Treat cask OS support as explicit data

[MikeMcQuaid]

• Align casks with formulae: platform support comes from depends_on :macos/:linux/macos: data rather than generation heuristics guessing intent from os stanzas, Linux checksums or on_linux blocks.
• Cask#to_hash_with_variations now emits variations for every valid OS/arch tag whenever on_system blocks exist, matching Formula#to_hash_with_variations; the Linux-specific gate and its sha256_set_for_linux? and on_linux_blocks_exist? tracking are removed. macOS-only casks publish truthful Linux variations, e.g. a null sha256, instead of omitting them.
• Cask::Installer gains a first-class unsupported-system error: API-loaded casks with no activatable artifact for the running system fail with "This cask is not available on macOS/Linux." instead of installing nothing. Audited casks always declare an activatable artifact for the systems they support, so missing artifacts in API data mean the system is unsupported. Source loads keep working for unaudited casks, e.g. naked containers.
• A sweep of the full homebrew/cask generation pipeline (all casks, both Linux tags, including internal per-tag payloads) confirmed no cask needs new depends_on annotations and nothing regresses.

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• Have you checked for other open Pull Requests for the same change?
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• Have you written new tests (excluding integration tests)? Here's an example.
• Have you successfully run brew lgtm (style, typechecking and tests) locally?

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• I did not use AI/LLM to create this PR, or I disclosed the tool/model below and reviewed its output; I did not attribute commits to AI and will answer maintainer questions and review comments myself without AI/LLM.

Fable 5 max with local review and testing.

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[Copilot]

Pull request overview

This PR shifts cask OS/arch support to be treated as explicit data (via depends_on :macos/:linux and related stanza data) rather than inferred heuristically, and updates API variation output and installer behavior to match that model.

Changes:

• Emit Cask#to_hash_with_variations entries for all valid OS/arch tags whenever on_system blocks exist (removing Linux-variation gating heuristics).
• Add Cask::Installer#check_supported_system to fail fast for API-loaded casks that have no activatable install artifacts for the current system.
• Remove now-unused Linux tracking (on_linux_blocks_exist?, sha256_set_for_linux?) from the DSL and Sorbet RBI; adjust specs accordingly.

Reviewed changes

Copilot reviewed 6 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
Library/Homebrew/test/cask/installer_spec.rb	Adds coverage for unsupported-system errors; adjusts API-loaded behavior expectations and fixtures.
Library/Homebrew/test/cask/cask_spec.rb	Updates expected variations output to include Linux tags for casks with on_system blocks.
Library/Homebrew/sorbet/rbi/dsl/cask/cask.rbi	Removes the RBI signature for on_linux_blocks_exist? after DSL tracking removal.
Library/Homebrew/on_system.rb	Removes @on_linux_blocks_exist bookkeeping from on_linux/on_system block setup.
Library/Homebrew/cask/installer.rb	Introduces check_supported_system into the installer requirements flow.
Library/Homebrew/cask/dsl.rb	Removes Linux-specific tracking accessors/state from the cask DSL.
Library/Homebrew/cask/cask.rb	Removes the Linux-variation skip heuristic so variations are generated for all OS/arch tags.

Files not reviewed (1)

• Library/Homebrew/sorbet/rbi/dsl/cask/cask.rbi: File type not supported

Suppressed comments (3)

Library/Homebrew/test/cask/installer_spec.rb:255

• This expectation hardcodes "macOS", but the installer chooses "macOS" vs "Linux" at runtime. The test should build the expected string dynamically so it passes on both macOS and Linux CI.

    it "treats uninstall-only artifacts as nothing to install" do
      zap_only_cask = Cask::Cask.new("with-zap-only", loaded_from_api: true) do
        version "1.0"
        sha256 :no_check
        url "https://brew.sh/x.zip"
        zap trash: "~/Library/Caches/brew-test"
      end
      expect do
        described_class.new(zap_only_cask).check_supported_system
      end.to raise_error(Cask::CaskError, "with-zap-only: This cask is not available on macOS.")
    end

Library/Homebrew/test/cask/installer_spec.rb:552

• This spec is asserting an exact call count for loaded_from_api?, which makes it sensitive to internal refactors (like adding another requirement check). Prefer stubbing with allow (or at least avoiding exact counts) so the test asserts behavior rather than implementation details.

      it "uninstalls cask" do
        source_caffeine = Cask::CaskLoader.load(path)
        expect(Homebrew::API::Cask).to receive(:source_download_cask).twice.and_return(source_caffeine)

        caffeine = Cask::CaskLoader.load(path)
        expect(caffeine).to receive(:loaded_from_api?).exactly(3).times.and_return(true)
        expect(caffeine).to receive(:caskfile_only?).twice.and_return(true)
        expect(caffeine).to receive(:installed_caskfile).once.and_return(invalid_path)

Library/Homebrew/cask/cask.rb:629

• Now that Linux variations are always emitted, casks that set url using #{version} inside macOS-only on_* blocks can produce Linux-variation URLs containing a double-slash (because version is nil on Linux tags). That makes the emitted Linux url misleading/invalid for API consumers; consider emitting null/omitting url (and url_specs) when version is nil for a given OS/arch tag.

      if dsl!.on_system_blocks_exist?
        begin
          OnSystem::VALID_OS_ARCH_TAGS.each do |bottle_tag|
            macos_requirements = [depends_on.macos, depends_on.maximum_macos].compact
            next if bottle_tag.macos? &&
                    macos_requirements.present? &&
                    !dsl!.depends_on_set_in_block? &&
                    macos_requirements.any? { |requirement| !requirement.allows?(bottle_tag.to_macos_version) }

            refresh_for_tag(bottle_tag) do
              to_h_with_language_variations.each do |key, value|
                next if HASH_KEYS_TO_SKIP.include? key
                next if value.to_s == hash[key].to_s

                variations[bottle_tag.to_sym] ||= {}
                variations[bottle_tag.to_sym][key] = value
              end

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