Update: Config File Improvements

designs/2019-eslintrc-improvements/diagrams.plantuml

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+@startuml current-deps
+rectangle "depending on fs" {
+  object CLIEngine
+  object Config
+  object Plugins
+  object GlobUtils
+}
+object Linter
+object ConfigCache
+object Environments
+object Rules
+object "config object" as ConfigData
+object "parser object" as Parser
+object "plugin object" as Plugin
+object "env object" as Environment
+object "rule object" as Rule
+
+CLIEngine *-->"1" Linter
+CLIEngine *-->"1" Config
+CLIEngine *-->"1" GlobUtils
+Config -->"1" Linter
+Config *-->"1" ConfigCache : "register"
+Config *-->"1" Plugins : "use"
+Config ..> ConfigData : "load"
+Config ..> Environments : "use"
+Plugins -->"1" Environments : "register"
+Plugins -->"1" Linter
+Plugins ..> Rules : "register"
+Plugins *-->"0..*" Plugin : "load"
+Linter *-->"*" Parser : "load"
+Linter *-->"1" Rules
+Linter *-->"1" Environments
+Linter ..> ConfigData : "use"
+Environments -->"0..*" Environment
+Rules -->"0..*" Rule
+Plugin *-->"0..*" Rule
+Plugin *-->"0..*" Environment
+@enduml
+
+@startuml new-deps
+rectangle "depending on fs" {
+  object CLIEngine
+  object FileEnumerator
+  object ConfigArrayFactory
+}
+object Linter
+object ConfigArray
+object ConfigArrayElement
+object "parser object" as Parser
+object "plugin object" as Plugin
+object "env object" as Environment
+object "rule object" as Rule
+
+CLIEngine *-->"1" Linter
+CLIEngine *-->"1" FileEnumerator
+Linter ..> ConfigArray
+FileEnumerator *-->"1" ConfigArrayFactory
+ConfigArrayFactory ..> ConfigArray : "load"
+ConfigArrayFactory ..> Parser : "load"
+ConfigArrayFactory ..> Plugin : "load"
+ConfigArray *-->"0..*" ConfigArrayElement
+ConfigArrayElement o-->"0..1" Parser
+ConfigArrayElement o-->"0..*" Plugin
+Plugin *-->"0..*" Rule
+Plugin *-->"0..*" Environment
+@enduml

designs/2019-eslintrc-improvements/major-01-additional-lint-targets.md

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+# Config File Improvements: Additional Lint Targets
+
+## Summary
+
+This proposal adds the ability to specify additional target files in configuration files. This enhancement will solve the pain that people have to use `--ext` option with wanted file extensions even if they use plugins.
+
+## Motivation
+
+People have to use `--ext` option or glob patterns to check wanted files even if they use plugins to support additional file types.
+
+```yml
+plugins:
+  - markdown
+  - html
+  - "@typescript-eslint"
+  - react
+  - vue
+```
+
+```bash
+# ESLint checks only `*.js`.
+eslint src docs
+
+# Needs `--ext` option
+eslint src docs --ext .js,.md,.html,.ts,.jsx,.tsx,.vue
+```
+
+## Detailed Design
+
+> Proof of Concept (implementation): https://github.com/eslint/eslint/tree/proof-of-concept/config-array-in-eslintrc
+
+This proposal enhances [there](README.md#additional-lint-targets).
+
+When `FileEnumerator` checks if a file is matched by the given extensions, additionally, it checks the file is matched by any of `files` property of configs (except `files` patterns which end with `*` to avoid unlimited).
+
+> [lib/lookup/file-enumerator.js#L338](https://github.com/eslint/eslint/blob/153640180a8944af3a1c488462ed30d0c215f5ed/lib/_lookup/file-enumerator.js#L338) in PoC.
+
+<table><td>
+💡 <b>Example</b>:
+<pre lang="yml">
+overrides:
+  - files: "*.ts"
+    extends: "plugin:@typescript-eslint/recommended"
+</pre>
+
+With the above config, `eslint .` command will check `*.ts` files additionally.
+</td></table>
+
+If a plugin has file extension processors, [it yields config array elements which have `overrides` property](README.md#yield-file-extension-processor-element), so this enhancement affects file extension processors.
+
+<table><td>
+💡 <b>Example</b>:
+<pre lang="yml">
+plugins:
+  - vue # has `.vue` processor
+</pre>
+
+With the above config, `eslint .` command will check `*.vue` files additionally.
+</td></table>
+
+The ignoring configuration (`.eslintignore`) is prior to this enhancement. If `.eslintignore` contains the additional target files, ESLint just ignores those as same as currently.
+
+## Documentation
+
+This enhancement needs migration guide because of a breaking change.
+
+- If your config contains `overrides` property or plugins which have file extension processors, `eslint` command with directory paths lints the files which are matched automatically. This may increase errors of your command.<br>
+  If you don't want to add file types to check, please use glob patterns instead of directory paths.
+
+This enhancement, so it needs to update some documents.
+
+- In the description of `--ext` CLI option, it should say that your config file may add file types automatically.
+- In the description of `overrides` property, it should say that the `overrides[].files` property adds target files automatically.
+- In the description of `plugins` property, it should say that plugins which have file extension processors add target files automatically.
+- In the "working with plugins" page, it should say that file extension processors add target files automatically.
+
+## Drawbacks
+
+- This is a breaking change.
+- Implicit behavior may make people confused.
+
+## Backwards Compatibility Analysis
+
+In the following situation, `eslint` command increases errors.
+
+- Their configuration has `overrides` property (with `files` property which doesn't end with `*`) or plugins which have file extension processors.
+- Their `eslint` command is using directory paths without `--ext` option.
+
+I guess that the impact is limited because people use `--ext` option or glob patterns if they use configuration which affects files other than `*.js`.
+
+## Alternatives
+
+- To add `extensions` property that behaves as same as `--est` option. Explicit reduces confusion. However, plugins and shareable configs cannot add the `extensions` property without the breaking change that drops old ESLint support.
+
+## Open Questions
+
+-
+
+## Frequently Asked Questions
+
+-
+
+## Related Discussions
+
+- [eslint/eslint#11223](https://github.com/eslint/eslint/issues/11223)