Inline Box drop glue

Box<T> drop glue is pretty simple and should not lead to blow-up problems.
Moreover, not inlining it leads to LLVM pessimizing some pointer-based
optimizations because it thinks the pointer passed to the drop glue could be
escaped.

Fixes rust-lang#46515.  However, this is not a great fix -- the problem still exists for
other smart pointers besides `Box`.

src/librustc_trans/mir/block.rs

@@ -116,6 +116,7 @@ impl<'a, 'tcx> MirContext<'a, 'tcx> {
  fn_ty: FnType<'tcx>,
  fn_ptr: ValueRef,
  llargs: &[ValueRef],
+ force_noinline: bool,
  destination: Option<(ReturnDest<'tcx>, mir::BasicBlock)>,
  cleanup: Option<mir::BasicBlock>
  | {
@@ -140,11 +141,7 @@ impl<'a, 'tcx> MirContext<'a, 'tcx> {
  } else {
  let llret = bcx.call(fn_ptr, &llargs, cleanup_bundle);
  fn_ty.apply_attrs_callsite(llret);
- if this.mir[bb].is_cleanup {
- // Cleanup is always the cold path. Don't inline
- // drop glue. Also, when there is a deeply-nested
- // struct, there are "symmetry" issues that cause
- // exponential inlining - see issue #41696.
+ if force_noinline {
  llvm::Attribute::NoInline.apply_callsite(llvm::AttributePlace::Function, llret);
  }
 
@@ -285,7 +282,18 @@ impl<'a, 'tcx> MirContext<'a, 'tcx> {
  FnType::of_instance(bcx.ccx, &drop_fn))
  }
  };
- do_call(self, bcx, fn_ty, drop_fn, args,
+
+ // Cleanup is always the cold path. Don't inline
+ // drop glue. Also, when there is a deeply-nested
+ // struct, there are "symmetry" issues that cause
+ // exponential inlining - see issue #41696.
+ // We have a special exception for `Box`, see #46515.
+ let force_noinline = if ty.is_box() {
+ ty.boxed_ty().needs_drop(bcx.tcx(), ty::ParamEnv::empty(traits::Reveal::All))
+ } else {
+ true
+ };
+ do_call(self, bcx, fn_ty, drop_fn, args, force_noinline,
  Some((ReturnDest::Nothing, target)),
  unwind);
  }
@@ -420,7 +428,7 @@ impl<'a, 'tcx> MirContext<'a, 'tcx> {
  let llfn = callee::get_fn(bcx.ccx, instance);
 
  // Translate the actual panic invoke/call.
- do_call(self, bcx, fn_ty, llfn, &args, None, cleanup);
+ do_call(self, bcx, fn_ty, llfn, &args, false, None, cleanup);
  }
 
  mir::TerminatorKind::DropAndReplace { .. } => {
@@ -597,7 +605,7 @@ impl<'a, 'tcx> MirContext<'a, 'tcx> {
  _ => span_bug!(span, "no llfn for call"),
  };
 
- do_call(self, bcx, fn_ty, fn_ptr, &llargs,
+ do_call(self, bcx, fn_ty, fn_ptr, &llargs, false,
  destination.as_ref().map(|&(_, target)| (ret_dest, target)),
  cleanup);
  }