compiler/rustc_mir_transform/src/remove_unneeded_drops.rs

   1 //! This pass replaces a drop of a type that does not need dropping, with a goto.
   2 //!
   3 //! When the MIR is built, we check `needs_drop` before emitting a `Drop` for a place. This pass is
   4 //! useful because (unlike MIR building) it runs after type checking, so it can make use of
   5 //! `Reveal::All` to provide more precies type information.
   6
   7 use crate::MirPass;
   8 use rustc_middle::mir::*;
   9 use rustc_middle::ty::TyCtxt;
  10
  11 use super::simplify::simplify_cfg;
  12
  13 pub struct RemoveUnneededDrops;
  14
  15 impl<'tcx> MirPass<'tcx> for RemoveUnneededDrops {
  16     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
  17         trace!("Running RemoveUnneededDrops on {:?}", body.source);
  18
  19         let did = body.source.def_id();
  20         let param_env = tcx.param_env_reveal_all_normalized(did);
  21         let mut should_simplify = false;
  22
  23         let (basic_blocks, local_decls) = body.basic_blocks_and_local_decls_mut();
  24         for block in basic_blocks {
  25             let terminator = block.terminator_mut();
  26             if let TerminatorKind::Drop { place, target, .. } = terminator.kind {
  27                 let ty = place.ty(local_decls, tcx);
  28                 if ty.ty.needs_drop(tcx, param_env) {
  29                     continue;
  30                 }
  31                 if !tcx.consider_optimizing(|| format!("RemoveUnneededDrops {:?} ", did)) {
  32                     continue;
  33                 }
  34                 debug!("SUCCESS: replacing `drop` with goto({:?})", target);
  35                 terminator.kind = TerminatorKind::Goto { target };
  36                 should_simplify = true;
  37             }
  38         }
  39
  40         // if we applied optimizations, we potentially have some cfg to cleanup to
  41         // make it easier for further passes
  42         if should_simplify {
  43             simplify_cfg(tcx, body);
  44         }
  45     }
  46 }