src/librustc_mir/util/alignment.rs

   1 use rustc::ty::{self, TyCtxt};
   2 use rustc::mir::*;
   3
   4 /// Returns `true` if this place is allowed to be less aligned
   5 /// than its containing struct (because it is within a packed
   6 /// struct).
   7 pub fn is_disaligned<'tcx, L>(
   8     tcx: TyCtxt<'tcx>,
   9     local_decls: &L,
  10     param_env: ty::ParamEnv<'tcx>,
  11     place: &Place<'tcx>,
  12 ) -> bool
  13 where
  14     L: HasLocalDecls<'tcx>,
  15 {
  16     debug!("is_disaligned({:?})", place);
  17     if !is_within_packed(tcx, local_decls, place) {
  18         debug!("is_disaligned({:?}) - not within packed", place);
  19         return false
  20     }
  21
  22     let ty = place.ty(local_decls, tcx).ty;
  23     match tcx.layout_raw(param_env.and(ty)) {
  24         Ok(layout) if layout.align.abi.bytes() == 1 => {
  25             // if the alignment is 1, the type can't be further
  26             // disaligned.
  27             debug!("is_disaligned({:?}) - align = 1", place);
  28             false
  29         }
  30         _ => {
  31             debug!("is_disaligned({:?}) - true", place);
  32             true
  33         }
  34     }
  35 }
  36
  37 fn is_within_packed<'tcx, L>(tcx: TyCtxt<'tcx>, local_decls: &L, place: &Place<'tcx>) -> bool
  38 where
  39     L: HasLocalDecls<'tcx>,
  40 {
  41     let mut cursor = &*place.projection;
  42     while let [proj_base @ .., elem] = cursor {
  43         cursor = proj_base;
  44
  45         match elem {
  46             // encountered a Deref, which is ABI-aligned
  47             ProjectionElem::Deref => break,
  48             ProjectionElem::Field(..) => {
  49                 let ty = Place::ty_from(&place.base, proj_base, local_decls, tcx).ty;
  50                 match ty.kind {
  51                     ty::Adt(def, _) if def.repr.packed() => {
  52                         return true
  53                     }
  54                     _ => {}
  55                 }
  56             }
  57             _ => {}
  58         }
  59     }
  60
  61     false
  62 }