Linter: Implement source-indentation rule

javascript/packages/linter/docs/rules/README.md

@@ -78,6 +78,7 @@ This page contains documentation for all Herb Linter rules.
 - [`html-require-script-nonce`](./html-require-script-nonce.md) - Require `nonce` attribute on script tags and helpers
 - [`html-tag-name-lowercase`](./html-tag-name-lowercase.md) - Enforces lowercase tag names in HTML
 - [`parser-no-errors`](./parser-no-errors.md) - Disallow parser errors in HTML+ERB documents
+- [`source-indentation`](./source-indentation.md) - Indent with spaces instead of tabs.
 - [`svg-tag-name-capitalization`](./svg-tag-name-capitalization.md) - Enforces proper camelCase capitalization for SVG elements
 - [`turbo-permanent-require-id`](./turbo-permanent-require-id.md) - Require `id` attribute on elements with `data-turbo-permanent`
 

javascript/packages/linter/docs/rules/source-indentation.md

@@ -0,0 +1,33 @@
+# Linter Rule: Indentation
+
+**Rule:** `source-indentation`
+
+## Description
+
+Detects indentation with tabs. Consistent use of spaces for indentation improves readability and avoids alignment issues across editors and tools.
+
+## Rationale
+
+Mixing tabs and spaces for indentation causes inconsistent visual formatting across different editors, tools, and environments. Tabs render at different widths depending on the viewer's settings, which can make code appear misaligned or harder to read. Standardizing on space indentation ensures that code appears the same regardless of editor or tool, diffs and code reviews display consistently, and the codebase maintains a uniform visual style.
+
+## Examples
+
+### ✅ Good
+
+```erb
+<div>
+  <p>Hello</p>
+</div>
+```
+
+### 🚫 Bad
+
+```erb
+<div>
+	<p>Hello</p>
+</div>
+```
+
+## References
+
+- [Shopify/erb_lint - `SpaceIndentation`](https://github.com/Shopify/erb_lint/blob/main/lib/erb_lint/linters/space_indentation.rb)

javascript/packages/linter/src/linter.ts

@@ -409,7 +409,8 @@ export class Linter {
     context = {
       ...context,
       validRuleNames: this.getAvailableRules().map(ruleClass => ruleClass.ruleName),
-      ignoredOffensesByLine
+      ignoredOffensesByLine,
+      indentWidth: context?.indentWidth ?? this.config?.formatter?.indentWidth
     }
 
     const regularRules = this.rules.filter(ruleClass => ruleClass.ruleName !== "herb-disable-comment-unnecessary")
@@ -522,6 +523,12 @@ export class Linter {
    */
   autofix(source: string, context?: Partial<LintContext>, offensesToFix?: LintOffense[], options?: { includeUnsafe?: boolean }): AutofixResult {
     const includeUnsafe = options?.includeUnsafe ?? false
+
+    context = {
+      ...context,
+      indentWidth: context?.indentWidth ?? this.config?.formatter?.indentWidth
+    }
+
     const lintResult = offensesToFix ? { offenses: offensesToFix } : this.lint(source, context)
 
     const parserOffenses: LintOffense[] = []

javascript/packages/linter/src/rules.ts

@@ -83,6 +83,8 @@ import { HTMLTagNameLowercaseRule } from "./rules/html-tag-name-lowercase.js"
 
 import { ParserNoErrorsRule } from "./rules/parser-no-errors.js"
 
+import { SourceIndentationRule } from "./rules/source-indentation.js"
+
 import { SVGTagNameCapitalizationRule } from "./rules/svg-tag-name-capitalization.js"
 
 import { TurboPermanentRequireIdRule } from "./rules/turbo-permanent-require-id.js"
@@ -171,6 +173,8 @@ export const rules: RuleClass[] = [
 
   ParserNoErrorsRule,
 
+  SourceIndentationRule,
+
   SVGTagNameCapitalizationRule,
 
   TurboPermanentRequireIdRule,

javascript/packages/linter/src/rules/index.ts

@@ -81,4 +81,6 @@ export * from "./html-require-closing-tags.js"
 export * from "./html-require-script-nonce.js"
 export * from "./html-tag-name-lowercase.js"
 
+export * from "./source-indentation.js"
+
 export * from "./svg-tag-name-capitalization.js"

javascript/packages/linter/src/rules/source-indentation.ts

@@ -0,0 +1,69 @@
+import { Location } from "@herb-tools/core"
+
+import { BaseSourceRuleVisitor, positionFromOffset } from "./rule-utils.js"
+import { SourceRule } from "../types.js"
+import type { UnboundLintOffense, LintOffense, LintContext, FullRuleConfig } from "../types.js"
+
+const START_BLANKS = /^[^\S\n]*\t[^\S\n]*/
+
+class SourceIndentationVisitor extends BaseSourceRuleVisitor {
+  protected visitSource(source: string): void {
+    const lines = source.split("\n")
+    let offset = 0
+
+    lines.forEach((line) => {
+      const match = line.match(START_BLANKS)
+
+      if (match) {
+        const start = positionFromOffset(source, offset)
+        const end = positionFromOffset(source, offset + match[0].length)
+        const location = new Location(start, end)
+
+        this.addOffense(
+          "Indent with spaces instead of tabs.",
+          location,
+        )
+      }
+
+      offset += line.length + 1
+    })
+  }
+}
+
+export class SourceIndentationRule extends SourceRule {
+  static autocorrectable = true
+  static ruleName = "source-indentation"
+  static introducedIn = this.version("unreleased")
+
+  get defaultConfig(): FullRuleConfig {
+    return {
+      enabled: true,
+      severity: "error"
+    }
+  }
+
+  check(source: string, context?: Partial<LintContext>): UnboundLintOffense[] {
+    const visitor = new SourceIndentationVisitor(this.ruleName, context)
+
+    visitor.visit(source)
+
+    return visitor.offenses
+  }
+
+  autofix(_offense: LintOffense, source: string, context?: Partial<LintContext>): string | null {
+    const indentWidth = context?.indentWidth ?? 2
+    const lines = source.split("\n")
+    const result = lines.map((line) => {
+      const match = line.match(START_BLANKS)
+
+      if (match) {
+        const replaced = match[0].replace(/\t/g, " ".repeat(indentWidth))
+        return replaced + line.substring(match[0].length)
+      }
+
+      return line
+    })
+
+    return result.join("\n")
+  }
+}

javascript/packages/linter/src/types.ts

@@ -242,6 +242,7 @@ export interface LintContext {
   validRuleNames: string[] | undefined
   ignoredOffensesByLine: Map<number, Set<string>> | undefined
   ignoreDisableComments: boolean | undefined
+  indentWidth: number | undefined
 }
 
 /**
@@ -251,7 +252,8 @@ export const DEFAULT_LINT_CONTEXT: LintContext = {
   fileName: undefined,
   validRuleNames: undefined,
   ignoredOffensesByLine: undefined,
-  ignoreDisableComments: undefined
+  ignoreDisableComments: undefined,
+  indentWidth: undefined
 } as const
 
 export abstract class SourceRule<TAutofixContext extends BaseAutofixContext = BaseAutofixContext> {