RFC: Configure analysis via .luaurc

rfcs/config-luaurc.md

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+# Configure analysis via .luaurc
+
+## Summary
+
+Introduces a way to configure type checker and linter using JSON-like .luaurc files
+
+## Motivation
+
+While Luau analysis tools try to provide sensible defaults, it's difficult to establish the rules that work for all code.
+For example, some packages may decide that unused variables aren't interesting; other packages may decide that all files should be using strict typechecking mode.
+
+While it's possible to configure some aspects of analysis behavior using --! comments, it can be cumbersome to replicate this in all files.
+
+## Design
+
+To solve this problem, we are going to introduce support for `.luaurc` files for users of command-line Luau tools.
+For a given .lua file, Luau will search for .luaurc files starting from the folder that the .lua file is in; all files in the ancestry chain will be parsed and their configuration
+applied. When multiple files are used, the file closer to the .lua file overrides the settings.
+
+.luaurc is a JSON file that can also contain comments and trailing commas. The file can have the following keys:
+
+- `"mode"`: type checking mode, can be one of "nocheck", "nonstrict", "strict"
+- `"language"`: legacy specification of type checking mode, contains `"mode"` key - e.g. `{ "language": { "mode": "strict" } }`. Supported for backwards compatibility with existing .robloxrc files.
+- `"lint"`: linting settings; points to an object that contains keys corresponding to the names of linting rules (see https://luau-lang.org/lint) or `"*"` that means "all rules"; the values must be one of "disabled", "enabled", "fatal".
+- `"globals"`: extra global values; points to an array of strings where each string names a global that the type checker and linter must assume is valid and of type `any`
+
+Example of a valid .luaurc file:
+
+```json
+{
+  "mode": "nonstrict"
+	"lint": {
+		"*": "fatal",
+		"LocalUnused": "disabled",
+	},
+  "globals": ["expect"] // TestEZ
+}
+```
+
+## Drawbacks
+
+The introduction of configuration files means that it's now impossible to type check or lint sources in isolation, which complicates the code setup.
+
+File-based JSON configuration may or may not map cleanly to environments that don't support files, such as Roblox Studio.
+
+Support for two ways to specify the mode, "mode" and "language"/"mode", complicates specification a bit.
+
+Making the default set of linting passes fatal requires enumerating them one by one, as "*": "fatal" means "enable all linting passes and make them fatal" (unlike -Werror).
+
+Making the type checking issues visible but non-fatal is impossible.
+
+## Alternatives
+
+It's possible to consider forcing users to specify the source settings via `--!` comments exclusively. This is problematic as it may require excessive amounts of annotation though, which this proposal aims to simplify.
+
+The format of the configuration file does not have to be JSON; for example, it can be a valid Luau source file which is the approach luacheck takes. This makes it more difficult to repurpose the .luaurc file to use third-party processing tools though, e.g. a package manager would need to learn how to parse Luau syntax to store configuration in .luaurc.