Only warn on rename when migration destination exists

[p-linnane]

brew edit <name> (and any other NamedArgs#to_paths caller) emits two rename warnings — one for Formula, one for Cask — when only one of the two actually exists at the migration destination:

Warning: Formula p-linnane/tap/pinprick was renamed to starhaven-io/tap/pinprick.
Warning: Cask p-linnane/tap/pinprick was renamed to starhaven-io/tap/pinprick.
Editing /opt/homebrew/Library/Taps/starhaven-io/homebrew-tap/Casks/pinprick.rb

NamedArgs#to_paths calls both Formulary.path(name) and Cask::CaskLoader.path(name) speculatively to resolve a name. Both lookups walk tap_formula_name_type / tap_cask_token_type, which unconditionally opoo for any matching tap_migrations.json entry — including when the migrated formula (or cask) does not exist at the destination tap.

This generalises the existing !(type == :migration && migrated_tap.core_cask_tap?) carve-out in FromNameLoader.try_new (and its cask twin) to all cross-type migrations between any taps: only warn when the migrated file actually exists at the destination (or, for the core tap, is published via the API).

After:

Warning: Cask p-linnane/tap/pinprick was renamed to starhaven-io/tap/pinprick.
Editing /opt/homebrew/Library/Taps/starhaven-io/homebrew-tap/Casks/pinprick.rb

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Claude Code (Opus 4.7) traced the duplicate warning to both Formulary.path and Cask::CaskLoader.path walking tap_formula_name_type/tap_cask_token_type from NamedArgs#to_paths, then drafted the destination-existence gate and the two new test contexts. I reviewed the diff, confirmed the carve-out matches the existing core_cask_tap? precedent, and verified ./bin/brew lgtm passes locally; reproduced the original double warning and the fixed single warning against a real tap_migrations.json setup.

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[Copilot]

Pull request overview

This PR updates Homebrew’s tap migration warning behavior so rename/migration warnings are only emitted when the migrated destination actually exists for the relevant type (formula vs cask), avoiding duplicate/incorrect warnings from speculative lookups (e.g., NamedArgs#to_paths calling both formula and cask resolution).

Changes:

• Gate formula migration/rename warnings on destination existence (filesystem, or API-published for core tap).
• Gate cask migration/rename warnings on destination existence (filesystem, or API-published for core cask tap).
• Add/adjust specs covering cross-type migrations (formula→cask and cask→formula) and fix a wording typo in a spec context.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
Library/Homebrew/formulary.rb	Only emits formula migration/rename warnings when the migrated formula destination exists (or is available via the API for core).
Library/Homebrew/cask/cask_loader.rb	Only emits cask migration/rename warnings when the migrated cask destination exists (or is available via the API for core cask).
Library/Homebrew/test/formulary_spec.rb	Adds coverage for “formula migration points to a cask-only destination tap” (no warning expected).
Library/Homebrew/test/cask/cask_loader_spec.rb	Adds coverage for “cask migration points to a formula-only destination tap” (no warning expected) and fixes a context typo.

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[MikeMcQuaid]

Thanks!