New: Changing Base Path of overrides and ignorePatterns (#37)

designs/2019-changing-base-path-in-config-files-that-cli-options-specify/README.md

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+- Start Date: 2019-09-18
+- RFC PR: https://github.com/eslint/rfcs/pull/37
+- Authors: Toru Nagashima <http://github.com/mysticatea>
+
+# Changing Base Path of `overrides` and `ignorePatterns` that CLI Options Specify
+
+## Summary
+
+This RFC changes the base path of `overrides`, `ignorePatterns`, and `.eslintignore` from the directory which contains the config file to the current working directory if the config file was specified by CLI options `--config` or `--ignore-path`.
+
+## Motivation
+
+Currently, the base path of `overrides`, `ignorePatterns`, and `.eslintignore` is the directory which contains the config file even if the config file was specified by CLI options `--config` or `--ignore-path`.
+
+```bash
+# the paths of 'overrides' and 'ignorePatterns' are relative to 'node_modules/my-config/'
+eslint lib --config node_modules/@me/my-config/.eslintrc.js
+# the paths in `.eslintignore` are relative to 'node_modules/my-config/'
+eslint lib --ignore-path node_modules/@me/my-config/.eslintignore
+```
+
+This is a barrier to use `--config`/`--ignore-path` option with shared files. If we change the base path to the current working directory, it will resolve this problem.
+
+### .gitignore and core.excludesFile
+
+`.eslintignore` has been designed as similar stuff to `.gitignore`. Therefore, it's better if the feature around `.eslintignore` is similar to `.gitignore`.
+
+Git doesn't have `--ignore-path`-like CLI option, but has `core.excludesFile` setting to specify additional ignore file.
+
+```bash
+# Use './config/ignore' file as '.gitignore'.
+git config core.excludesFile config/ignore
+```
+
+In this case, the base path of the paths in `config/ignore` is the repository root. For example, `/node_modules` in `config/ignore` is `./node_modules` rather than `./config/ignore/node_modules`. This behavior is different from our `--ignore-path` option.
+
+## Detailed Design
+
+It changes the base path of the following patterns to the current working directory:
+
+- In the file which is specified by the `--config` option:
+    - `overrides[i].files`
+    - `overrides[i].excludedFiles`
+    - `ignorePatterns`
+- In the file which is specified by the `--ignore-path` option:
+    - file patterns.
+
+```bash
+# the paths of 'overrides' and 'ignorePatterns' are relative to './'
+eslint lib --config node_modules/@me/my-config/.eslintrc.js
+# the paths in `.eslintignore` are relative to './'
+eslint lib --ignore-path node_modules/@me/my-config/.eslintignore
+```
+
+This RFC *doesn't* change the resolving logic of `extends`, `parser`, and `plugins`.
+
+### Implementation
+
+It modifies [`createCLIConfigArray()` function](https://github.com/eslint/eslint/blob/869f96aa87c4f990f54e1eeccb0e3f7dbd66e6c2/lib/cli-engine/cascading-config-array-factory.js#L133-L165). In the function, it modifies the elements of the config arrays which are created from the `ignorePath` (corresponds to `--ignore-path`) and the `specificConfigPath` (corresponds to `--config`). It modifies `element.criteria.basePath` (corresponds to `overrides`) and `element.ignorePattern.basePath` (corresponds to `ignorePatterns` and `.eslintignore`) to `cwd`.
+
+## Documentation
+
+This change needs the migration guide because of a breaking change.
+
+This change needs to update the following documents:
+
+- the "[Configuration Based on Glob Patterns » Relative glob patterns](https://github.com/eslint/eslint/blob/869f96aa87c4f990f54e1eeccb0e3f7dbd66e6c2/docs/user-guide/configuring.md#relative-glob-patterns)" section.
+- the "[Ignoring Files and Directories » `ignorePatterns` in config files](https://github.com/eslint/eslint/blob/869f96aa87c4f990f54e1eeccb0e3f7dbd66e6c2/docs/user-guide/configuring.md#ignorepatterns-in-config-files)" section.
+- the "[Ignoring Files and Directories » `.eslintignore`](https://github.com/eslint/eslint/blob/869f96aa87c4f990f54e1eeccb0e3f7dbd66e6c2/docs/user-guide/configuring.md#eslintignore)" section.
+    - As a side note, this section is outdated a bit. It said "Paths are relative to `.eslintignore` location or the current working directory.", but it has been changed since ESLint 4.0.0. Currently, it's "Paths are relative to `.eslintignore` location."
+
+## Drawbacks
+
+This is a breaking change. It can break the current workflow.
+
+For example, if a user runs ESLint on variety directories with the same config by `--config` and `--ignore-path`, the workflow will be broken. The user has to change the path to target files instead of the working directory.
+
+## Backwards Compatibility Analysis
+
+This is a breaking change.
+
+The behavior of `--config` and `--ignore-path` options will be changed.
+
+## Alternatives
+
+### Prior Arts
+
+- [`.gitignore` and `core.excludesFile`](#gitignore-and-coreexcludesfile) is similar to this RFC's behavior.
+
+## Related Discussions
+
+- https://github.com/eslint/eslint/issues/6759
+- https://github.com/eslint/eslint/issues/11558
+- https://github.com/eslint/eslint/issues/12278