compiler/rust: impl AsSlice for Box

Acked-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Reviewed-by: Seán de Búrca <leftmostcat@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/37315>

src/compiler/rust/as_slice.rs

@@ -1,6 +1,8 @@
 // Copyright © 2024 Collabora, Ltd.
 // SPDX-License-Identifier: MIT
 
+use std::ops::Deref;
+use std::ops::DerefMut;
 use std::ops::Index;
 
 pub enum AttrList<T: 'static> {
@@ -26,3 +28,20 @@ pub trait AsSlice<T> {
     fn as_mut_slice(&mut self) -> &mut [T];
     fn attrs(&self) -> AttrList<Self::Attr>;
 }
+
+impl<T, A> AsSlice<T> for Box<A>
+where
+    A: AsSlice<T>,
+{
+    type Attr = A::Attr;
+
+    fn as_slice(&self) -> &[T] {
+        self.deref().as_slice()
+    }
+    fn as_mut_slice(&mut self) -> &mut [T] {
+        self.deref_mut().as_mut_slice()
+    }
+    fn attrs(&self) -> AttrList<Self::Attr> {
+        self.deref().attrs()
+    }
+}