Formatter: fix attribute spacing lost on elements nested in ERB blocks

javascript/packages/formatter/test/html/content-preserving-tags.test.ts

@@ -178,6 +178,18 @@ describe("@herb-tools/formatter - content preserving tags", () => {
     expect(formatter.format(result)).toEqual(result)
   })
 
+  test("preserves attribute spacing on an element nested inside an ERB block (#2142)", () => {
+    const source = `<pre><% if condition %><span class="x">x</span><% end %></pre>`
+    const result = formatter.format(source)
+    expect(result).toEqual(`<pre><% if condition %><span class="x">x</span><% end %></pre>`)
+  })
+
+  test("preserves spacing between multiple attributes on an element nested inside an ERB block", () => {
+    const source = `<pre><% if condition %><span class="x" id="y">x</span><% end %></pre>`
+    const result = formatter.format(source)
+    expect(result).toEqual(`<pre><% if condition %><span class="x" id="y">x</span><% end %></pre>`)
+  })
+
   test("preserves textarea with ERB control flow", () => {
     const source = dedent`
       <textarea>

javascript/packages/printer/src/identity-printer.ts

@@ -1,5 +1,5 @@
 import { Printer } from "./printer.js"
-import { getNodesBeforePosition, getNodesAfterPosition } from "@herb-tools/core"
+import { getNodesBeforePosition, getNodesAfterPosition, isWhitespaceNode } from "@herb-tools/core"
 
 import type * as Nodes from "@herb-tools/core"
 
@@ -44,13 +44,40 @@ export class IdentityPrinter extends Printer {
       this.write(node.tag_name.value)
     }
 
-    this.visitChildNodes(node)
+    // Without `track_whitespace: true` the parser doesn't emit a node for the
+    // whitespace that separates the tag name from the first attribute, or the
+    // whitespace between attributes, so reconstructing children back-to-back
+    // would merge them together (e.g. `<span class="x">` becoming
+    // `<spanclass="x">`). Restore a single separating space wherever the
+    // previous node's end position doesn't line up with the next node's start.
+    //
+    // WhitespaceNode children already print their own whitespace (including
+    // synthetic ones inserted by autofixers, whose location doesn't line up
+    // with the surrounding nodes), so they're skipped here to avoid writing a
+    // duplicate separating space in addition to the whitespace they print.
+    let previousEnd = node.tag_name?.location.end ?? node.tag_opening?.location.end
+
+    node.children.forEach(child => {
+      if (previousEnd && !isWhitespaceNode(child) && !this.samePosition(previousEnd, child.location.start)) {
+        this.write(" ")
+      }
+
+      this.visit(child)
+
+      if (!isWhitespaceNode(child)) {
+        previousEnd = child.location.end
+      }
+    })
 
     if (node.tag_closing) {
       this.write(node.tag_closing.value)
     }
   }
 
+  private samePosition(a: Nodes.Position, b: Nodes.Position): boolean {
+    return a.line === b.line && a.column === b.column
+  }
+
   visitHTMLCloseTagNode(node: Nodes.HTMLCloseTagNode): void {
     if (node.tag_opening) {
       this.write(node.tag_opening.value)