Add ability to filter/skip by tags using a special tag: prefix

[gmedori]

While Xcode has had the ability to filter based on tags, this ability has been missing from the command line. This PR introduces a special prefix you can apply to your --filter and --skip option arguments: tag:.

When prefixing your argument with tag:, rather than treating the following characters as a regular expression, it's treated as a string literal that matches exactly against your test tags. For example, suppose you define a tag in your code as follows.

extension Tag {
  @Tag static var integrationTest: Self
}

If you then add that tag to some suites or tests, you'll be able to run those tests exclusively by running:

swift test --filter tag:integrationTest

Resolves #591.
Resolves rdar://132989780.

CleanShot.2026-02-05.at.16.21.38.mp4

Modifications:

This PR enhances the nested testFilter function inside of configurationForEntryPoint(from:) function. The testFilter function now:

• Processes any arguments prefixed with tag: as string literals to be interpreted as tags
• Handles escaping the tag: prefix with the following syntax: tag\:myRegex to force the entire string to be read as a regex (as it would be normally prior to this change). This allows for filtering on test names that contain tag:.
• Continues to process regexes as it did before this change, once all the tag-based arguments are processed.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[grynspan]

Holding off on approval because this will also need a Swift Evolution proposal and possibly changes to Swift Package Manager. But in terms of the technical implementation, seems reasonable. :)

[grynspan]

I'd suggest adding some more comments in the new code explaining what it's doing.

[gmedori]

I added a bunch more comments in eab4679, I think it should be clearer now! Let me know if I've overshot, or if we need more. Still mentally calibrating the right balance 🙂

[grynspan]

As above. Can we combine the code for filter and skip? Looks basically identical?

[gmedori]

I've gone and just made the testFilter nested function more comprehensive to DRY this up. It's a nested function and didn't have any doc comments before, so I left it that way, but let me know if doc comments on it would be beneficial. I think the inline comments are sufficient since it's not a publicly visible symbol and the inline comments do a decent job.

eab4679

[gmedori]

Adding test coverage to this one before I take it out of draft status!

[gmedori]

Just updated with some tests and fleshed out the PR description! The Swift Evolution proposal for this change is currently a WIP. Waiting on some folks to proofread it before I put it up because it's my very first one.