fixed vec mut slice

sub/core/src/arrays.rs

@@ -52,6 +52,11 @@ impl<const MAX_LEN: usize, T: Sized> FixedVec<MAX_LEN, T> {
         unsafe { core::slice::from_raw_parts(self.buffer.as_ptr() as *const T, self.length) }
     }
 
+    pub fn as_slice_mut(&mut self) -> &mut [T] {
+        // length can't be larger than MaxLen, use unsafe to dodge the panic
+        unsafe { core::slice::from_raw_parts_mut(self.buffer.as_mut_ptr() as *mut T, self.length) }
+    }
+
     pub fn clear(&mut self) {
         self.length = 0;
     }