Fix incorrect selector transformation when a substring of a selector matches another local classname

.changeset/flat-humans-hammer.md

@@ -0,0 +1,5 @@
+---
+'@vanilla-extract/css': patch
+---
+
+Fixes a bug that caused invalid selectors to be generated when adjacent classnames contained a substring equal to another local classname

packages/css/src/transformCss.test.ts

@@ -2299,6 +2299,61 @@ describe('transformCss', () => {
       }
     `);
   });
+
+  it('should handle adjacent classnames containing a separate local classname as a substring', () => {
+    // Note that `classname2` starts and ends with the same character, so when two `classname1`s are
+    // adjacent, the resulting string will contain `classname2` as a substring
+    const classname1 = 'debugName_hash1';
+    const classname2 = 'debugName_hash1d';
+
+    expect(
+      transformCss({
+        composedClassLists: [],
+        localClassNames: [classname1, classname2],
+
+        cssObjs: [
+          {
+            type: 'local',
+            selector: classname1,
+            rule: {
+              selectors: {
+                ['&&']: {
+                  background: 'black',
+                },
+                [`${classname2}&`]: {
+                  background: 'orange',
+                },
+                [`&${classname2}&`]: {
+                  background: 'orange',
+                },
+                [`${classname2}${classname2}&`]: {
+                  background: 'orange',
+                },
+              },
+            },
+          },
+          {
+            type: 'local',
+            selector: classname2,
+            rule: {},
+          },
+        ],
+      }).join('\n'),
+    ).toMatchInlineSnapshot(`
+      .debugName_hash1.debugName_hash1 {
+        background: black;
+      }
+      .debugName_hash1d.debugName_hash1 {
+        background: orange;
+      }
+      .debugName_hash1.debugName_hash1d.debugName_hash1 {
+        background: orange;
+      }
+      .debugName_hash1d.debugName_hash1d.debugName_hash1 {
+        background: orange;
+      }
+    `);
+  });
 });
 
 endFileScope();

packages/css/src/transformCss.ts

@@ -317,11 +317,20 @@ class Stylesheet {
       const [endIndex, [firstMatch]] = results[i];
       const startIndex = endIndex - firstMatch.length + 1;
 
-      if (startIndex >= lastReplaceIndex) {
-        // Class names can be substrings of other class names
-        // e.g. '_1g1ptzo1' and '_1g1ptzo10'
-        // If the startIndex >= lastReplaceIndex, then
-        // this is the case and this replace should be skipped
+      // Class names can be substrings of other class names
+      // e.g. '_1g1ptzo1' and '_1g1ptzo10'
+      //
+      // Additionally, concatenated classnames can contain substrings equal to other classnames
+      // e.g. '&&' where '&' is 'debugName_hash1' and 'debugName_hash1d' is also a local classname
+      // Before transforming the selector, this would look like `debugName_hash1debugName_hash1`
+      // which contains the substring `debugName_hash1d`’.
+      //
+      // In either of these cases, the last replace index will occur either before or within the
+      // current replacement range (from `startIndex` to `endIndex`).
+      // If this occurs, we skip the replacement to avoid transforming the selector incorrectly.
+      const skipReplacement = lastReplaceIndex <= endIndex;
+
+      if (skipReplacement) {
         continue;
       }