New: RuleTester test isolation with only

designs/2020-rule-tester-only/README.md

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+- Repo: eslint/eslint
+- Start Date: 2020-12-27
+- RFC PR: (leave this empty, to be filled in later)
+- Authors: Brandon Mills ([@btmills](https://github.com/btmills))
+
+# `RuleTester` test isolation with `only`
+
+## Summary
+
+<!-- One-paragraph explanation of the feature. -->
+
+`RuleTester` currently lacks a built-in way for developers to isolate individual tests during development.
+Temporarily deleting or commenting out the other tests is tedious.
+This adds an optional `only` property to run individual tests in isolation.
+
+## Motivation
+
+<!-- Why are we doing this? What uses does it support? What is the expected
+outcome? -->
+
+When developers are working on a rule, we can run its tests with `npm run test:cli tests/lib/rules/my-rule.js`.
+Debugging sometimes requires focusing on a particular test and running it in isolation to avoid unrelated noise from other tests.
+
+Tools like Mocha already offer `it.only()` for low-friction test isolation.
+Developers may already be familiar with that API.
+Exposing a similar API via `RuleTester` would improve the ergonomics of debugging tests.
+This API is beneficial to any approach from `console.log("HERE");` to interactive debugging with breakpoints.
+
+## Detailed Design
+
+<!--
+   This is the bulk of the RFC.
+
+   Explain the design with enough detail that someone familiar with ESLint
+   can implement it by reading this document. Please get into specifics
+   of your approach, corners, and examples of how the change will be
+   used. Be sure to define any new terms in this section.
+-->
+
+Add an optional boolean `only` property to `RuleTester`'s `ValidTestCase` and `InvalidTestCase` types.
+Add `only` to the other `RuleTester`-only properties in the `RuleTesterParameters` array to exclude it from configuration schema validation.
+
+To allow using `RuleTester` with a custom test framework other than Mocha, parallel `RuleTester`'s existing `describe` and `it` implementations for `itOnly`:
+
+1. Declare an `IT_ONLY` `Symbol`, set it as a static property on `RuleTester`, and initialize it to `null`.
+1. Add an `itOnly` `set` accessor that sets `RuleTester[IT_ONLY]`.
+1. Add an `itOnly` `get` accessor.
+   1. If `RuleTester[IT_ONLY]` is set, return it.
+   2. If global `it` and `it.only` are functions, return `it.only`. *TODO: Is `this` necessary? Could return a wrapper instead.*
+   3. Throw an error:
+      1. If either `RuleTester[DESCRIBE]` or `RuleTester[IT]` is customized, recommend setting a custom `RuleTester.itOnly`.
+      2. If global `it` is a function, the current test framework does not support `only`.
+      3. Otherwise recommend installing a test framework like Mocha so that `only` can be used.
+
+At the end of `RuleTester`'s `run()` method, for each valid and invalid item, if `only` is `true`, call `RuleTester.itOnly` instead of `RuleTester.it`.
+
+## Documentation
+
+<!--
+    How will this RFC be documented? Does it need a formal announcement
+    on the ESLint blog to explain the motivation?
+-->
+
+The [RuleTester section](https://eslint.org/docs/developer-guide/nodejs-api#ruletester) in our Node.js API docs will need to include a description of the `only` property on test case objects.
+
+The [Unit Tests](https://eslint.org/docs/developer-guide/unit-tests) page in the developer guide will also include a note about this next to the [Running Individual Tests](https://eslint.org/docs/developer-guide/unit-tests#running-individual-tests) section.
+
+## Drawbacks
+
+<!--
+    Why should we *not* do this? Consider why adding this into ESLint
+    might not benefit the project or the community. Attempt to think 
+    about any opposing viewpoints that reviewers might bring up. 
+
+    Any change has potential downsides, including increased maintenance
+    burden, incompatibility with other tools, breaking existing user
+    experience, etc. Try to identify as many potential problems with
+    implementing this RFC as possible.
+-->
+
+This exposes new API surface.
+That is mitigated by the implementation still being a wrapper around Mocha's existing API like the rest of `RuleTester`.
+
+Deleted or commented-out tests are hard to miss in diffs.
+An extra `only: true` could more easily sneak through, accidentally disabling all other tests.
+We could mitigate this by always exiting with 1 when `only: true` is used even if the test itself passes, though this diverges from Mocha's behavior.
+We could instead write a custom internal rule or add a rule to `eslint-plugin-eslint-plugin` to flag `only` in committed code.
+
+## Backwards Compatibility Analysis
+
+<!--
+    How does this change affect existing ESLint users? Will any behavior
+    change for them? If so, how are you going to minimize the disruption
+    to existing users?
+-->
+
+This change is backwards compatible.
+`RuleTester` currently fails any tests that include unrecognized properties like `only`, so we have no risk of breaking existing tests.
+
+If someone has customized `RuleTester` with a custom `it()` method, we cannot assume that `it.only()` has the same semantics as Mocha.
+Their existing tests will still work, but if they wish to use this new `only` property, they would need to provide a custom `itOnly()` alongside `it()`.
+
+## Alternatives
+
+<!--
+    What other designs did you consider? Why did you decide against those?
+
+    This section should also include prior art, such as whether similar
+    projects have already implemented a similar feature.
+-->
+
+This builds upon Mocha's established `it.only()` API as prior art.
+
+The status quo has two alternatives:
+
+1. We can temporarily delete or comment out the other tests. This is tedious.
+1. We can filter through noise by scrolling through `console.log()` output or using conditional breakpoints, though the latter is not always possible.
+
+## Open Questions
+
+<!--
+    This section is optional, but is suggested for a first draft.
+
+    What parts of this proposal are you unclear about? What do you
+    need to know before you can finalize this RFC?
+
+    List the questions that you'd like reviewers to focus on. When
+    you've received the answers and updated the design to reflect them, 
+    you can remove this section.
+-->
+
+1. We could define a static `only()` convenience method on `RuleTester`.
+Given, for example, a valid string test `"var foo = 42;"`, it is easier to convert to `RuleTester.only("var foo = 42;")`, which is equivalent to `{ code: "var foo = 42;", only: true }`.
+The helper sets `only: true` on a test case argument after converting string cases to objects if necessary.
+For invalid cases that are already objects, adding `only: true` is likely easier than using the helper.
+1. Should using `only` cause the process to exit `1` even if tests pass as described in "Drawbacks"?
+1. In the `RuleTester.itOnly` `get` accessor, if `RuleTester[IT_ONLY]` is not customized and the global `it.only` is not a function, is throwing an error the right choice?
+We could instead pass through to `RuleTester.it`, ignoring `only` if it's not supported.
+1. Do we need to support `skip`, the inverse of `only`?
+
+## Help Needed
+
+<!--
+    This section is optional.
+
+    Are you able to implement this RFC on your own? If not, what kind
+    of help would you need from the team?
+-->
+
+I can implement this myself.
+
+## Frequently Asked Questions
+
+<!--
+    This section is optional but suggested.
+
+    Try to anticipate points of clarification that might be needed by
+    the people reviewing this RFC. Include those questions and answers
+    in this section.
+-->
+
+## Related Discussions
+
+<!--
+    This section is optional but suggested.
+
+    If there is an issue, pull request, or other URL that provides useful
+    context for this proposal, please include those links here.
+-->
+
+- This was suggested as an alternative solution to [RFC67](https://github.com/eslint/rfcs/pull/67) and [eslint/eslint#13625](https://github.com/eslint/eslint/issues/13625) because that change may not be possible as currently described.