feat(isolated-declarations)!: Rewrite declare module X to declare namespace X

crates/oxc_isolated_declarations/src/declaration.rs

@@ -127,14 +127,16 @@ impl<'a> IsolatedDeclarations<'a> {
             return decl.clone_in(self.ast.allocator);
         };
 
+        // Follows https://github.com/microsoft/TypeScript/pull/54134
+        let kind = TSModuleDeclarationKind::Namespace;
         match body {
             TSModuleDeclarationBody::TSModuleDeclaration(decl) => {
                 let inner = self.transform_ts_module_declaration(decl);
                 self.ast.alloc_ts_module_declaration(
                     decl.span,
                     decl.id.clone_in(self.ast.allocator),
                     Some(TSModuleDeclarationBody::TSModuleDeclaration(inner)),
-                    decl.kind,
+                    kind,
                     self.is_declare(),
                 )
             }
@@ -144,7 +146,7 @@ impl<'a> IsolatedDeclarations<'a> {
                     decl.span,
                     decl.id.clone_in(self.ast.allocator),
                     Some(TSModuleDeclarationBody::TSModuleBlock(body)),
-                    decl.kind,
+                    kind,
                     self.is_declare(),
                 )
             }

crates/oxc_isolated_declarations/tests/fixtures/module-to-namespace.ts

@@ -0,0 +1,27 @@
+// Test module to namespace conversion
+export module Foo {
+    export var a = 2;
+    export function bar(): void {}
+}
+
+// Nested modules
+export module Outer {
+    export module Inner {
+        export var x = 1;
+    }
+}
+
+// Should preserve namespace as-is
+export namespace Baz {
+    export var c = 3;
+}
+
+// Should preserve global as-is
+declare global {
+    interface GlobalTest {}
+}
+
+// Should preserve string modules as-is
+declare module "test-module" {
+    export var d = 4;
+}

crates/oxc_isolated_declarations/tests/snapshots/module-to-namespace.snap

@@ -0,0 +1,30 @@
+---
+source: crates/oxc_isolated_declarations/tests/mod.rs
+input_file: crates/oxc_isolated_declarations/tests/fixtures/module-to-namespace.ts
+---
+```
+==================== .D.TS ====================
+
+// Test module to namespace conversion
+export declare namespace Foo {
+	var a: number;
+	function bar(): void;
+}
+// Nested modules
+export declare namespace Outer {
+	namespace Inner {
+		var x: number;
+	}
+}
+// Should preserve namespace as-is
+export declare namespace Baz {
+	var c: number;
+}
+// Should preserve global as-is
+declare global {
+	interface GlobalTest {}
+}
+// Should preserve string modules as-is
+declare module "test-module" {
+	var d = 4;
+}