fix(linter): fix useImportExtensions handling of index files (#7209)

Co-authored-by: Arend van Beelen jr. <arend@arendjr.nl>

Author: patrickshipe

.changeset/petite-pans-fetch.md

@@ -0,0 +1,19 @@
+---
+"@biomejs/biome": patch
+---
+
+Resolved an overcorrection in [`useImportExtensions`](https://biomejs.dev/linter/rules/use-import-extensions/) when importing explicit index files.
+
+Imports that explicitly reference an index file are now preserved and no longer rewritten to nested index paths.
+
+#### Example
+
+```diff
+// Before
+-      import "./sub/index";
++      import "./sub/index/index.js";
+
+// After
+-      import "./sub/index";
++      import "./sub/index.js";
+```

crates/biome_js_analyze/src/lint/correctness/use_import_extensions.rs

@@ -251,8 +251,9 @@ fn get_extensionless_import(
     };
 
     let is_index_file = resolved_stem.is_some_and(|stem| stem == "index");
+    let import_path_ends_with_index = path.file_name().is_some_and(|name| name == "index");
 
-    let new_path = if is_index_file {
+    let new_path = if is_index_file && !import_path_ends_with_index {
         let mut path_parts = path.as_str().split('/');
 
         // Remove trailing slash and useless path segment.

crates/biome_js_analyze/tests/specs/correctness/useImportExtensions/invalid.ts

@@ -1,5 +1,6 @@
 import "./sub/foo";
 import "./sub/bar/";
+import "./sub/index";
 
 // Guaranteed resolve to 'index.js' file
 import './sub/bar/../'