[Glimmer] improve text/mustache formatting (#6206)

CHANGELOG.unreleased.md

@@ -104,7 +104,50 @@ Previously, Prettier added line breaks between text and mustaches which resulted
 </p>
 ```
 
+### Handlebars: Improve comment formatting ([#6206] by [@gavinjoyce])
+
+Previously, Prettier would sometimes ignore whitespace when formatting comments.
+
+<!-- prettier-ignore -->
+```hbs
+// Input
+<div>
+  {{! Foo }}
+  {{#if @foo}}
+    Foo
+  {{/if}}
+
+  {{! Bar }}
+  {{#if @bar}}
+    Bar
+  {{/if}}
+</div>
+
+// Output (Prettier stable)
+<div>
+  {{! Foo }}
+  {{#if @foo}}
+    Foo
+  {{/if}}{{! Bar }}{{#if @bar}}
+    Bar
+  {{/if}}
+</div>
+
+// Output (Prettier master)
+<div>
+  {{! Foo }}
+  {{#if @foo}}
+    Foo
+  {{/if}}
+  {{! Bar }}
+  {{#if @bar}}
+    Bar
+  {{/if}}
+</div>
+```
+
 [#6209]: https://github.com/prettier/prettier/pull/6209
 [#6186]: https://github.com/prettier/prettier/pull/6186
+[#6186]: https://github.com/prettier/prettier/pull/6206
 [@duailibe]: https://github.com/duailibe
 [@gavinjoyce]: https://github.com/gavinjoyce

src/language-handlebars/printer-glimmer.js

@@ -35,19 +35,22 @@ const voidTags = [
 function printChildren(path, options, print) {
   return concat(
     path.map((childPath, childIndex) => {
+      const childNode = path.getValue();
       const isFirstNode = childIndex === 0;
       const isLastNode =
         childIndex == path.getParentNode(0).children.length - 1;
       const isLastNodeInMultiNodeList = isLastNode && !isFirstNode;
+      const isWhitespace = isWhitespaceNode(childNode);
 
-      if (isLastNodeInMultiNodeList) {
+      if (isWhitespace && isLastNodeInMultiNodeList) {
         return concat([print(childPath, options, print)]);
       } else if (
         isFirstNode ||
         isPreviousNodeOfSomeType(childPath, [
           "ElementNode",
           "CommentStatement",
-          "MustacheCommentStatement"
+          "MustacheCommentStatement",
+          "BlockStatement"
         ])
       ) {
         return concat([softline, print(childPath, options, print)]);

tests/glimmer/__snapshots__/jsfmt.spec.js.snap

@@ -293,6 +293,69 @@ singleQuote: true
 ================================================================================
 `;
 
+exports[`comment.hbs 1`] = `
+====================================options=====================================
+parsers: ["glimmer"]
+printWidth: 80
+                                                                                | printWidth
+=====================================input======================================
+<div>
+  {{! Foo }}
+  {{#if @foo}}
+    Foo
+  {{/if}}
+
+  {{! Bar }}
+  {{#if @bar}}
+    Bar
+  {{/if}}
+</div>
+=====================================output=====================================
+<div>
+  {{! Foo }}
+  {{#if @foo}}
+    Foo
+  {{/if}}
+  {{! Bar }}
+  {{#if @bar}}
+    Bar
+  {{/if}}
+</div>
+================================================================================
+`;
+
+exports[`comment.hbs 2`] = `
+====================================options=====================================
+parsers: ["glimmer"]
+printWidth: 80
+singleQuote: true
+                                                                                | printWidth
+=====================================input======================================
+<div>
+  {{! Foo }}
+  {{#if @foo}}
+    Foo
+  {{/if}}
+
+  {{! Bar }}
+  {{#if @bar}}
+    Bar
+  {{/if}}
+</div>
+=====================================output=====================================
+<div>
+  {{! Foo }}
+  {{#if @foo}}
+    Foo
+  {{/if}}
+  {{! Bar }}
+  {{#if @bar}}
+    Bar
+  {{/if}}
+</div>
+================================================================================
+`;
+
 exports[`component.hbs 1`] = `
 ====================================options=====================================
 parsers: ["glimmer"]

tests/glimmer/comment.hbs

@@ -0,0 +1,11 @@
+<div>
+  {{! Foo }}
+  {{#if @foo}}
+    Foo
+  {{/if}}
+
+  {{! Bar }}
+  {{#if @bar}}
+    Bar
+  {{/if}}
+</div>