fetch: add --all-platforms to fetch every variant

[p-linnane]

brew fetch --cask --all-platforms <cask> downloads every supported variant of a cask in one command, to verify that all URLs resolve and checksums match without manually iterating --os/--arch.

It iterates the full OS/arch matrix (every macOS version × arm/intel, plus Linux) and each language block, then downloads each distinct URL once (many combinations resolve to the same file). This covers casks whose URL varies by macOS version — e.g. itsycal resolves to 0.14.1 on Catalina and 0.15.12 on Big Sur and newer — not just a fixed set of platform corners.

Notes:

• --all-platforms is equivalent to --os=all --arch=all and reuses the existing os_arch_combinations machinery, so it also works for <formula>e (fetching every bottle); the language and distinct-URL handling are cask-specific.
• Under --all-platforms, a cask without on_system blocks resolves identically everywhere, so it collapses to a single combination (existing --os=all/--arch=all behaviour is unchanged).
• Casks that legitimately omit some platforms (e.g. an on_linux block with only x86_64_linux) are tolerated rather than raising, mirroring the bump-cask-pr rescue pattern.
• OS/arch cannot be simulated for casks loaded from the API, so an opoo points to HOMEBREW_NO_INSTALL_FROM_API (languages are unaffected). This matches the pre-existing behaviour of --os=all --arch=all.
• Profiling note: cask reloading under simulation is ~1 ms each; the dominant cost is one curl --head redirect resolution per unique URL, inherent to resolving a download's cached filename.

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• AI was used to generate or assist with generating this PR.

AI (Claude Code) assisted with the implementation and this description. The cask iteration is extracted into a cask_downloads helper with unit tests asserting the distinct-URL set (sha256-os → 4 URLs) and the single-combination collapse (local-caffeine → 1). Verified locally: brew lgtm passes, and the full-matrix + language iteration resolves the expected distinct URLs for source casks (itsycal → 2 macOS-version URLs, git-credential-manager → 4 of 15 combinations, firefox → 102 language URLs); API-loaded casks emit the warning.

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[Copilot]

Pull request overview

Adds a --all-platforms switch to brew fetch that expands os_arch_combinations to the 4 base [:macos, :linux] × [:intel, :arm] combos (vs. the ~24 expanded by --os=all --arch=all), primarily as a shorthand for verifying multi-platform cask URLs/SHAs. Also makes the cask iteration tolerant of CaskInvalidError / CaskUnreadableError when the original cask uses on_system blocks, mirroring the existing rescue pattern in bump-cask-pr.

Changes:

• New --all-platforms switch on brew fetch, conflicting with --os, --arch, and --bottle-tag.
• Shared Args#os_arch_combinations short-circuits to BASE_OS_OPTIONS × ARCH_OPTIONS when --all-platforms is set.
• Cask loop renames the inner reloaded cask to loaded_cask, rescues invalid/unreadable cask errors for system-block casks, and skips combos that produce a nil reload.

Reviewed changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated 1 comment.

File	Description
Library/Homebrew/cmd/fetch.rb	Adds the switch, conflict declarations, and the rescue/loaded_cask handling in the cask iteration.
Library/Homebrew/cli/args.rb	Adds the --all-platforms short-circuit in the shared os_arch_combinations helper.
Library/Homebrew/sorbet/rbi/dsl/homebrew/cmd/fetch_cmd.rbi	Regenerated DSL RBI exposing all_platforms? on FetchCmd::Args.

Files not reviewed (1)

• Library/Homebrew/sorbet/rbi/dsl/homebrew/cmd/fetch_cmd.rbi: Language not supported

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[MikeMcQuaid]

Code looks fine, thanks!

Adds a --all-platforms switch to brew fetch that iterates [:macos, :linux] × [:intel, :arm] (four combos), as a shorthand for verifying multi-platform cask URLs and SHA-256 checksums in a single command. --os=all --arch=all already exists but iterates every macOS codename (~24 combos), which is excessive when cask URLs typically don't vary by macOS version.

This feels like a bit of a potential footgun in future.

I think this should handle the case where cask URLs do vary by the macOS version and do something along the lines of e.g. getting each possible variant of the URL across all potential platforms and trying to download the unique URLs that result. May want/need to handle languages as part of this too.

If the system simulation part of this is too slow over all the possible combos: would be good to profile and speed that up as it shouldn't need to be.

I think --all would be an acceptable shorthand for this, too.

[Copilot]

Pull request overview

Copilot reviewed 7 out of 8 changed files in this pull request and generated 4 comments.

Files not reviewed (1)

• Library/Homebrew/sorbet/rbi/dsl/homebrew/cmd/fetch_cmd.rbi: Language not supported

[Copilot]

Pull request overview

Copilot reviewed 8 out of 9 changed files in this pull request and generated 2 comments.

Files not reviewed (1)

• Library/Homebrew/sorbet/rbi/dsl/homebrew/cmd/fetch_cmd.rbi: Language not supported

[p-linnane]

@MikeMcQuaid This should be good to go based off your feedback and some help from Copilot. Would like you to do another review since there was a decent amount of changes.

[MikeMcQuaid]

Thanks!