fix(formatter): print decorators on rest parameters (#28731)

oxfmt deletes a decorator on a rest parameter. This happens with the
default settings, and oxfmt reports no error.

```ts
class C {
  m(@Dec() ...args: string[]) {}
}

// before
class C {
  m(...args: string[]) {}
}
```

The parser and codegen support these decorators (#18938, #20132), but
the formatter printed only the rest element and its type annotation.
This PR prints the decorators in the same way as for `FormalParameter`.

The new fixture `ts/parameters/rest-decorators.ts` covers single and
multiple decorators, `@(await p)`, a decorator after another parameter,
and long parameter lists. The output matches Prettier 3.9.10 for every
case.

---

AI assistance was used for this change. I reviewed and tested it.

---------

Co-authored-by: leaysgur <6259812+leaysgur@users.noreply.github.com>

crates/oxc_formatter/src/print/decorators.rs

@@ -35,7 +35,7 @@ impl<'a> Format<'a, JsFormatContext<'a>> for AstNode<'_, 'a, ArenaVec<'a, Decora
                 );
             }
             // Parameter decorators
-            AstNodes::FormalParameter(_) => {
+            AstNodes::FormalParameter(_) | AstNodes::FormalParameterRest(_) => {
                 write!(f, should_expand_decorators(self, f).then_some(expand_parent()));
             }
             AstNodes::ExportDeclaration(_) | AstNodes::ExportDefaultDeclaration(_) => {

crates/oxc_formatter/src/print/mod.rs

@@ -1103,8 +1103,17 @@ impl<'a> FormatWrite<'a> for AstNode<'_, 'a, BindingPattern<'a>> {
 
 impl<'a> FormatWrite<'a> for AstNode<'_, 'a, FormalParameterRest<'a>> {
     fn write(&self, f: &mut JsFormatter<'_, 'a>) {
-        write!(f, [self.rest()]);
-        write!(f, self.type_annotation());
+        let content = format_with(|f| {
+            write!(f, [self.rest()]);
+            write!(f, self.type_annotation());
+        });
+        // Decorators are laid out as on `FormalParameter`, an undecorated rest parameter stays ungrouped
+        let decorators = self.decorators();
+        if decorators.is_empty() {
+            write!(f, [content]);
+        } else {
+            write!(f, [group(&decorators), group(&content)]);
+        }
     }
 }
 

crates/oxc_formatter/tests/fixtures/ts/parameters/rest-decorators.ts

@@ -0,0 +1,20 @@
+// Decorators on a rest parameter are printed like those on other parameters
+class C { m(@d ...args: unknown[]) {} }
+class C2 { constructor(@Inject(TOKEN) ...deps: any[]) {} }
+class C3 { m(@d x, @e ...rest) {} }
+class C4 { async rest(@(await p) ...args: unknown[]) {} }
+class C5 { m(@d @e ...args) {} }
+class C6 {
+  constructor(
+    @Inject(VERY_LONG_TOKEN_NAME_FOR_DEPENDENCY_INJECTION) private readonly a: A,
+    @Inject(ANOTHER_VERY_LONG_TOKEN_NAME) ...rest: unknown[]
+  ) {}
+}
+class C7 { m(@d
+  ...args) {} }
+// Comments around the decorator keep their place
+class C8 { m(@d /* c */ ...args) {} }
+class C9 { m(@d // c
+  ...args) {} }
+class C10 { m(@d ...args // c
+) {} }

crates/oxc_formatter/tests/fixtures/ts/parameters/rest-decorators.ts.snap

@@ -0,0 +1,122 @@
+---
+source: crates/oxc_formatter/tests/fixtures/mod.rs
+---
+==================== Input ====================
+// Decorators on a rest parameter are printed like those on other parameters
+class C { m(@d ...args: unknown[]) {} }
+class C2 { constructor(@Inject(TOKEN) ...deps: any[]) {} }
+class C3 { m(@d x, @e ...rest) {} }
+class C4 { async rest(@(await p) ...args: unknown[]) {} }
+class C5 { m(@d @e ...args) {} }
+class C6 {
+  constructor(
+    @Inject(VERY_LONG_TOKEN_NAME_FOR_DEPENDENCY_INJECTION) private readonly a: A,
+    @Inject(ANOTHER_VERY_LONG_TOKEN_NAME) ...rest: unknown[]
+  ) {}
+}
+class C7 { m(@d
+  ...args) {} }
+// Comments around the decorator keep their place
+class C8 { m(@d /* c */ ...args) {} }
+class C9 { m(@d // c
+  ...args) {} }
+class C10 { m(@d ...args // c
+) {} }
+
+==================== Output ====================
+------------------
+{ printWidth: 80 }
+------------------
+// Decorators on a rest parameter are printed like those on other parameters
+class C {
+  m(@d ...args: unknown[]) {}
+}
+class C2 {
+  constructor(@Inject(TOKEN) ...deps: any[]) {}
+}
+class C3 {
+  m(@d x, @e ...rest) {}
+}
+class C4 {
+  async rest(@(await p) ...args: unknown[]) {}
+}
+class C5 {
+  m(@d @e ...args) {}
+}
+class C6 {
+  constructor(
+    @Inject(VERY_LONG_TOKEN_NAME_FOR_DEPENDENCY_INJECTION)
+    private readonly a: A,
+    @Inject(ANOTHER_VERY_LONG_TOKEN_NAME) ...rest: unknown[]
+  ) {}
+}
+class C7 {
+  m(
+    @d
+    ...args
+  ) {}
+}
+// Comments around the decorator keep their place
+class C8 {
+  m(@d /* c */ ...args) {}
+}
+class C9 {
+  m(
+    @d // c
+    ...args
+  ) {}
+}
+class C10 {
+  m(
+    @d ...args // c
+  ) {}
+}
+
+-------------------
+{ printWidth: 100 }
+-------------------
+// Decorators on a rest parameter are printed like those on other parameters
+class C {
+  m(@d ...args: unknown[]) {}
+}
+class C2 {
+  constructor(@Inject(TOKEN) ...deps: any[]) {}
+}
+class C3 {
+  m(@d x, @e ...rest) {}
+}
+class C4 {
+  async rest(@(await p) ...args: unknown[]) {}
+}
+class C5 {
+  m(@d @e ...args) {}
+}
+class C6 {
+  constructor(
+    @Inject(VERY_LONG_TOKEN_NAME_FOR_DEPENDENCY_INJECTION) private readonly a: A,
+    @Inject(ANOTHER_VERY_LONG_TOKEN_NAME) ...rest: unknown[]
+  ) {}
+}
+class C7 {
+  m(
+    @d
+    ...args
+  ) {}
+}
+// Comments around the decorator keep their place
+class C8 {
+  m(@d /* c */ ...args) {}
+}
+class C9 {
+  m(
+    @d // c
+    ...args
+  ) {}
+}
+class C10 {
+  m(
+    @d ...args // c
+  ) {}
+}
+
+===================== End =====================