Remove feature maybe_uninit_slice

Summary: Trying to make starlark compile on stable or at least understand why it is not possible. So removing non-essential "feature" dependencies from starlark.

Reviewed By: ndmitchell

Differential Revision: D40893208

fbshipit-source-id: 5e20d1ae6913242afebd1250e77763bbfdb656e4

Author: stepancheg

starlark/src/collections/alloca.rs

@@ -144,7 +144,7 @@ impl Alloca {
             for x in data.iter_mut() {
                 x.write(init());
             }
-            let data = unsafe { MaybeUninit::slice_assume_init_mut(data) };
+            let data = unsafe { &mut *(data as *mut [MaybeUninit<T>] as *mut [T]) };
             k(data)
         })
     }
@@ -176,7 +176,7 @@ impl Alloca {
             let _x_drop_guard = DropSliceGuard(x);
             let y = MaybeUninit::write_slice_cloned(y_uninit, y);
             let _y_drop_guard = DropSliceGuard(y);
-            let xy = unsafe { MaybeUninit::slice_assume_init_mut(xy) };
+            let xy = unsafe { &mut *(xy as *mut [MaybeUninit<T>] as *mut [T]) };
             k(xy)
         })
     }

starlark/src/lib.rs

@@ -354,7 +354,6 @@
 #![feature(const_type_id)]
 #![cfg_attr(fbcode_build, feature(core_intrinsics))]
 #![feature(generic_associated_types)]
-#![feature(maybe_uninit_slice)]
 #![feature(maybe_uninit_write_slice)]
 #![feature(ptr_metadata)]
 #![feature(thread_local)]