Correctly identify which backslashes are removed by glimmer (#12302)

* Correctly identify which backslashes are removed by glimmer

* Add tests for #12298

* Don't use ?? syntax

* Add a changelog entry for #12302

* Add a test for #11401

* Move backslash tests to the escape directory

* Backslash test shouldn't have a self-closing p

changelog_unreleased/handlebars/12302.md

@@ -0,0 +1,21 @@
+#### Correctly identify which backslashes are removed by glimmer (#12302 by @MattTheNub)
+
+This caused Prettier to unnecessarily add backslashes every time a file was formatted.
+
+<!-- prettier-ignore -->
+```hbs
+{{! Input }}
+<p>\</p>
+<p>\\</p>
+<p>\\\</p>
+
+{{! Prettier stable }}
+<p>\\</p>
+<p>\\\</p>
+<p>\\\\</p>
+
+{{! Prettier main }}
+<p>\</p>
+<p>\\</p>
+<p>\\\</p>
+```

src/language-handlebars/parser-glimmer.js

@@ -12,8 +12,21 @@ function addBackslash(/* options*/) {
   return {
     name: "addBackslash",
     visitor: {
-      TextNode(node) {
-        node.chars = node.chars.replace(/\\/, "\\\\");
+      All(node) {
+        const childrenOrBody = node.children || node.body;
+        if (childrenOrBody) {
+          for (let i = 0; i < childrenOrBody.length - 1; i++) {
+            if (
+              childrenOrBody[i].type === "TextNode" &&
+              childrenOrBody[i + 1].type === "MustacheStatement"
+            ) {
+              childrenOrBody[i].chars = childrenOrBody[i].chars.replace(
+                /\\$/,
+                "\\\\"
+              );
+            }
+          }
+        }
       },
     },
   };

tests/format/handlebars/escape/__snapshots__/jsfmt.spec.js.snap

@@ -1,5 +1,37 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`backslashes.hbs format 1`] = `
+====================================options=====================================
+parsers: ["glimmer"]
+printWidth: 80
+                                                                                | printWidth
+=====================================input======================================
+<p>\\</p>
+<p>\\\\</p>
+<p>\\\\\\</p>
+<p>\\\\\\ \\\\{{non-escaped-moustache}}</p>
+
+=====================================output=====================================
+<p>\\</p>
+<p>\\\\</p>
+<p>\\\\\\</p>
+<p>\\\\\\ \\\\{{non-escaped-moustache}}</p>
+================================================================================
+`;
+
+exports[`backslashes-in-attributes.hbs format 1`] = `
+====================================options=====================================
+parsers: ["glimmer"]
+printWidth: 80
+                                                                                | printWidth
+=====================================input======================================
+<p data-attr="backslash \\ in an attribute"></p>
+
+=====================================output=====================================
+<p data-attr="backslash \\ in an attribute"></p>
+================================================================================
+`;
+
 exports[`html-entities.hbs format 1`] = `
 ====================================options=====================================
 parsers: ["glimmer"]

tests/format/handlebars/escape/backslashes-in-attributes.hbs

@@ -0,0 +1 @@
+<p data-attr="backslash \ in an attribute"></p>

tests/format/handlebars/escape/backslashes.hbs

@@ -0,0 +1,4 @@
+<p>\</p>
+<p>\\</p>
+<p>\\\</p>
+<p>\\\ \\{{non-escaped-moustache}}</p>