Add a needs_drop intrinsic

Closes #2055

src/rustc/middle/trans/native.rs

@@ -801,6 +801,10 @@ fn trans_intrinsic(ccx: @crate_ctxt, decl: ValueRef, item: @ast::native_item,
       "addr_of" {
         Store(bcx, get_param(decl, first_real_arg), fcx.llretptr);
       }
+      "needs_drop" {
+        Store(bcx, C_bool(ty::type_needs_drop(ccx.tcx, tp_ty)),
+              fcx.llretptr);
+      }
     }
     build_return(bcx);
     finish_fn(fcx, lltop);

src/rustc/middle/trans/type_use.rs

@@ -78,7 +78,7 @@ fn type_uses_for(ccx: @crate_ctxt, fn_id: def_id, n_tps: uint)
             let flags = alt check i.ident {
               "size_of" | "align_of" | "init" |
               "reinterpret_cast" { use_repr }
-              "get_tydesc" { use_tydesc }
+              "get_tydesc" | "needs_drop" { use_tydesc }
               "forget" | "addr_of" { 0u }
             };
             uint::range(0u, n_tps) {|n| cx.uses[n] |= flags;}

src/rustc/middle/typeck.rs

@@ -1899,6 +1899,7 @@ fn check_intrinsic_type(ccx: @crate_ctxt, it: @ast::native_item) {
                             param(ccx, 1u)) }
       "addr_of" { (1u, [arg(ast::by_ref, param(ccx, 0u))],
                    ty::mk_imm_ptr(tcx, param(ccx, 0u))) }
+      "needs_drop" { (1u, [], ty::mk_bool(tcx)) }
       other {
         tcx.sess.span_err(it.span, "unrecognized intrinsic function: `" +
                           other + "`");