New: Accessing cwd from formatters (#57)

* New: Accessing `cwd` from formatters

* update head

designs/2020-cwd-in-formatters/README.md

@@ -0,0 +1,73 @@
+- Repo: eslint/eslint
+- Start Date: 2020-06-05
+- RFC PR: https://github.com/eslint/rfcs/pull/57
+- Authors: Toru Nagashima (https://github.com/mysticatea)
+
+# Accessing `cwd` from formatters
+
+## Summary
+
+This RFC lets formatters can access `cwd` in order to make relative paths or do something like.
+
+## Motivation
+
+Our Node.js API has the `cwd` option, and ESLint uses it while linting. But formatters cannot access the `cwd` option. It prevents formatters from making relative paths for readability.
+
+## Detailed Design
+
+Pass `cwd` to the `cwd` property of the second argument, then formatters can use it.
+
+```js
+module.exports = (results, context) => {
+  console.log(context.cwd) // → the absolute path to the current working directory.
+  console.log(context.rulesMeta) // → the metadata of the used rules (it exists currently).
+}
+```
+
+### Implementation
+
+The adapter object that the `ESLint.prototype.loadFormatter` method makes gives the `cwd`.
+
+> ```diff
+> @@ -575,7 +575,7 @@ class ESLint {
+>             throw new Error("'name' must be a string");
+>         }
+>
+> -       const { cliEngine } = privateMembersMap.get(this);
+> +       const { cliEngine, options } = privateMembersMap.get(this);
+>         const formatter = cliEngine.getFormatter(name);
+>
+>         if (typeof formatter !== "function") {
+> @@ -595,6 +595,9 @@ class ESLint {
+>                 results.sort(compareResultsByFilePath);
+>
+>                 return formatter(results, {
+> +                   get cwd() {
+> +                       return options.cwd;
+> +                   },
+>                     get rulesMeta() {
+>                         if (!rulesMeta) {
+>                             rulesMeta = createRulesMeta(cliEngine.getRules());
+> ```
+>
+> https://github.com/eslint/eslint/commit/43a7e207ca8c0b816d4fba2b4b290f761c33adb4
+
+## Documentation
+
+We should update the "[Working with Custom Formatters](https://eslint.org/docs/developer-guide/working-with-custom-formatters)" page to mention the `cwd`.
+
+## Drawbacks
+
+Nothing in particular.
+
+## Backwards Compatibility Analysis
+
+This addition is backward compatible.
+
+If custom formatters want to use the `cwd` property and continue to support old versions of ESLint, they must check if the `cwd` property exists or not.
+
+## Alternatives
+
+## Related Discussions
+
+- https://github.com/eslint/eslint/issues/13376 - Output relative paths rather than absolute paths