Add max-len rule to enforce configurable line length limits for Markdown entities

.changeset/max-len.md

@@ -0,0 +1,5 @@
+---
+"eslint-plugin-markdown-preferences": minor
+---
+
+feat: add `markdown-preferences/max-len` rule

README.md

@@ -138,10 +138,26 @@ The rules with the following 💄 are included in the `standard` config.
 
 <!--RULES_TABLE_START-->
 
+### Layout & Formatting Rules
+
+<!-- eslint-disable markdown-links/no-dead-urls -- Auto generated -->
+
+<!-- prettier-ignore-start -->
+
+| Rule ID                                                                                                           | Description                                          | Fixable | Config |
+| :---------------------------------------------------------------------------------------------------------------- | :--------------------------------------------------- | :-----: | :----: |
+| [markdown-preferences/max-len](https://ota-meshi.github.io/eslint-plugin-markdown-preferences/rules/max-len.html) | enforce maximum length for various Markdown entities |         |   💄   |
+
+<!-- prettier-ignore-end -->
+
+<!-- eslint-enable markdown-links/no-dead-urls -- Auto generated -->
+
 ### Preference Rules
 
 - Rules to unify the expression and description style of documents.
 
+<!-- eslint-disable markdown-links/no-dead-urls -- Auto generated -->
+
 <!-- prettier-ignore-start -->
 
 | Rule ID                                                                                                                                                           | Description                                                | Fixable | Config |
@@ -157,10 +173,14 @@ The rules with the following 💄 are included in the `standard` config.
 
 <!-- prettier-ignore-end -->
 
+<!-- eslint-enable markdown-links/no-dead-urls -- Auto generated -->
+
 ### Notation Rules
 
 - Rules related to notation styles in Markdown.
 
+<!-- eslint-disable markdown-links/no-dead-urls -- Auto generated -->
+
 <!-- prettier-ignore-start -->
 
 | Rule ID                                                                                                                                                               | Description                                                                                | Fixable | Config |
@@ -185,10 +205,14 @@ The rules with the following 💄 are included in the `standard` config.
 
 <!-- prettier-ignore-end -->
 
+<!-- eslint-enable markdown-links/no-dead-urls -- Auto generated -->
+
 ### Whitespace Rules
 
 - Rules related to whitespace styles in Markdown.
 
+<!-- eslint-disable markdown-links/no-dead-urls -- Auto generated -->
+
 <!-- prettier-ignore-start -->
 
 | Rule ID                                                                                                                                                           | Description                                                                    | Fixable | Config |
@@ -212,10 +236,14 @@ The rules with the following 💄 are included in the `standard` config.
 
 <!-- prettier-ignore-end -->
 
+<!-- eslint-enable markdown-links/no-dead-urls -- Auto generated -->
+
 ### Decorative Rules
 
 - Rules related to visual or stylistic decorations in Markdown.
 
+<!-- eslint-disable markdown-links/no-dead-urls -- Auto generated -->
+
 <!-- prettier-ignore-start -->
 
 | Rule ID                                                                                                                                                                   | Description                                                                               | Fixable | Config |
@@ -234,6 +262,8 @@ The rules with the following 💄 are included in the `standard` config.
 
 <!-- prettier-ignore-end -->
 
+<!-- eslint-enable markdown-links/no-dead-urls -- Auto generated -->
+
 <!--RULES_TABLE_END-->
 
 <!--RULES_SECTION_END-->

docs/rules/index.md

@@ -10,6 +10,16 @@ The rules with the following 💄 are included in the `plugin.configs.standard`
 
 <!-- This file is automatically generated in tools/update-docs-rules-index.js, do not change! -->
 
+## Layout & Formatting Rules
+
+<!-- prettier-ignore-start -->
+
+| Rule ID                                      | Description                                          | Fixable | Config |
+| :------------------------------------------- | :--------------------------------------------------- | :-----: | :----: |
+| [markdown-preferences/max-len](./max-len.md) | enforce maximum length for various Markdown entities |         |   💄   |
+
+<!-- prettier-ignore-end -->
+
 ## Preference Rules
 
 - Rules to unify the expression and description style of documents.

docs/rules/max-len.md

@@ -0,0 +1,124 @@
+---
+pageClass: "rule-details"
+sidebarDepth: 0
+title: "markdown-preferences/max-len"
+description: "enforce maximum length for various Markdown entities"
+---
+
+# markdown-preferences/max-len
+
+> enforce maximum length for various Markdown entities
+
+- ❗ <badge text="This rule has not been released yet." vertical="middle" type="error"> **_This rule has not been released yet._** </badge>
+- ⚙️ This rule is included in `plugin.configs.standard`.
+
+## 📖 Rule Details
+
+This rule enforces a configurable maximum length for different Markdown entities such as headings, paragraphs, list items, blockquotes, table rows, and optionally code blocks.
+
+Overly long Markdown elements reduce readability, make diffs harder to review, and complicate maintenance. By enforcing reasonable maximum lengths for various entities, documentation remains cleaner, easier to navigate, and more consistent across the project.
+
+<!-- prettier-ignore-start -->
+
+<!-- eslint-skip -->
+
+```md
+<!-- eslint markdown-preferences/max-len: ['error', { heading: 80, paragraph: 120 }] -->
+
+<!-- ✓ GOOD -->
+
+## This heading is within the limit
+
+This is a paragraph that fits comfortably within the specified maximum length limit.
+
+- A list item that is short enough
+
+<!-- ✗ BAD -->
+
+## This is a very long heading that exceeds the default eighty character maximum length
+
+This is an extremely long paragraph that goes on and on and on well beyond the one hundred and twenty character maximum length limit and should be broken up.
+
+- This is a very long list item that definitely exceeds the default one hundred and twenty character maximum length limit and should be reported
+```
+
+<!-- prettier-ignore-end -->
+
+## 🔧 Options
+
+This rule accepts an object with the following properties:
+
+```json
+{
+  "markdown-preferences/max-len": [
+    "error",
+    {
+      "heading": 80,
+      "paragraph": 120,
+      "listItem": 120,
+      "blockquote": 120,
+      "tableRow": 120,
+      "codeBlock": null,
+      "ignoreUrls": true
+    }
+  ]
+}
+```
+
+### Entity-Specific Options
+
+- `heading` (default: `80`): Maximum line length for headings
+- `paragraph` (default: `120`): Maximum line length for paragraphs
+- `listItem` (default: `120`): Maximum line length for list items
+- `blockquote` (default: `120`): Maximum line length for blockquotes
+- `tableRow` (default: `120`): Maximum line length for table rows
+- `codeBlock` (default: `null`): Maximum line length for code blocks. Set to `null` to ignore code blocks (recommended)
+
+### URL Handling
+
+- `ignoreUrls` (default: `true`): When enabled, lines containing URLs are ignored if:
+  - The URL is longer than the maximum line length, or
+  - The line would be within the limit without the URL
+
+This option works similarly to [`@stylistic/eslint-plugin`'s `max-len` rule](https://eslint.style/rules/max-len#ignoreurls).
+
+### Notes on Code Blocks
+
+**Note:** Code blocks are ignored by default (`codeBlock: null`) because:
+
+1. Code inside code blocks should be linted using language-specific linters via ESLint's language plugins
+2. The `@eslint/markdown` plugin supports linting code blocks as their respective languages
+3. This rule cannot understand the syntax inside code blocks (e.g., comments, strings)
+
+If you need to enforce line length for code blocks in Markdown, you can:
+- Use the language-specific ESLint configuration for code blocks (recommended):
+  ```js
+  {
+    files: ["**/*.md/*.js"],
+    rules: {
+      "@stylistic/max-len": ["error", { "ignoreComments": true }]
+    }
+  }
+  ```
+- Set the `codeBlock` option to enforce a simple line length limit (not syntax-aware)
+
+For more information, see the [advanced configuration guide](https://github.com/eslint/markdown/blob/main/docs/processors/markdown.md#advanced-configuration).
+
+## 📚 Further Reading
+
+- [Markdownlint MD013](https://github.com/DavidAnson/markdownlint/blob/main/doc/Rules.md#md013) - Similar rule in markdownlint
+- [@stylistic/eslint-plugin max-len](https://eslint.style/rules/max-len) - ESLint stylistic rule for code
+
+## 👫 Related Rules
+
+- [no-multiple-empty-lines](./no-multiple-empty-lines.md) - Disallow multiple empty lines
+
+## 🔍 Implementation
+
+<!-- eslint-disable markdown-links/no-dead-urls -- Auto generated -->
+
+- [Rule source](https://github.com/ota-meshi/eslint-plugin-markdown-preferences/blob/main/src/rules/max-len.ts)
+- [Test source](https://github.com/ota-meshi/eslint-plugin-markdown-preferences/blob/main/tests/src/rules/max-len.ts)
+- [Test fixture sources](https://github.com/ota-meshi/eslint-plugin-markdown-preferences/tree/main/tests/fixtures/rules/max-len)
+
+<!-- eslint-enable markdown-links/no-dead-urls -- Auto generated -->

src/configs/standard.ts

@@ -39,6 +39,7 @@ export const rules: Linter.RulesRecord = {
   "markdown-preferences/link-paren-spacing": "error",
   "markdown-preferences/link-title-style": "error",
   "markdown-preferences/list-marker-alignment": "error",
+  "markdown-preferences/max-len": "error",
   "markdown-preferences/no-implicit-block-closing": "error",
   "markdown-preferences/no-laziness-blockquotes": "error",
   "markdown-preferences/no-multi-spaces": "error",

src/rule-types.ts

@@ -139,6 +139,11 @@ export interface RuleOptions {
    * @see https://ota-meshi.github.io/eslint-plugin-markdown-preferences/rules/list-marker-alignment.html
    */
   'markdown-preferences/list-marker-alignment'?: Linter.RuleEntry<MarkdownPreferencesListMarkerAlignment>
+  /**
+   * enforce maximum length for various Markdown entities
+   * @see https://ota-meshi.github.io/eslint-plugin-markdown-preferences/rules/max-len.html
+   */
+  'markdown-preferences/max-len'?: Linter.RuleEntry<MarkdownPreferencesMaxLen>
   /**
    * disallow trailing punctuation in headings.
    * @see https://ota-meshi.github.io/eslint-plugin-markdown-preferences/rules/no-heading-trailing-punctuation.html
@@ -433,6 +438,16 @@ type MarkdownPreferencesLinkTitleStyle = []|[{
 type MarkdownPreferencesListMarkerAlignment = []|[{
   align?: ("left" | "right")
 }]
+// ----- markdown-preferences/max-len -----
+type MarkdownPreferencesMaxLen = []|[{
+  heading?: number
+  paragraph?: number
+  listItem?: number
+  blockquote?: number
+  tableRow?: number
+  codeBlock?: (number | null)
+  ignoreUrls?: boolean
+}]
 // ----- markdown-preferences/no-heading-trailing-punctuation -----
 type MarkdownPreferencesNoHeadingTrailingPunctuation = []|[{
   punctuation?: (string | {