Add eslint-plugin-lit proposal

rfcs/NNNN-eslint-plugin-lit.md

@@ -0,0 +1,100 @@
+---
+Status: Active
+Champions: @justinfagnani @43081j
+PR: 
+---
+
+# @lit-labs/eslint-plugin-lit
+
+An TypeScript ESLint plugin for Lit projects that performs type-aware linting and augmented type-checking of LitElement components and standalone lit-html templates.
+
+## Objective
+
+Assist developers in writing correct Lit components and templates with a fully-supported, first-party tool.
+
+This RFC proposes that we bring James Garbut's eslint-plugin-lit work into the Lit monorepo, with some major changes, and combining it with our other first-party tooling:
+
+- Use the typescript-eslint infrastructure
+- Base the analysis on @lit-labs/analyzer
+- Add template type-checking rules such as those found in runem's [lit-analyzer](https://github.com/runem/lit-analyzer/tree/master/packages/lit-analyzer)
+
+### Goals
+- Type-check lit-html template bindings
+- Check lit-html templates for well-formedness and other errors
+- Check LitElemenent & ReactiveElement property and style declarations
+- Offer value to IDE users that only have an ESLint plugin, and not an otherwise Lit-specific IDE plugin
+- Make it easy to add new rules to lint usage of other first-party Lit libraries like Context or Task
+
+### Non-Goals
+- Take over the non-linting feature of runem's lit-analyzer work - such as syntax-highlighting, auto-complete - which belong in a Language Server
+
+## Motivation
+
+### Tool maintenance and ownership
+
+We want to start bringing Lit tooling into the Lit monorepo to keep it more up-to-date, make it easier for the Lit team to contribute to, and hopefully expand the set of community contributors.
+
+### Convergence of linting tools
+
+Right now there are two main linters available for Lit: eslint-plugin-lit and lit-analyzer. They bother have different sets of rules. lit-analyzer performs TypeScript type-checking on templates, and eslint-plugin-lit has more syntactic checks.
+
+We would like to merge the functionality into one linter. and have that be a plugin for typescript-eslint. This will let existing ESLint users add the plugin to easily check Lit projects locally, in CI, and in editors with no additional IDE plugins of CI scripts.
+
+## Detailed Design
+
+_TODO_
+
+## Implementation Considerations
+
+### Implementation Plan
+
+_TODO:_
+
+There are two main options:
+
+1. Copy the eslint-plugin-lit codebase into a package in the monorepo
+2. Start with a clean typescript-eslint project since there are some tasks about getting typescript-eslint and @lit-labs/analyzer integrated that might be easier in a new project.
+
+### Backward Compatibility
+
+_TODO_:
+
+It's probably tractable to maintain backwards-compatibility with eslint-plugin-lit, though some rules could possibly be deprecated.
+
+It might be harder to maintain backwards-compatibility with lit-analyzer which has its own configuration and comment syntax for disabling rules. We could likely approach a 1-1 mapping or rules so that migration is easy.
+
+### Testing Plan
+
+We should copy the test suites from both eslint-plugin-lit and lit-analyzer.
+
+### Performance and Code Size Impact
+
+No impact on the client code.
+
+### Interoperability
+
+These tools are Lit-specific and not intended to be used on other web component libraries.
+
+Some of the rules will apply to ReactiveElement (checking properties, etc). If we see a new ReactiveElement-based library out there, we can consider supporting it somehow.
+
+Interoperability is a consideration for analyzing _dependencies_. We do not wish to create a meta-analyzer that can host plugins to analyze web component dependencies from arbitrary libraries. We instead want to consumer custom element manifests from dependencies and expect dependencies to publish them.
+
+Being strict about this could be a regression to users if dependencies have not yet published manifests. We may want a (temporary?) option to analyze Lit dependencies until publishing manifests is a norm in the ecosystem.
+
+### Security Impact
+
+No negative impact on client-side code.
+
+We may be able to add security-related rule like disallowing `javascript:` in attributes.
+
+### Documentation Plan
+
+We need the tools documented on lit.dev in the **Tools and Workflows** section.
+
+## Downsides
+
+It is more work to move this project into the Lit monorepo.
+
+## Alternatives
+
+We could keep Lit tooling as-is. The current situation is a maintenance burden and confusing to users.