turn rustc_box into an intrinsic

compiler/rustc_hir_analysis/src/check/intrinsic.rs

@@ -86,6 +86,7 @@ pub fn intrinsic_operation_unsafety(tcx: TyCtxt<'_>, intrinsic_id: LocalDefId) -
         | sym::assert_inhabited
         | sym::assert_zero_valid
         | sym::assert_mem_uninitialized_valid
+        | sym::box_new
         | sym::breakpoint
         | sym::size_of
         | sym::min_align_of
@@ -606,6 +607,8 @@ pub fn check_intrinsic_type(
 
             sym::ub_checks => (0, 0, Vec::new(), tcx.types.bool),
 
+            sym::box_new => (1, 0, vec![param(0)], Ty::new_box(tcx, param(0))),
+
             sym::simd_eq
             | sym::simd_ne
             | sym::simd_lt