feat(patch): add --preview flag to show diff without saving

[robobun]

Summary

This PR adds a --preview flag to bun patch and bun patch-commit commands that allows users to see what a patch will contain before committing to it.

• Prints the patch diff to stdout without saving the patch file
• Does not modify package.json
• Works with both bun patch --preview <path> and bun patch-commit --preview <path>

Motivation

When creating patches for dependencies (e.g., porting upstream bug fixes), users often want to create minimal patches. Currently, the only way to see the patch content is to run bun patch --commit, which saves the patch file and modifies package.json. If the patch needs adjustment, users must:

1. Undo everything (revert bun.lock and package.json, delete the .patch file)
2. Reinstall node_modules
3. Run bun patch again and reapply changes
4. Run bun patch --commit again

With --preview, users can iterate on their changes and preview the diff multiple times without any side effects.

Usage

# Preview what a patch will look like
bun patch --preview node_modules/package

# Or with patch-commit
bun patch-commit --preview node_modules/package

Test plan

• Added tests for --preview flag
• Verified preview prints diff to stdout
• Verified preview does not create patches directory
• Verified preview does not modify package.json
• Verified "No changes detected" message works in preview mode
• Verified existing --commit functionality still works

Closes #26463

🤖 Generated with Claude Code

[robobun]

^{Updated 5:00 PM PT - Jan 26th, 2026}

❌ Your commit 76163e20 has 3 failures in Build #35895 (All Failures):

• test/js/bun/perf/static-initializers.test.ts - 1 failing on 🍎 14 x64
• test/integration/next-pages/test/dev-server.test.ts - 1 failing on 🍎 14 aarch64
• test/js/sql/tls-sql.test.ts - 9 failing on 🪟 2019 x64
• test/js/sql/tls-sql.test.ts - 9 failing on 🪟 2019 x64-baseline
• test/js/sql/tls-sql.test.ts - 9 failing on 🐧 25.04 aarch64
• test/js/sql/tls-sql.test.ts - 9 failing on 🐧 3.22 aarch64
• test/js/sql/tls-sql.test.ts - 9 failing on 🐧 13 aarch64
• test/js/sql/tls-sql.test.ts - 9 failing on 🐧 3.22 x64-baseline
• test/js/sql/tls-sql.test.ts - 9 failing on 🐧 3.22 x64
• test/js/sql/tls-sql.test.ts - 9 failing on 🐧 25.04 x64
• test/js/sql/tls-sql.test.ts - 9 failing on 🐧 13 x64
• test/js/sql/tls-sql.test.ts - 9 failing on 🐧 25.04 x64-baseline
• test/js/sql/tls-sql.test.ts - 9 failing on 🐧 13 x64-baseline
• test/js/sql/tls-sql.test.ts - 9 failing on 🍎 14 aarch64
• test/js/sql/tls-sql.test.ts - 9 failing on 🍎 13 aarch64
• test/js/sql/tls-sql.test.ts - 9 failing on 🍎 14 x64
• test/js/sql/tls-sql.test.ts - 9 failing on 🍎 13 x64

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🧪   To try this PR locally:

bunx bun-pr 26466

That installs a local version of the PR into your bun-26466 executable, so you can run:

bun-26466 --bun

[coderabbitai]

Walkthrough

The pull request adds a --preview flag to bun patch and bun patch-commit commands, enabling users to view the generated patch diff without modifying files or saving patch entries. When preview mode is active, the patch contents are printed to stdout and the operation exits before any filesystem changes occur.

Changes

Cohort / File(s)	Summary
CLI and Options Parsing src/install/PackageManager/CommandLineArguments.zig, src/install/PackageManager/PackageManagerOptions.zig	Introduces --preview flag to patch command arguments and wires it through the PatchOpts data structure. Adds preview: bool = false field to the commit variant of patch_features, updates CLI parsing to populate the preview boolean, and extends public parameter sets to recognize the new option in help text and argument parsing.
Patch Implementation src/install/PackageManager/patchPackage.zig	Adds preview mode logic to doPatchCommit that prints generated patch contents to stdout and exits early, bypassing patch file creation and filesystem modifications while preserving error handling for stdout operations.
Test Coverage test/cli/install/bun-patch.test.ts	Adds comprehensive test suite validating that bun patch --preview and bun patch-commit --preview print diffs without creating patch files, modifying patches directory, or updating package.json.patchedDependencies, including scenarios for modified and unmodified packages.

Suggested reviewers

• zackradisic
• pfgithub

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: adding a --preview flag to patch commands that shows diffs without saving.
Description check	✅ Passed	The description covers both required sections (What does this PR do and How did you verify) with comprehensive details about the feature, motivation, usage, and test verification.
Linked Issues check	✅ Passed	The PR fully implements the feature requested in #26463: a --preview flag that prints diffs without modifying state, supporting both patch and patch-commit commands with comprehensive test coverage.
Out of Scope Changes check	✅ Passed	All changes are directly related to implementing the --preview flag feature for patch commands as specified in the linked issue, with no unrelated modifications.

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