compiler/rustc_mir_transform/src/remove_zsts.rs

   1 //! Removes assignments to ZST places.
   2
   3 use crate::MirPass;
   4 use rustc_middle::mir::tcx::PlaceTy;
   5 use rustc_middle::mir::{Body, LocalDecls, Place, StatementKind};
   6 use rustc_middle::ty::{self, Ty, TyCtxt};
   7
   8 pub struct RemoveZsts;
   9
  10 impl<'tcx> MirPass<'tcx> for RemoveZsts {
  11     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
  12         // Avoid query cycles (generators require optimized MIR for layout).
  13         if tcx.type_of(body.source.def_id()).is_generator() {
  14             return;
  15         }
  16         let param_env = tcx.param_env(body.source.def_id());
  17         let (basic_blocks, local_decls) = body.basic_blocks_and_local_decls_mut();
  18         for block in basic_blocks.iter_mut() {
  19             for statement in block.statements.iter_mut() {
  20                 if let StatementKind::Assign(box (place, _)) = statement.kind {
  21                     let place_ty = place.ty(local_decls, tcx).ty;
  22                     if !maybe_zst(place_ty) {
  23                         continue;
  24                     }
  25                     let layout = match tcx.layout_of(param_env.and(place_ty)) {
  26                         Ok(layout) => layout,
  27                         Err(_) => continue,
  28                     };
  29                     if !layout.is_zst() {
  30                         continue;
  31                     }
  32                     if involves_a_union(place, local_decls, tcx) {
  33                         continue;
  34                     }
  35                     if tcx.consider_optimizing(|| {
  36                         format!(
  37                             "RemoveZsts - Place: {:?} SourceInfo: {:?}",
  38                             place, statement.source_info
  39                         )
  40                     }) {
  41                         statement.make_nop();
  42                     }
  43                 }
  44             }
  45         }
  46     }
  47 }
  48
  49 /// A cheap, approximate check to avoid unnecessary `layout_of` calls.
  50 fn maybe_zst(ty: Ty<'_>) -> bool {
  51     match ty.kind() {
  52         // maybe ZST (could be more precise)
  53         ty::Adt(..) | ty::Array(..) | ty::Closure(..) | ty::Tuple(..) | ty::Opaque(..) => true,
  54         // definitely ZST
  55         ty::FnDef(..) | ty::Never => true,
  56         // unreachable or can't be ZST
  57         _ => false,
  58     }
  59 }
  60
  61 /// Miri lazily allocates memory for locals on assignment,
  62 /// so we must preserve writes to unions and union fields,
  63 /// or it will ICE on reads of those fields.
  64 fn involves_a_union<'tcx>(
  65     place: Place<'tcx>,
  66     local_decls: &LocalDecls<'tcx>,
  67     tcx: TyCtxt<'tcx>,
  68 ) -> bool {
  69     let mut place_ty = PlaceTy::from_ty(local_decls[place.local].ty);
  70     if place_ty.ty.is_union() {
  71         return true;
  72     }
  73     for elem in place.projection {
  74         place_ty = place_ty.projection_ty(tcx, elem);
  75         if place_ty.ty.is_union() {
  76             return true;
  77         }
  78     }
  79     return false;
  80 }