.hs	Adds tests for DNF simplification/negation behavior.
cabalscan/src/CabalScan/Rules.hs	Introduces PackageOutput, Flag, and configurable/select representations in JSON output.
cabalscan/src/CabalScan/RuleGenerator.hs	Adds conditional traversal and DNF→select/config-group translation.
cabalscan/src/CabalScan/DNF.hs	New DNF representation + simplification utilities.
cabalscan/src/Cabal/Cabal_9_4.hs	Switches to GenericPackageDescription APIs needed for conditionals/flags.
cabalscan/src/Cabal/Cabal_9_2.hs	Same as above for Cabal 9.2.
cabalscan/src/Cabal/Cabal_9_0.hs	Same as above for Cabal 9.0.
cabalscan/src/Cabal/Cabal_8_10.hs	Same as above for Cabal 8.10 (+ flag/ConfVar compatibility).
cabalscan/exe/Main.hs	Outputs aggregated PackageOutput instead of [RuleInfo].
cabalscan/BUILD.bazel	Tweaks test timeout and deps for new DNF tests.
MODULE.bazel	Refactors versions/overrides and aligns rules_nixpkgs version usage.
.github/workflows/workflow.yaml	Updates CI to rely on .bazelrc.local and adds package-d flag check.
.bazelrc	Sets locale env for tests involving Unicode output.

---

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[Copilot]

keptSelects is declared but never used, which will fail go test/go vet compilation. Please remove it or use it (e.g., if you intended to treat only select() calls specially).

[Copilot]

In Merge, any *bzl.CallExpr that is marked # keep is treated as a “select” and preserved wholesale, even if it’s not actually a select(...) call (e.g. glob(...)). This can unintentionally keep unrelated call expressions and change merge behavior. Consider checking e.X is Ident{Name:"select"} (or selects.config_setting_group if needed) before treating it as a keep-worthy select expression, otherwise fall back to the element/whole-expression logic.

[Copilot]

For unsupported/complex conditions, dnfToSelect currently emits a Select with a key of "not-implemented" and uses traceShow. This will produce invalid/non-resolvable select conditions in generated BUILD files and also introduces debug logging in normal runs. Prefer failing generation with a clear error (or returning a best-effort default branch only), and remove traceShow/traceStack debugging from production output paths.