fix(useArrowFunction): preserve directives in the code fix

CHANGELOG.md

@@ -186,6 +186,21 @@ our [guidelines for writing a good changelog entry](https://github.com/biomejs/b
 
   Contributed by @Conaclos
 
+- [useArrowFunction](https://biomejs.dev/linter/rules/use-arrow-function/) now preserves directives ([#4530](https://github.com/biomejs/biome/issues/4530)).
+
+  Previously the rule removed the directives when a function expression was turned into an arrow function.
+  The rule now correctly keeps the directives.
+
+  ```diff
+  - const withDirective = function () {
+  + const withDirective = () => {
+	    "use server";
+	    return 0;
+    }
+  ```
+
+  Contributed by @Conaclos
+
 ### Parser
 
 #### Bug fixes

crates/biome_js_analyze/src/lint/complexity/use_arrow_function.rs

@@ -331,9 +331,14 @@ impl Visitor for AnyThisScopeVisitor {
 
 /// Get a minimal arrow function body from a regular function body.
 fn to_arrow_body(body: JsFunctionBody) -> AnyJsFunctionBody {
+    let directives = body.directives();
     let body_statements = body.statements();
-    // () => { ... }
     let early_result = AnyJsFunctionBody::from(body);
+    if !directives.is_empty() {
+        // The function body has at least one directive.
+        // e.g. `function() { "directive"; return 0; }`
+        return early_result;
+    }
     let Some(AnyJsStatement::JsReturnStatement(return_statement)) = body_statements.iter().next()
     else {
         return early_result;

crates/biome_js_analyze/tests/specs/complexity/useArrowFunction/invalid.ts

@@ -50,4 +50,9 @@ const call = function () {}();
 const staticMember = function(a) {}.bind(null, 0);
 const computedMember = function(a) {}["bind"](null, 0);
 const logical = false || function () {};
-const binary = false + function () {};
+const binary = false + function () {};
+
+const withDirective = function () {
+	"use server";
+	return 0;
+}

crates/biome_js_analyze/tests/specs/complexity/useArrowFunction/invalid.ts.snap

@@ -1,6 +1,7 @@
 ---
 source: crates/biome_js_analyze/tests/spec_tests.rs
 expression: invalid.ts
+snapshot_kind: text
 ---
 # Input
 ```ts
@@ -57,6 +58,12 @@ const staticMember = function(a) {}.bind(null, 0);
 const computedMember = function(a) {}["bind"](null, 0);
 const logical = false || function () {};
 const binary = false + function () {};
+
+const withDirective = function () {
+	"use server";
+	return 0;
+}
+
 ```
 
 # Diagnostics
@@ -520,6 +527,7 @@ invalid.ts:52:26 lint/complexity/useArrowFunction  FIXABLE  ━━━━━━
   > 52 │ const logical = false || function () {};
        │                          ^^^^^^^^^^^^^^
     53 │ const binary = false + function () {};
+    54 │ 
   
   i Function expressions that don't use this can be turned into arrow functions.
   
@@ -530,6 +538,7 @@ invalid.ts:52:26 lint/complexity/useArrowFunction  FIXABLE  ━━━━━━
     52    │ - const·logical·=·false·||·function·()·{};
        52 │ + const·logical·=·false·||·(()·=>·{});
     53 53 │   const binary = false + function () {};
+    54 54 │   
   
 
 ```
@@ -543,6 +552,8 @@ invalid.ts:53:24 lint/complexity/useArrowFunction  FIXABLE  ━━━━━━
     52 │ const logical = false || function () {};
   > 53 │ const binary = false + function () {};
        │                        ^^^^^^^^^^^^^^
+    54 │ 
+    55 │ const withDirective = function () {
   
   i Function expressions that don't use this can be turned into arrow functions.
   
@@ -552,8 +563,37 @@ invalid.ts:53:24 lint/complexity/useArrowFunction  FIXABLE  ━━━━━━
     52 52 │   const logical = false || function () {};
     53    │ - const·binary·=·false·+·function·()·{};
        53 │ + const·binary·=·false·+·(()·=>·{});
+    54 54 │   
+    55 55 │   const withDirective = function () {
   
 
 ```
 
+```
+invalid.ts:55:23 lint/complexity/useArrowFunction  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
 
+  ! This function expression can be turned into an arrow function.
+  
+    53 │ const binary = false + function () {};
+    54 │ 
+  > 55 │ const withDirective = function () {
+       │                       ^^^^^^^^^^^^^
+  > 56 │ 	"use server";
+  > 57 │ 	return 0;
+  > 58 │ }
+       │ ^
+    59 │ 
+  
+  i Function expressions that don't use this can be turned into arrow functions.
+  
+  i Safe fix: Use an arrow function instead.
+  
+    53 53 │   const binary = false + function () {};
+    54 54 │   
+    55    │ - const·withDirective·=·function·()·{
+       55 │ + const·withDirective·=·()·=>·{
+    56 56 │   	"use server";
+    57 57 │   	return 0;
+  
+
+```