Fixed [NavigationRailDestination]'s label opacity while disabled not …

…being coherent with the icon (#132345)

Fixing the opacity of the NavigationRailDestination widget label while
it is disabled, right now it doesn't get affected by the disabled
attribute, which doesn't match the icon that gets affected

* flutter/flutter#132344

I believe this PR should be marked as [test-exempt]

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] All existing and new tests are passing.

packages/flutter/lib/src/material/navigation_rail.dart

@@ -594,7 +594,9 @@ class _RailDestination extends StatelessWidget {
       child: icon,
     );
     final Widget styledLabel = DefaultTextStyle(
-      style: labelTextStyle,
+      style: disabled
+        ? labelTextStyle.copyWith(color: theme.colorScheme.onSurface.withOpacity(0.38))
+        : labelTextStyle,
       child: label,
     );
 

packages/flutter/test/material/navigation_rail_test.dart

@@ -3290,6 +3290,47 @@ void main() {
     tester.pumpAndSettle();
   });
 
+  testWidgetsWithLeakTracking("Destination's label with the right opacity while disabled", (WidgetTester tester) async {
+    await _pumpNavigationRail(
+      tester,
+      navigationRail: NavigationRail(
+        selectedIndex: 0,
+        destinations: const <NavigationRailDestination>[
+          NavigationRailDestination(
+            icon: Icon(Icons.favorite_border),
+            selectedIcon: Icon(Icons.favorite),
+            label: Text('Abc'),
+          ),
+          NavigationRailDestination(
+            icon: Icon(Icons.bookmark_border),
+            selectedIcon: Icon(Icons.bookmark),
+            label: Text('Bcd'),
+            disabled: true,
+          ),
+        ],
+        onDestinationSelected: (int index) {},
+        labelType: NavigationRailLabelType.all,
+      ),
+    );
+
+    await tester.pumpAndSettle();
+
+    double? defaultTextStyleOpacity(String text) {
+      return tester.widget<DefaultTextStyle>(
+        find.ancestor(
+          of: find.text(text),
+          matching: find.byType(DefaultTextStyle),
+        ).first,
+      ).style.color?.opacity;
+    }
+
+    final double? abcLabelOpacity = defaultTextStyleOpacity('Abc');
+    final double? bcdLabelOpacity = defaultTextStyleOpacity('Bcd');
+
+    expect(abcLabelOpacity, 1.0);
+    expect(bcdLabelOpacity, closeTo(0.38, 0.01));
+  });
+
   group('Material 2', () {
     // These tests are only relevant for Material 2. Once Material 2
     // support is deprecated and the APIs are removed, these tests