compiler/rustc_mir_transform/src/remove_zsts.rs

   1 //! Removes assignments to ZST places.
   2
   3 use crate::MirPass;
   4 use rustc_middle::mir::{Body, StatementKind};
   5 use rustc_middle::ty::{self, Ty, TyCtxt};
   6
   7 pub struct RemoveZsts;
   8
   9 impl<'tcx> MirPass<'tcx> for RemoveZsts {
  10     fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
  11         sess.mir_opt_level() > 0
  12     }
  13
  14     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
  15         // Avoid query cycles (generators require optimized MIR for layout).
  16         if tcx.type_of(body.source.def_id()).is_generator() {
  17             return;
  18         }
  19         let param_env = tcx.param_env(body.source.def_id());
  20         let basic_blocks = body.basic_blocks.as_mut_preserves_cfg();
  21         let local_decls = &body.local_decls;
  22         for block in basic_blocks {
  23             for statement in block.statements.iter_mut() {
  24                 if let StatementKind::Assign(box (place, _)) | StatementKind::Deinit(box place) =
  25                     statement.kind
  26                 {
  27                     let place_ty = place.ty(local_decls, tcx).ty;
  28                     if !maybe_zst(place_ty) {
  29                         continue;
  30                     }
  31                     let Ok(layout) = tcx.layout_of(param_env.and(place_ty)) else {
  32                         continue;
  33                     };
  34                     if !layout.is_zst() {
  35                         continue;
  36                     }
  37                     if tcx.consider_optimizing(|| {
  38                         format!(
  39                             "RemoveZsts - Place: {:?} SourceInfo: {:?}",
  40                             place, statement.source_info
  41                         )
  42                     }) {
  43                         statement.make_nop();
  44                     }
  45                 }
  46             }
  47         }
  48     }
  49 }
  50
  51 /// A cheap, approximate check to avoid unnecessary `layout_of` calls.
  52 fn maybe_zst(ty: Ty<'_>) -> bool {
  53     match ty.kind() {
  54         // maybe ZST (could be more precise)
  55         ty::Adt(..)
  56         | ty::Array(..)
  57         | ty::Closure(..)
  58         | ty::Tuple(..)
  59         | ty::Alias(ty::Opaque, ..) => true,
  60         // definitely ZST
  61         ty::FnDef(..) | ty::Never => true,
  62         // unreachable or can't be ZST
  63         _ => false,
  64     }
  65 }