Formatter: Skip formatting Scaffold Templates (#771)

This pull request adds a visitor to detect if a given ParseResult
contains escaped `<%%` or `<%%=` tags, which are typically used in Rails
Scaffold Templates.

In the case of a scaffold template we abort the formatting and just
return the original source, since we cannot reliably tell how to format
a scaffold template properly, yet.

Resolves #673

Author: marcoroth

javascript/packages/formatter/src/formatter.ts

@@ -1,5 +1,6 @@
 import { FormatPrinter } from "./format-printer.js"
 
+import { isScaffoldTemplate } from "./scaffold-template-detector.js"
 import { resolveFormatOptions } from "./options.js"
 
 import type { Config } from "@herb-tools/config"
@@ -57,7 +58,9 @@ export class Formatter {
    */
   format(source: string, options: FormatOptions = {}, filePath?: string): string {
     let result = this.parse(source)
+
     if (result.failed) return source
+    if (isScaffoldTemplate(result)) return source
 
     const resolvedOptions = resolveFormatOptions({ ...this.options, ...options })
 

javascript/packages/formatter/src/scaffold-template-detector.ts

@@ -0,0 +1,33 @@
+import { Visitor } from "@herb-tools/core"
+import type { ERBContentNode, ParseResult } from "@herb-tools/core"
+
+export const isScaffoldTemplate = (result: ParseResult): boolean => {
+  const detector = new ScaffoldTemplateDetector()
+
+  detector.visit(result.value)
+
+  return detector.hasEscapedERB
+}
+
+/**
+ * Visitor that detects if the AST represents a Rails scaffold template.
+ * Scaffold templates contain escaped ERB tags (<%%= or <%%)
+ * and should not be formatted to preserve their exact structure.
+ */
+export class ScaffoldTemplateDetector extends Visitor {
+  public hasEscapedERB = false
+
+  visitERBContentNode(node: ERBContentNode): void {
+    const opening = node.tag_opening?.value
+
+    if (opening && opening.startsWith("<%%")) {
+      this.hasEscapedERB = true
+
+      return
+    }
+
+    if (this.hasEscapedERB) return
+
+    this.visitChildNodes(node)
+  }
+}

javascript/packages/formatter/test/erb/scaffold-templates.test.ts

@@ -0,0 +1,74 @@
+import { describe, test, expect, beforeAll } from "vitest"
+import { Herb } from "@herb-tools/node-wasm"
+import { Formatter } from "../../src"
+
+import dedent from "dedent"
+
+let formatter: Formatter
+
+describe("ERB scaffold templates", () => {
+  beforeAll(async () => {
+    await Herb.load()
+
+    formatter = new Formatter(Herb, {
+      indentWidth: 2,
+      maxLineLength: 80
+    })
+  })
+
+  test("preserves entire document with escaped ERB output tags", () => {
+    const source = '<%%=content%%>'
+    const result = formatter.format(source)
+
+    expect(result).toEqual(source)
+  })
+
+  test("preserves entire document with escaped ERB logic tags", () => {
+    const source = '<%%if condition%%>'
+    const result = formatter.format(source)
+
+    expect(result).toEqual(source)
+  })
+
+  test("preserves entire document with escaped ERB tags and spaces", () => {
+    const source = '<%%=  content  %%>'
+    const result = formatter.format(source)
+
+    expect(result).toEqual(source)
+  })
+
+  test("preserves mixed escaped and regular ERB tags", () => {
+    const source = dedent`
+      <div>
+        <%%=   spaced_escaped  %%>
+        <%=normal%>
+      </div>
+    `
+    const result = formatter.format(source)
+
+    expect(result).toEqual(source)
+  })
+
+  test("preserves scaffold template from issue #673 exactly as-is", () => {
+    const source = dedent`
+      <%# frozen_string_literal: true %>
+      <%%= simple_form_for(@<%= singular_table_name %>) do |f| %>
+        <%%= f.error_notification %>
+        <%%= f.error_notification message: f.object.errors[:base].to_sentence if f.object.errors[:base].present? %>
+
+        <div class="form-inputs">
+        <%- attributes.each do |attribute| -%>
+          <%%= f.<%= attribute.reference? ? :association : :input %> :<%= attribute.name %> %>
+        <%- end -%>
+        </div>
+
+        <div class="form-actions">
+          <%%= f.button :submit %>
+        </div>
+      <%% end %>
+    `
+    const result = formatter.format(source)
+
+    expect(result).toBe(source)
+  })
+})